UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO 
POSGRADO EN CIENCIAS BIOLÓGICAS 
INSTITUTO DE GEOLOGÍA 
SISTEMÁTICA 
(PROYECTO) 
VARIACIONES MORFOLÓGICAS EN EL COMPLEJO CHANOS LACEPÈDE, 1803 
(GONORYNCHIFORMES, CHANIDAE) DEL PALEOCENO DE CHIAPAS, MÉXICO 
TESIS 
(POR ARTÍCULO CIENTÍFICO) 
TWO CONTEMPORANEOUS MORPHS OF FOSSIL CHANOS LACEPÈDE, 1803 
(GONORYNCHIFORMES, CHANIDAE) FROM PALEOCENE (DANIAN) OUTCROPS 
NEAR PALENQUE (MEXICO) REVEALED BY GEOMETRIC MORPHOMETRICS 
INDICATE CONSERVATISM IN MILKFISHES AFTER THE K/PG BOUNDARY 
QUE PARA OPTAR POR EL GRADO DE: 
MAESTRO EN CIENCIAS BIOLÓGICAS  
PRESENTA: 
ALBERTO GUADARRAMA PÉREZ 
 
TUTOR PRINCIPAL DE TESIS: DR. KLEYTON MAGNO CANTALICE SEVERIANO  
                                                           INSTITUTO DE GEOLOGÍA, UNAM 
COMITÉ TUTOR: DR. LUIS FERNANDO DEL MORAL FLORES 
 FACULTAD DE ESTUDIOS SUPERIORES IZTACALA, UNAM 
 DR. MIGUEL ANGEL TORRES MARTÍNEZ 
                                   INSTITUTO DE GEOLOGÍA, UNAM 
 
CIUDAD UNIVERSITARIA, CD. MX., ENERO 2025 
 
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PROTESTA UNIVERSITARIA DE INTEGRIDAD Y 
HONESTIDAD ACADEMICA y PROfESIONAL 
(GraduaaOo con trabajo esa'ilo) 
De c:oo, .... "idad mn lo dlspueslo en los afilaD 87, frllCdón V, del EslalulO GenenI, &El, pri'ner 
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Que prnsenl6 para obteoor el grado de Mo\ESTRO(A) EN CIENCIAS BIOlóGICAS, es otlglJlllI. de 
mi all\orla y lo ¡"alioli oon rlgOl' metodológloo exigklo por el P'OQrllffill de Posgrado en Ciencias 
BIológicas, dtanóo IIIs fU(lnles de Ideas, 1,)IIOS,lmégen.es, gláfic:oo¡ u 111m lopa de Q/)re$ &mpIeada$ 
pon :su deHrTOIO, 
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UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO 
POSGRADO EN CIENCIAS BIOLÓGICAS 
INSTITUTO DE GEOLOGÍA 
SISTEMÁTICA 
(PROYECTO) 
VARIACIONES MORFOLÓGICAS EN EL COMPLEJO CHANOS LACEPÈDE, 1803 
(GONORYNCHIFORMES, CHANIDAE) DEL PALEOCENO DE CHIAPAS, MÉXICO 
TESIS 
(POR ARTÍCULO CIENTÍFICO) 
TWO CONTEMPORANEOUS MORPHS OF FOSSIL CHANOS LACEPÈDE, 1803 
(GONORYNCHIFORMES, CHANIDAE) FROM PALEOCENE (DANIAN) OUTCROPS 
NEAR PALENQUE (MEXICO) REVEALED BY GEOMETRIC MORPHOMETRICS 
INDICATE CONSERVATISM IN MILKFISHES AFTER THE K/PG BOUNDARY 
QUE PARA OPTAR POR EL GRADO DE: 
MAESTRO EN CIENCIAS BIOLÓGICAS  
PRESENTA: 
ALBERTO GUADARRAMA PÉREZ 
 
TUTOR PRINCIPAL DE TESIS: DR. KLEYTON MAGNO CANTALICE SEVERIANO  
                                                           INSTITUTO DE GEOLOGÍA, UNAM 
COMITÉ TUTOR: DR. LUIS FERNANDO DEL MORAL FLORES 
 FACULTAD DE ESTUDIOS SUPERIORES IZTACALA, UNAM 
 DR. MIGUEL ANGEL TORRES MARTÍNEZ 
                                   INSTITUTO DE GEOLOGÍA, UNAM 
 
CIUDAD UNIVERSITARIA, CD. MX., ENERO 2025 
  
 
COORDINACIÓN GENERAL DE ESTUDIOS DE POSGRADO 
CooRDINACIOtI DEL POSGRADO EN CIENCIAS BIOLOGICAS 
ENTIDAD INSTITUTO DE GEOLoolA 
OFICIO: CGEP/cPC8ilGELIII810J202' 
ASUNTO: anclo "" J~rad<l 
M. " n C Ivonn" R ~mi r u W"nc" 
DirKtor~ G" .... ral d" Administración Escolar, UNAM 
Pr es"n t" 
Me permito informar a usted que en la reunión ordinaria (Vi"ual) del Cornité Académico (!el Posgrado en 
Ciencias Bjo¡ógicas. celebrada el dla O" d" novi"mbr" de 2024 se aprobó el sigoiente jurado para el 
examen de grado de MAESTRO EN CIENCIAS BIOLÓGICAS en 01 campo de conocimiento de 
Sistem ~ tic~ 001 alumno GUADARRAMA PF;REZ ALBERTO con nIlmero de cuenla 3Ut312S4 por la 
modalidad de gfaduación de l esis por articulo ci"ntilico mulado: "Two con t"mpor~neD<J s morphs 01 
lonil CII~nos L~ ce~ d e, 1803 (GonoryncllifDmll>s, Cllan¡d~,,) Irom Paleocen" (Dani~n) outcrops 
near Palenque (Muico) revealed by geometric morpllometrics Indicale conservatism in mil~lishe l 
afler Ih" KlPg boundary", que es producto del proy&Clo realizado en la maestrta que lleva por tllulo: 
"Variaciones morfológicas en el complejo Ch",no. L..acepede. 1803 (Gonorynchilormes. Chanidae) 
del Paleoceno de Clliapal , Mexico", ambos rea lizados bajo la dirección de( DR. KLEYTON MAGNO 
CANTAUCE SEVERIANO, quedando integrado de la sigoiente manera' 
Presidenl .. : 
Vocal' 
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Vocal' 
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DRA. CLAUDIA PATRICIA ORNELAS GARclA 
DR. AORIAN FELIPE GONZALEZ ACOSTA 
DAA MARISOL MONTELlANO BAlLESTEROS 
DR. JAIRO ANDR!:S ARROYAVE GUTIIORREZ 
DR. LUIS FERNANDO DEL MORAL FLORES 
Sin otro particular. me es IIrato enviarle un cordia l saludo. 
ATENTAMENTE 
"POR MI RAZA HABLARÁ EL ESpIRITU" 
Ciudad UnNefS~ari a . Cd. M~ .• a 26 de OO\Iiemlml de 2024 
COORDINADOR DEL PROGRAMA 
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Agradecimientos institucionales 
 
Agradezco primeramente al Posgrado en Ciencias Biológicas (UNAM), por las puertas que 
abrió y los apoyos recibidos; por la formación acádemica y personal derivados de los estudios 
de maestría que han sido invaluables como científico.  
 
A todos los apoyos ecónomicos brindados para la realización del proyecto. Empezando por el 
CONACHYT y su beca de estudios de posgrado con número de CVU 1191092, la cual fue mi 
fuente de manutención durante dos años. Al Proyecto de investigación DGAPA-PAPIIT  
206123, por mi beca de terminación, apoyos para congresos y trabajos de campo. Al Programa 
de Apoyo a los Estudios de Posgrado (PAEP) 2023, por su apoyo parcial en la realización de 
una estancia corta de investigación en el extranjero.  
 
A mi tutor principal, el Dr. Kleyton Magno Cantalice Severiano, quien dirigió esta 
investigación y siempre estuvo comprometido con mi formación y proyecto. A los miembros 
del cómite tutor, el Dr. Miguel Angel Torres Martínez y el Dr. Luis Fernando Del Moral Flores, 
por sus distintas asesorías sobre el posgrado, geología e ictiología durante cuatro semestres. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
Agradecimientos a titulo personal  
A mi casa por más de diez años, la Universidad Nacional Autónoma de México (UNAM), tiene 
mi eterna gratitud por permitirme estudiar gratuitamente todo este tiempo.  
 
Al Instituto de Geología por el acceso a sus instalaciones y la colección Nacional de 
Paleontología.  
 
A los sindolaes, Dr. Luis Fernando Del Moral Flores, Dra. Marisol Montellano Ballesteros, 
Dra. Claudia Patricia Ornelas García, Dr. Jairo Andrés Arroyave Gutiérrez, y al Dr. Adrian 
Felipe Gonzalez Acosta, por su disposición y atención durante revisiones y el proceso de 
titulación. En especial al Dr. Adrian, cuyas revisiones mejoraron ampliamente la parte en 
castellano de esta tesis. 
 
Al enlace del Posgrado del Intituto de Geología, Maria Rodríguez Jiménez, por su accesibilidad 
y atención durante cinco semestres. 
 
A la M. en C. Violeta Romero por catalogar los ejemplares en la Colección Nacional de 
Paleontología del Instituto de Geología, UNAM. 
 
Al Dr. César Espinoza, quien me enseñó tanto la parte teórica de morfometría geométrica, 
como a usar el paquete geomorph de R.  
 
Al Dr. Paulo Brito, por facilitarme el acceso a los Chanidae de Brasil resguardados por la 
Universidade do Estado do Rio de Janeiro (UERJ, Brasil), que usé como material comparativo 
y por sus varias perspectivas en la evolución de Chanidae.  
 
Al M. en F. Adrían Flores y al Dr. Omar Ávalos; a la Dra. Ana Barahona y a la Dra. Erica 
Torrens; a la Dra. Ivonne Garzón y al Dr. Alejandro Oceguera. Sus cursos fueron parte 
fundamental de mi formación y de este proyecto. 
 
 
 
  
A mis nuevos amigxs y colegas que conocí durante el posgrado, Mariana, Dana, Sebastían, 
Scarlett, Minerva, Octavio, Oscar, Pablo, Aurora, Diego, David, Andrea, Melissa y Elsa, 
quienes le dieron identidad y felicidad a esta etapa de mi vida.  
 
A los amigos que conocí en mi estancia en Brasil, quienes me hicieron sentir bienvenido y 
como en casa, en particular a Mariana ‘Irene’, Danilo y Gabrielle.  
 
A mis más cercanos amigos del IGL, UNAM, mis queridos Metzeri y Bernardo, quienes fueron 
las personas más influyentes en mi vida estos semestres. 
 
A mis antiguos amigxs, por todos los años, por todas las cosas. Abril, Beto(s), Carlos, Erasmo, 
América, Rodrigo ‘Rai’, Rodrigo, Adán, Victor, Rodolfo, Manuel, Eduardo, Mauricio(s), 
Fernando, Valente, Alexsa, y especialmente a Raquel.  
 
A toda mi familia. A mis hermanos. A mis abuelos. A mis tíos y tías, primos y primas. Han 
sido unos dolorosos años para todos, gracias por cuidarme. 
 
Finalmente, a mi difunto padre Fernando y a mi madre Laura. Quienes en tantos sentidos son 
responsables de poder seguir luchando por mi sueño de una carrera en la acádemia.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
Dedicada a mi madre, Laura Pérez Gómez 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
Índice 
Resumen………………………………………………….……………………………………1 
Abstract……………………………………………………………………………………......2 
Introducción general…………………………………………………………………………...3 
Manuscrito del Artículo de investigación …...………………………………………….…….7 
Introduction…………………………………………………………………………....9 
Material and methods……………………………...…………………………………10 
 Phylogenetic analysis………………………………………………………………..11 
 Geometric morphometrics………………………………………………………..….12 
  Data acquisition and curation……………………………………………………….12 
  Cluster analyses………………………………………………………………..........14 
  Nomenclatural Acts…………………………………………………………………16 
Results…………………………..……………………………………………………16 
 Systematic Paleontology……………………….…….………………………………16 
 Description……………………………………..…………………………………….18 
  General body features………………………………………………………………..18 
  Neurocranium………………………………………………………………………..19 
  Circumorbital series………………………………………………………………....22 
  Jaws………………………………………………………………………………….22 
  Suspensorium……………………………………………………………………..…24 
  Opercular series……………………………………………………………………...25 
  Hyoid and branchiostegal regions…………………………………………………...25 
  Vertebral column and intramuscular bones………………………………………….26 
  Pectoral girdle and fin……………………………………………………………….27 
  Pelvic girdle and fin…………………………………………………………………28 
  Dorsal fin…………………………………………………………………………….29 
  Anal fin………………………………………………………………………………32 
  Caudal skeleton……………………………………………………………………...33 
  Scales………………………………………………………………………………...35 
 Phylogenetic analysis…………………………………………………….…………..35 
 Geometric morphometrics…………………………………………………………...38 
Discussion…………………………………………………………...……………….43 
   †Chanos chautus sp. nov. systematic position……………………………………….43 
†Chanos chautus sp. nov. and other fossil Chanidae…………………………………45 
The morphotypes and Chanos chanos………………………………………………..47 
Specimens as juveniles……..………………………………………………………...50 
The Chanos morphotypes from Palenque…………………………………………….52 
Head and depth variations………………………..…………………………………..56 
Conclusion……………………………………………………………………………57 
Acknowledgments……………………………………………………………………58 
Funding……………………………………………………………………………….59 
References……………………………………………………………………………60 
Discusión general y Conclusiones generales………………………………………………....71 
Referencias Bibliográficas…...………………………………………………………………74 
Anexo (Supporting Information)…………………………………………………………......80 
 1 
Resumen 
En este trabajo se estudió material fósil de peces de la familia Chanidae (Teleostei, 
Gonorynchiformes) de las localidades del Paleoceno (Daniano) División del Norte y Belisario 
Domínguez próximas al sitio arqueológico de Palenque (Chiapas, México). El análisis 
filogenético indica que los ejemplares fósiles están estrechamente relacionados con la especie 
actual Chanos chanos, una de las sinapomorfias del género Chanos es un pleurostilo o 
complejo caudal que no se encuentra en ningún género fósil de la familia. La comparación 
entre fósiles y actuales reveló una notable similitud estructural de casi todos los huesos visibles. 
A pesar de su similitud cualitativa, se encontró señal de variación cuantitativa con herramientas 
de morfometría geométrica en la muestra fósil; Chanos chanos es muy variable en rasgos 
morfométricos y merísticos. Se lidió con la torsión post-mortem y los landmarks perdidos en 
ciertos ejemplares para maximizar el tamaño de muestra. Los análisis revelan la presencia de 
dos morfotipos, uno con cabeza más grande cuerpo más profundo y otro un con cabeza pequeña 
y cuerpo delgado que coexistieron en tiempo y espacio, al encontrarse en el mismo nivel y en 
ambas localidades. Se puso a prueba el efecto alométrico y se exploraron diferentes escenarios 
que podrían explicar los patrones observados, como dimorfismo sexual o la existencia de dos 
especies simpátricas para los morfotipos, y forrajeo diferencial para las variaciones de la 
mandíbula, cabeza y profundidad. Además de la estasis morfológica del esqueleto, desde el 
límite Cretácico-Paleógeno, Chanos ha conservado el rasgo de historia de vida de migración 
de juveniles a cuerpos de agua transicionales que les sirven como guardería. Dada su estructura 
paleontológica y tafonómica, las localidades muestran influencia tanto de ambientes marinos 
como transicionales. 
 
 
 
 2 
Abstract 
We described chanid material from the Paleocene (Danian) localities of División del Norte and 
Belisario Domínguez near the archeological site of Palenque, Chiapas State, southeastern 
Mexico. The parsimony-based morphological phylogeny indicates that the specimens are 
closely related to the extant milkfish, Chanos chanos (Teleostei, Ostariophysi), and the 
comparative anatomy reveals a remarkable qualitative similarity of almost every visible bone. 
Among the synapomorphies for the genus Chanos, is a pleurostyle or caudal complex, which 
is missing in all other chanid fossils. Extant milkfish are highly variable in meristic and 
morphometric traits, and we found a signal for quantitative variation with geometric 
morphometrics tools in the fossil sample. We first dealt with post-mortem body torsion and 
missing landmarks. The main analysis shows a pattern of two forms present in both localities. 
A group of specimens shows a bigger head and deeper body than the slender and smaller head 
of the rest, implying that two types of milkfish coexisted in time and space. We tested for 
allometry and explored scenarios that can explain the patterns, such as sexual dimorphism or 
two sympatric and closely related species for the morphotypes, and differential resource 
utilization for the jaw, head, and depth variations. Furthermore, we argue that, alongside the 
morphological stasis, Chanos has conserved the life history trait of fry migration towards near-
shore nurseries through protracted time (~63mybp). We infer the fish were juvenile, and the 
paleontological assemblage and taphonomy suggest that the localities exhibit the influence of 
both marine and transitional environments. 
 
 
 
 
 3 
Introducción general 
El orden Gonorynchiformes (=Anotophysi) es un pequeño grupo (39 especies) de peces del 
superorden Ostariophysi (Actinopterygi, Otocephala) grupo hermano al clado extremadamente 
diverso (≈11 000 especies) Otophysi (Nakatani et al., 2011; Kalumba et al., 2023; Murray et 
al., 2023; Near y Thacker, 2024).  
Dependiendo de la fuente, los resultados difieren sobre cómo se relacionan entre sí las 
tres familias: Chanidae (1 especie eurihalina), Gonorynchidae (1 género, 5 especies marinas) y 
Kneriidae (5 géneros, 37 especies de agua dulce) (Kalumba et al., 2023; Murray et al., 2023). 
Hipotésis filogenéticas a partir de datos morfológicos incluyen a Chanidae como grupo 
hermano de Gonorynchidae + Kneriidae (Fink y Fink, 1981, 1996; Grande, 1994, 1996; Grande 
y Poyato-Ariza, 1995, 1999; Poyato-Ariza et al., 2010). Mientras que, estudios moleculares de 
datos mitocondriales y nucleares incluyen consistentemente a Gonorynchidae como grupo 
hermano de Chanidae + Kneriidae; resultado similar a partir de datos moleculares y 
morfológicos combinados en análisis de evidencia total (Lavoué et al., 2005, 2012; Davis et 
al., 2013; Near et al., 2014).  
Chanos chanos (Fabricius, 1755), chano, o sábalote es la única especie actual de la 
familia Chanidae. Siendo eurihalino, puede ser encontrado en agua dulce, salobre, marina e 
incluso hipersalina. Hábita en aguas marinas someras >20° C del Indo-Pacífico, desde la costa 
oriental de África hasta la costa occidental de América (Bagarinao, 1994). Es uno de los peces 
más importantes en la acuicultura asiática; los humanos se han aprovechado de su historia de 
vida y tolerancia, adaptando lagunas costeras como criaderos desde hace más de 500 años 
(Bagarinao, 1991; Coad et al., 2015). México exportó 3795 toneladas de sábalote provenientes 
de la pesca en 2016, principalmente a Vietnam (SIAP, 2017). 
En el registro fósil, los peces de la familia Chanidae están presentes desde el Berriasiano 
(Cretácico Temprano) hasta el Burdigaliano (Cenozoico, Mioceno) (Fara et al., 2010). El pico 
 4 
de diversidad a nivel género (≈10) ocurrió durante el Cretácico Temprano, quedando uno o dos 
géneros relicto entrando en el Ceneozoico (Fara et al., 2010; Taverne et al., 2019). En general 
Chanidae es un grupo morfológicamente homogéneo, la etimología de los nombres de los 
géneros †Prochanos Bassani, 1882, †Parachanos Arambourg y Schneegans, 1935, 
†Francischanos Ribeiro et al., 2022, †Cabindachanos Taverne, De Putter, Mees, Smith, 2019, 
y †Vangus Murray, Brinkman, Friedman, Krause, 2023 –la palabra para el sábalote en ciertas 
partes de Madagascar (Murray et al., 2023)–, aluden a su gran parecido y relación con C. 
chanos. Patterson (1984: p167) incluso describió los cráneos de †Tharrhias Jordan y Branner, 
1908 y C. chanos como casi idénticos. 
 Se han reportado cuatro especies fósiles del género Chanos: †Chanos torosus 
Danil’chenko, 1968 (Paleoceno, Thanetiano) de Turkmenistán; †Chanos forcipatus Kner y 
Steindachner, 1863 (Eoceno, Lutetiano tardío-Barteniano temprano) de Italia; †Chanos brevis 
(Heckel 1854) y †Chanos zignoi Kner y Steindachner, 1863, (Oligoceno, Chattiano-Rupeliano) 
también de Italia (Fara et al., 2010). Sin embargo, sus descripciones son superficiales y su 
estatus taxonómico es problemático. Por ejemplo, las diagnosis de las especies descritas para 
Italia se basan en diferencias en el número de vertebras totales, respectivamente 45, 40 y 35 
(Fara et al., 2010).  
Los datos morfométricos y merísticos se han usado desde Cuvier para discriminar 
especies de peces (Sidlauskas et al., 2011). La comparación de dichos datos se convirtió en 
práctica regular para mediados del siglo XIX, y para el siglo XX se había estandarizado un 
sistema general de medidas lineales: longitudes, anchos y alturas (Sidlauskas et al., 2011). El 
análisis de la información basada en estos datos de distancias analizados con estadística 
multivariada también se les conoce como morfometría clásica o tradicional (Adams et al., 
2004). 
 5 
La morfometría se revolucionó a finales del siglo XX con la llegada de métodos para 
analizar la variación de la forma con sistemas de coordenadas Cartesianas, conocidos como 
morfometría geométrica (Rohlf y Slice, 1990). Los métodos de morfometría geométrica son 
conocidos por su poder estadístico y su fácil visualización (Sidlauskas et al., 2011). Cada 
landmark representa un punto en el plano cartesiano con nombre y coordenadas (Kershbaumer 
y Sturmbauer, 2011). Los landmarks tipo 1 y 2 son homólogos (correspondencia 
biológica/topológica verdadera) entre especies, mientras que las medidas lineales no lo son 
(Strauss y Bookstein, 1982; Kershbaumer y Sturmbauer, 2011). A su vez, los datos de 
morfometría geométrica son multidimensionales, llegando a captar aspectos más integrativos 
de la forma que las distancias lineales, como áreas, perímetros, ángulos o volúmenes en 
sistemas 3D (Kershbaumer y Sturmbauer, 2011).  
Por dicho poder estadístico, en peces se ve acentuado el problema del arqueamiento del 
cuerpo causado por el rigor mortis u otros efectos tafonómicos; el cual se debe atender para 
evitar introducir variación no intrínseca en análisis multivariados (Sidlauskas et al., 2011; San 
Román et al., 2024). Hasta la fecha, el único trabajo basado en morfometría geométrica con 
peces del orden Gonorynchiformes corresponde a los géneros fósiles de la familia Chanidae 
†Rubiesichthys Wenz, 1984 y †Gordichthys Poyato-Ariza, 1994  del Cretácico Temprano de 
España; que fueron usados en un modelo experimental dirigido a evaluar diferentes métodos 
de ‘unbending’ o destorsión que buscan lidiar con el arqueamiento (San Román et al., 2024).  
Algunos análisis multivariados usando datos merísticos y morfométricos tradicionales 
han sido publicados sobre C. chanos en el contexto de variación geográfica (Winans, 80, 85; 
SriHari et al., 2019a,b); recientemente en Kneria Steindachner, 1866, donde se delimitaron 
especies crípticas (Kalumba et al., 2023) y en el género fósil †Rubiesichthys Wenz, 1984, donde 
se concluyó dimorfismo sexual (Poyato-Ariza, 2005).  
 6 
La posible presencia de más de un tipo de Gonorynchiformes en los dépositos del 
Paleoceno de Chiapas, México, fue reportada por Alvarado-Ortega et al. (2015). Se conocen 
dos localidades danianas al norte del estado, Belisario Domínguez (17° 25' 28.60" N y 91° 58' 
46.80" W) y División del Norte (17° 16' 12.17" N y 97° 40' 40.7" W); 9.5 km al sur y 6 km al 
suroeste de la ciudad de Palenque respectivamente (Figura 1). Son canteras de dificil acceso, 
de apenas un par de metros de profundidad. Además, en ambas el suelo intemperiza los estratos 
superiores, dificultando el levantamiento de una columna estatrigráfica. Las rocas son 
sedimentarias, conformadas por arcillas y limos en una matriz rica en carbonatos, o rocas 
margas. La edad de ≈63 millones de años antes del presente fue determinada a partir de un 
estudio isotópico 𝑆𝑟!"/𝑆𝑟!# usando dientes de Pycnodontiformes (Cuevas-García y Alvarado-
Ortega, 2009; Alvarado-Ortega et al., 2015). Por su localización y litología deben pertenecer a 
la Formación Tenajapa o a la Formación Lacandón; ambas son lateralmente continuas y sus 
límites no están definidos (Alvarado-Ortega et al., 2015). Los estratos del Paleoceno cubren 
discordantemente a los del Crétacico (Formación La Angostura), cubiertos discordantemente 
por las formaciones El Bosque y Lomut del Eoceno (Alvarado-Ortega et al., 2015; Martens y 
Sierra-Roja, 2021). 
 
 
 7 
 
Figura 1. Mapa de la zona de estudio. En amarillo la unidad Tenejapa-Lacandón (Paleoceno), 
en blanco [abajo] Formación El bosque (Eoceno), en violeta Formación Tulijá (Mioceno), en 
café Formación Lomut (Eoceno), en gris sedimentos recientes [arriba].  
 
La investigación presentada en el siguiente artículo se centra en dos principales 
cuestiones. En primer lugar las relaciones filogenéticas del material proveniente de BD y DN, 
poniendo a prueba la hipotesis inicial de un pez chánido; parte de la investigación 
principalmente desarrollada durante la licenciatura (Guadarrama, 2022). Como parte del 
proyecto de maestría, dada la alta variación en las proporciones de la cabeza entre ejemplares, 
se puso a prueba la presencia de más de una forma de pez chánido.  
 
 
Manusctrito del Artículo de investigación 
Enviado a PLOS ONE. Versión octubre 2024, incluye modificaciones sugeridas por el revisor 
anónimo. 
 8 
Two contemporaneous morphs of fossil Chanos Lacepède, 
1803 (Gonorynchiformes, Chanidae) from Paleocene (Danian) 
outcrops near Palenque (Mexico) revealed by geometric 
morphometrics indicate conservatism in milkfishes after the 
K/Pg boundary 
Alberto Guadarrama1, 2 *   and Kleyton M Cantalice2  
 
1 Posgrado en Ciencias Biológicas, Universidad Nacional Autónoma de México, Coyoacán, 
Ciudad de México, México. 
2 Departamento de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de 
México; Coyoacán, Ciudad de México, México. 
 
*Corresponding author 
E-mail: guadarrama_alberto@ciencias.unam.mx (AG) 
 
 
 
 
 
 
 
 9 
Introduction 
Gonorynchiformes is an order of teleostean fishes composed of three families: Chanidae, 
Gonorynchidae, and Kneriidae. This clade is the sister group to the remaining ostariophysians, 
the fishes with functioning Weberian Apparatus gathered into Otophysi: Cypriniformes, 
Gymnotiformes, Characiformes, Siluriformes, and Cithariniformes [1]. In contrast to 
otophysians, a group that includes over a quarter of extant fish diversity, fossil and extant 
diversity of Gonorynchiformes is depauperate [1-3]. They probably have a Pangean or Tethyan 
origin and appear in the fossil record during the Berriasian (Lower Cretaceous) period [4]. 
The milkfish Chanos chanos (Fabricius, 1755) is the sole extant Chanidae. It is a large, 
long-lived, pelagic, generalist, euryhaline, plastic, and pantropical Indo-Pacific fish [5, 6]. 
Chanos chanos can show statistically significant morphometric and meristic variability 
between distant allopatric populations, just as between close panmictic populations [7-10]. In 
the nineteenth century, multiple junior synonyms of this species were coined based on 
geographical variations [6]. 
The fossil record of Chanidae comprises around 13 genera and 16 species distributed 
in South America, Africa, Asia, and Europe, spanning from the Lower Cretaceous (Berriasian) 
to the Miocene (Burdigalian) [2, 11-16]. The morphological features of all the chanid genera 
are roughly similar [14]. Recently, detailed anatomical studies resulted in newly recognized 
taxa [12-14], but the validity of several fossil species remains questionable [2]. Several named 
morphs, such as †Dastilbe crandalli  Jordan, 1910, †D. batai Gayet, 1989, and †D. elongatus 
Silva-Santos, 1947, which differ in morphometric proportions [2, 17-21]; †Chanos brevis 
(Haeckel, 1854), †C. zignoi Kner and Steindachner, 1863, and †C. forcipatus Kner and 
Steindachner, 1863 which differ in vertebrae count, are considered junior synonyms of their 
type, and their differences are generally explained by geographical variation [2, 17]. The 
 10 
species †Rubiesicthys gregalis Wenz, 1984 has two coexisting morphs, inferred to be a case of 
sexual dimorphism [22]. 
Here, we present newly described chanid fossil forms, retrieved from two small and 
shallow paleontological sites 9.5 km south and 6 km southeast respectively to Palenque, 
Chiapas State, southeastern Mexico. These sites are the Belisario Domínguez (BD) and the 
División del Norte (DN) quarries, previously reported by different authors [e.g., 23, 24] as 
Danian (≈ 63 mybp) outcrops of the Paleocene Tenejapa-Lacandón geological unit. These two 
formations are contemporaneous and laterally continuous, deposited in the periphery of a 
shallow carbonated platform [25]. Both quarries show parallel and finely laminated carbonated 
strata of yellow-cream-colored marls in an 8 to 15 cm sequence. Little is known about other 
geological aspects of these localities, and no stratigraphic column exists. 
The paleontological structure is similar in both localities, consisting primarily of plant 
and fish fossils. Currently, the fish assemblage of these sites includes Pycnodontiformes, 
Anguilliformes, Clupeiformes, and different acanthomorph clades [e.g., 23, 26]. The quality of 
preservation and geographical-temporal distribution of these fossils near the Chicxulub impact 
crater and the K/Pg boundary associated with the mass extinction event highlights the 
importance of these localities in the understanding of the origin and tempo of the modern 
marine fish fauna [23, 27]. 
 
Material and methods 
The specimens selected to perform this work are the best-preserved, most-complete, and least-
deformed for a total of 54 specimens housed in the Colección Nacional de Paleontología 
(CNP), Instituto de Geología, Universidad Nacional Autónoma de México, at Mexico City, 
Mexico. (see Supporting information, S1 PDF). No permits were required for the described 
study, which complied with all relevant regulations. Most of these fossils required little 
 11 
preparation, but we removed rock sediments from some of the fossils with fine brushes, 
needles, and air scribes under a stereoscopic microscope. 
Additionally, we made rubber molds of the holotype and selected specimens. One 
specimen of extant C. chanos from the CNP, CMR 1259, was cleared and stained following 
Esguícero and Bockmann [28] protocol for the comparative anatomy analysis. The 
nomenclature of osteological elements follows Grande and Poyato-Ariza [29], Grande and 
Arratia [30], Poyato-Ariza et al. [31], and Koch and Moritz [32]. 
 
Phylogenetic analysis 
We were first interested in the relationships within Chanidae of the examined material, testing 
the original hypothesis of a chanid and gonorynchiform fish. For this, we performed a 
parsimony-based phylogenetic analysis based on the morphological matrix of Ribeiro et al. 
[13], which works upon the original in Poyato-Ariza et al. [31] and the next iteration of Amaral 
and Brito [11]. Being a revision of chanid relationships, it excludes myologic characters and 
most of the Kneriidae and Gonorynchidae taxa. 
We added a single composite coding of †Chanos chautus sp. nov. in Mesquite version 
3.81 [33], as the individual coding of morphotypes would be identical. We modified the state 
0 to 1 of character 75 (abutting contact of anterior neural arches) for C. chanos per our 
observations of the recent material. Since their predorsal neural arches are autogenous and 
large, especially the first five, they are in contact with those adjacent anteriorly and posteriorly 
without overlapping. 
Parsimony analysis and character mapping were performed in TNT v.1.6 [34], 
indicating †Diplomystus Cope, 1877 as the outgroup as explained in the original work [31]. 
Characters were unweighted and treated as unordered. The maximum memory for trees was 
increased to 10,000. Per the small number of terminal taxa (15), we analyzed the dataset by the 
 12 
exact tree search implicit enumeration (or branch and bound), which finds all optimal 
topologies. The clade consistency was obtained by bootstrap analysis with 1,000 
pseudoreplications. The discrete matrix is available in Supporting information (S1 file). 
 
Geometric morphometrics 
Data acquisition and curation 
Only laterally preserved and complete or semi-complete specimens (44) from the original 
comparative anatomy sample were selected for geometric morphometrics (GM). We allowed 
a maximum of four lost landmarks, whether completely absent or preservationally distorted; 
eight specimens had one missing landmark (mlm), one had two mlm, three had three mlm, and 
one had four mlm (see Supporting information, S1 PDF). Fifteen specimens were not bent, but 
we allowed specimens with different degrees of dorsoventral torsion. To keep spurious 
ontogenetic effects to a minimum we also restricted our dataset to fish with a standard length 
(SL) >35 mm. 
Standardized high-resolution photos (e.g., with the same tripod, same light, and same 
settings) were taken with a Nikon DSLR D5500 camera and an AF-S Micro NIKKOR 60 mm 
lens to prevent any deformation of images and perspective. Images of specimens preserved on 
their right side were mirror-reflected to standardize all specimens on their left side. Images in 
.jpg format were converted to .tps format with tpsUtil [35]. 
Fifteen landmarks, 14 homologous (biological/topological correspondence), and one 
reference point (14) [36, 37] were digitized in tpsDig2 [38]: (1) snout tip, (2) posterior end of 
frontals, (3) origin of the dorsal fin, (4) insertion (end) of the dorsal fin, (5) anteriormost dorsal 
procurrent ray of the caudal fin, (6) end of the vertebral column (pleurostyle and hypural plate 
1), (7) anteriormost ventral procurrent ray of the caudal fin, (8) insertion of the anal fin, (9) 
origin of the anal-fin, (10) origin of the pelvic-fin rays, (11) origin of the pectoral-fin rays, (12) 
 13 
cleithrum-supracleithrum articulation (posteriormost edge of the opercle), (13) interopercle-
subopercle articulation (anteroventralmost edge of opercle), (14) angle between the arms of the 
preopercle’s ridge, and (15) ascending process of parasphenoid (posteroventral margin of the 
orbit). 
A table describing the type, position, definition, and further notes on precise landmark 
placement is available in Supporting information (S1 PDF). This skeletal anatomy landmark 
system uses other 2D ostariophysan fish landmark systems as reference [e.g., 39]. During 
design, we accounted for the general preservation of the specimens while sufficiently 
characterizing the shape and its variation. 
The lost landmarks of 15 specimens of the raw dataset (S2 File) were estimated with 
the estimate.missing() function of geomorph [40, 41] in RStudio Build 554 [42]. Variations of 
tpsDig2 and tpsUtil [35, 38, 43] unbending procedure were performed to correct dorsoventral 
torsion. This protocol consists of adding extra landmarks on a midline, in this case, 8 or 10 
landmarks, whether considering only postcranial torsion or along all of SL, respectively) along 
the spine; tps then calculates the perpendicular deviations on a straight line according to a cubic 
or quadratic curve fit. Finally, landmarks 16 to 25 were removed for further analysis. Four 
specimens had particularly low (<0.25) regression scores and did not adjust to any unbending 
variation, and thus were not used in the main analyses, leaving a final n of 40 for GM in the 
not-bent + unbent sample (S5 File). Selected landmark-based linear morphometrics 
(interlandmark distance) measurements were calculated using the measurement tool of tpsDig2 
[37] (see Supporting information, S1 PDF). 
Bent or incomplete specimens are sometimes removed from the datasets [e.g., 38] as 
they introduce spurious variation. Nonetheless, different unbending protocols have yielded 
success in minimizing the effect of torsion [44-46]; among these, tps-unbending was found to 
be the best option for dealing with arching in fossils, especially for multivariate analyses, as 
 14 
assessed in the Chanidae of Las Hoyas (Cretaceous, Spain) [47]. As in this study, they also 
found that most specimens adjusted better to a cubic rather than a quadratic fit, probably as 
arching in fossils is often irregularly sigmoidal [47]. Also, Arbour and Brown [48] 
demonstrated with different manipulations of empirical datasets that estimation can be 
preferred to landmark or specimen deletion in the majority of cases, particularly in datasets 
with low disparity and if using the regression method (implemented in estimate.missing() 
function) as their estimates fell within the digitization error range.  
 
Cluster analyses 
All further GM analyses were performed in RStudio Build 554 [42] with the R package 
geomorph v.4.0 [40, 41]. The concerns regarding replicability derived from preservation and 
ambiguity of landmark definition were addressed by testing digitization error [49] in the 
estimated sample. This was calculated by performing a non-parametric and distance-based 
Procrustes analysis of variance (ANOVA), comparing digitization replicates of the total sample 
(bent and not-bent) taken on separate days (one month apart, different researcher, AG, and KC) 
through the procD.lm() function and 999 iterations and default settings, using as linear model: 
Procrustes coordinates (shape) explained by the independent variables days and individuals. 
The function uses Procrustes distances among specimens as a measure of SS instead of 
covariance matrices, preferable for small sample sizes. It evaluates such values through 
permutation and a test-statistic analog to Fishers’s F-ratio [50, 51]. 
For the main analyses, position, orientation, and scale effects were removed using the 
generalized Procrustes superimposition (GPA) method using the gpagen() function, producing 
only shape variables [46, 52]. Using the plotOutliers() function, we searched for outliers, which 
plots specimens according to their Procrustes distance from the mean shape. A principal 
component analysis (PCA) of Procrustes coordinates was performed to identify the major 
 15 
patterns of shape variation [53-55] using the gm.prcomp() function in the not bent (n=15), 
‘total’ (n=44, not-bent and bent), and ‘unbent’ (n=40, not-bent and unbent) datasets (S2-5 
Files). We visualized the morphospace and described shape changes for the first three PCs 
(differences between minimum and maximum) through the plotRefToTarget() function. Up to 
this point, no a priori groups were considered, as our null hypothesis was a single variable 
shape. Since visualization signaled two unambiguous groups (no-overlapping) on the unbent 
dataset (see Results), 23 specimens of henceforth morphotype 1 (M1) and 17 of morphotype 2 
(M2), we decided to test the significance of differences between the two and the effect of size, 
despite the small sample sizes. We, therefore, performed a Procrustes ANOVA through the 
procD.lm() function with 99 (exploratory run, per the small sample) and with 999 permutations 
and default settings, the linear model being: shape explained by morphotypes and log-
transformed centroid size (CS) (i.e., does shape change with size?) and the interaction between 
the two kinds of independent variables (i.e., do morphotypes differ in size?). CS is traditionally 
used as a proxy of size in geometric morphometrics [e.g., 56-58], it has no relation to shape 
and is defined as the square root of the sum of the squared distances of all landmarks to their 
centroid [36, 59]. We visualized the allometric relationship of shape and size according to our 
ANOVA fit via plotAllometry() (PredLine method, a regression) with CS as the size predictor 
for the fitted PC shape values. 
As we opted for a Procrustes permutational ANOVA because of our sample size, we 
used the whole shape instead of using only the principal axes of variation (variance-covariance 
matrices), whether PCs or relative warps (RWs) [e.g., 45]. We also opted for PCA and not 
canonical variates analysis (CVA) [e.g., 60, 61], as it requires a priori grouping while 
maximizing differences between these groups [62, 63]. We did not add locality as a variable as 
¾ of specimens come from BD and 10 (seven M1, three M2) from DN (See Supporting 
 16 
information, S1 PDF). Geometric morphometrics datasets and the R code are available in 
Supporting information (S2-S6 Files). 
 
Nomenclatural Acts 
The electronic edition of this article conforms to the requirements of the amended International 
Code of Zoological Nomenclature, and hence the new name contained herein is available under 
that Code from the electronic edition of this article. This published work and the nomenclatural 
acts it contains have been registered in ZooBank, the online registration system for the ICZN. 
The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information 
viewed through any standard web browser by appending the LSID to the prefix 
""http://zoobank.org/"". The LSID for this publication is: urn:lsid:zoobank.org:pub: 95765763-
6EA6-4C00-9094-3F767306C392. The electronic edition of this work was published in a 
journal with an ISSN, and has been archived and is available from the following digital 
repositories: PubMed Central and LOCKSS. 
 
Results 
Systematic Paleontology 
 
Division TELEOSTEI Müller, 1845 
Infracohort OSTARIOPHYSI Sagemehl, 1885 
Series ANOTOPHYSI Rosen and Greenwood, 1970 
Order GONORYNCHIFORMES Regan, 1909 
Family CHANIDAE Jordan, 1887 
Subfamily CHANINAE sensu Poyato-Ariza et al., 2010 
 17 
Tribe CHANINI sensu Poyato-Ariza et al., 2010 
Genus CHANOS Lacepède, 1803 
†Chanos chautus sp. nov. (Fig 1) 
 
 
Fig 1. †Chanos chautus sp. nov. holotype, IGM 13970. 
 
LSID: urn:lsid:zoobank.org:act:EB151209-9E8C-4F0D-A444-B529ECE16CC3 
 
Diagnosis. †Chanos chautus sp. nov. differs from extant C. chanos by a combination of 
features, including the lack of the anterior notch in the mesethmoid, with longer posterior wings 
arranged closer to each other, and its lateral wings less developed; a shorter predorsal length of 
56.14-63.49% of SL, and a smaller number of vertebrae, 35 to 42. 
 
Derivation of the name. In honor of the precolonial Mayan city of Palenque, the composite 
specific epithet derives from two words of the Mayan language: ka’ meaning two, and utus 
meaning face. Therefore, referencing the morphotypes and their head variation, the complete 
name means ‘the two-faced Chanos’. 
 18 
 
Holotype. IGM 13970, 147.87 mm of SL. A M1 specimen preserved on its right side with most 
of the skull bones preserved but the tail is missing. 
Paratypes. IGM 13978, a complete M2 specimen preserved on its right side, 115.22 mm of 
SL. IGM 13977, a M1 specimen without anal fin, 55.03 mm of SL, and IGM 13984 a complete 
M2 specimen with a detailed caudal skeleton, 53.99 mm of SL; both preserved near one another 
in the same slab. 
Referred material. 50 specimens, recorded as IGM13971-IGM14013 (see S1 PDF for a list 
with locality and SL of all material). 
Type locality and horizon. Danian (63mybp, Paleocene) strata of Tenejapa or Lacandón 
formations. Belisario Domínguez (17° 25' 28.60" N y 91° 58' 46.80" O) and División del Norte 
(17° 16' 12.17" N y 97° 40' 40.7" O) quarries, Chiapas State, southeastern Mexico.  
 
Description 
General body features 
Standard length ranges from 35.8 to 147 mm, selected landmark-based linear morphometrics 
are shown in Table 1. The body shape is fusiform. The head is triangular, with a small terminal 
mouth, and large eyes. The head length and depth occupy 26.58-43.58% and 14.65-24.42% of 
the SL, respectively. The maximum body depth is near the dorsal fin and represents 20.8-22.1% 
of the SL. The dorsal fin is in the middle of the body, originating at 56.14-63.49% of the SL, 
whereas the anal fin is short and located far in the back of the trunk, its origin being at 76.68-
90.68 % of the SL. The paired fins are triangular and lie near the ventral edge of the abdomen. 
The pelvic fin is abdominal, positioned opposite to the middle part of the dorsal fin, and barely 
exceeds its posterior end. The caudal peduncle is shallow. The caudal fin is homocercal, deeply 
forked, and exceeds the maximum body height. 
 19 
 
Table 1. Landmark-based linear morphometrics measurements. 
 Head 
length 
SL% [1-5] 
Head 
depth 
SL% [2-
13] 
Body 
depth 
SL% [3-
10] 
Predorsal 
length 
SL% [1-3] 
Preanal 
length 
SL% [1-9] 
Caudal 
peduncle 
depth 
SL% [5-7] 
†Chanos 
chautus sp. 
nov. n=40 
26.58-
43.58 
(33.65) 
14.79-
24.42 
(19.26) 
13.9-
26.79 
(20.76) 
56.14-
63.49 
(58.61) 
76.68-
90.68 
(83.06) 
5.26-12.42 
(8.72) 
†Chanos 
chautus sp. 
nov. M1 
(n=23) 
26.73-
31.75 
(29.03) 
14.65-
18.8 
(16.87) 
13.9-
20.95 
(18.02) 
56.14-
61.53 
(57.94) 
79.91-
86.52 
(82.70) 
5.26-10.12 
(8.11) 
†Chanos 
chautus sp. 
nov. M2 
(n=17) 
35.62-
43.58 
(39.90) 
20.32-
24.46 
(22.5) 
22.09-
26.79 
(24.46) 
57.3-63.49 
(59.5) 
79.59-
90.68 
(83.55) 
7.83-12.42 
(9.55) 
Measurements were taken as the inter-landmark distance shown in square brackets; the mean 
of measurements are shown in brackets. 
 
Neurocranium 
In dorsal view, the skull roof is triangular, about two times wider than long, and rostrally 
tapered (Fig 2). In its dorsal view, the mesethmoid is a complex star-like shaped bone, with a 
rounded anterior end, a pair of wings or lateral processes, and a posterior pair of long stout 
processes overlapped by the anterior end of both frontal bones. Lateral ethmoids can only be 
 20 
identified by their position and part of their wing protruding posterior to the mesethmoid, 
separating the nasal capsule from the orbit. 
 
 
ptt 
Sob(?) 
l.e( ?) 
Den+Ang 
Ecpt 
10 mm B 
Rb 
 
 21 
Fig 2. Head of the holotype IGM 13970. (A) Photography under UV light. (B) Schematic 
drawing. Abbreviations: Ang, anguloarticular; Cl, cleithrum; Co, coracoid; C.r, Cranial rib; 
Den, dentary; Entp, entopterygoid, Epc, epicentrals, Epn, epineurals; Fr, frontals; F.r, Fin rays; 
Hm, hyomandibula; Io, infraorbital; Iop, interopercle; L.e, lateral ethmoid; Mes, mesethmoid; 
Mpt, metapterygoid; Mx, maxilla; Na, nasal; N.a., neural arch; Ns, neural spines; Op, 
opercular; Pa, parietal; Pal, autopalatine; Pas, parasphenoid; Pop, preopercular; Pro, prootic; 
Pto, pterosphenoid; Ptt, postemporal; Rar, retroarticular; Rb, rib; Rd, radial; Sca, scapula; Scl, 
supracleithrum; Sob, supraorbital; Soc, supraoccipital; Sop, subopercle; Sph, sphenotic; V, 
vertebrae centrum. 
 
The frontals are the largest bones in the skull and form the majority of the cranium in 
dorsal view; they exhibit a straight interfrontal suture, and together form an arrowhead-shaped 
cranial surface with a deep and broad interfrontal depression extending from their ethmoidal 
end up to the posterior part of the orbit (Fig 2B). Each frontal lateral edge exhibits a medial 
constriction behind the orbit and above the sphenotic bone. The posterior edge includes a small 
and rounded medial notch in contact with the supraoccipital bone. 
Most of the bones in the postorbital region of the skull are not well preserved. The skull 
shows the latero-parietal condition, in which parietals are separated by the supraoccipital bone. 
Parietals are reduced, flat, smooth, and square-like bones. In the dorsal view, the supraoccipital 
is a complex hexagonal or star-like bone that sutures the frontals anteriorly, the exoccipitals 
posteroventrally, and the parietals laterally. The supraoccipital has an elongated and bifid 
posterior crest with two sets of 10-12 brush-like filaments distally. 
The sphenotic is a small, somewhat triangular bone that borders the dorsoposterior part 
of the orbit and sutures the anterior pterotic edge. Lateroventrally articulated to the respective 
frontal and parietal, the pterotics are elongated and complex bones. They exhibit a foramen and 
 22 
have distal main branch-like projections posteriorly. The exoccipitals form a flat arrowhead-
like structure pointing posteriorly, overlapping the basioccipital. The parasphenoid is a slender, 
straight, toothless bar-like structure that extends through the lower part of the orbit and reaches 
the ethmoid part of the skull. It has an ascending process at the posterior edge of the orbit. 
There are large cephalic ribs in the posterodorsal region of the skull, but its articulation is 
ambiguous. Other elements such as vomer, intercalary, extrascapular, prootic, and epiotic 
cannot be described unambiguously, all seem to be present but hidden or fragmented. 
 
Circumorbital series 
Overall, the bones of the circumorbital series are poorly preserved. However, fragments of an 
indeterminate number of infraorbitals cover the dorsal region of the cheek and part of the 
postorbital region of the skull. A single thick supraorbital forms the anterodorsal border of the 
orbit (Fig 2B). A small and flat lacrimal bone probably covers the ethmoid region of this fish. 
 
Jaws 
As in all gonorynchiforms, the jaw lacks teeth. The premaxilla is a concave-convex half-moon-
like bone without an ascending process. It has a nearly straight ventral edge and a spiny-like 
posteroventral process that partly envelops the maxilla (Fig 3B). The majority of M2 specimens 
seem to have a marginally protruding snout, perhaps by a slight anterior displacement of the 
premaxilla. The maxilla is a complex bone with a dorsoventral constriction separating it into 
two parts. The anterior end is T-shaped, with a very shallow maxillary facet and process for 
the articulation with autopalatine just before the ventral and dorsal anterior spiny processes. In 
contrast, the posterior part forms a broad and oblong posterior expansion. In M2 the maxilla 
appears to be bate-shaped, slightly straighter and more elongated than the more curved and 
compact posterior section of M1. Furthermore, M1 possesses the maxillary facet and process 
 23 
for articulation with the autopalatine not visible in any of M2 specimens. However, only 
fragmented maxillae were found. 
 
 
Pmx 
Mx 
Den+Aa 
Q 
Pop 
Ecpt 
Hh 
eh 
Hm Supop 
Op 
5 mm B 
 
Rar 
 24 
Fig 3. Jaws and suspensorium of IGM 13974. (A) Photography under UV light. (B) 
Schematic drawing. Abbreviations: Aa, anguloarticular; Ch, ceratohyal; Den, dentary; Ecpt, 
ectopterygoid; Enpt, endopterygoid; Hh, hypohials; Hm, hyomadibula; Io, infraorbital; Mpt, 
metapterygoid; Op, opercular; Pal, autopalatine; Pmx, premaxilla; Q, quadrate; Rar, 
retroarticular; Supop, suprapreopercle; Sy, symplectic. 
 
The general shape of the lower jaw is triangular and consists of the dentary and 
anguloarticular-retroarticular bones (Fig 3B). The dentary has a shallow symphysis and a high 
and rounded coronoid process occupying the posterior two-thirds of this bone. As in most 
Chanidae, the coronoid process has a notch in its anterior ascending border (Fig 3B). The 
alveolar part of the dentary is thick, low, and tapered anteriorly. In lingual view, the 
anguloarticular shows a tightly U-shaped suture with the dentary and posteriorly forms an 
articular process in which a distal small facet for the quadrate protrudes. 
 
Suspensorium 
The autopalatine is an elongated and thick bone with a shallow medial constriction and a 
relatively broad anterior end. The endopterygoid is large and has a rectangular shape that 
extends below the orbit. The ectopterygoid is a slender curved bone located between the ventral 
part of the autopalatine, and the anteroventral edge of the endopterygoid (Fig 3B). These three 
elements always appear fused, at least by cartilage. The metapterygoid is located posteroventral 
to the endopterygoid and has a somewhat rectangular shape. (Fig 3B). The quadrate-lower jaw 
joint is slightly anterior to the orbit. The quadrate is somewhat triangular and has a spherical, 
conspicuous articular head forming its anterior tip. The anterior border is straight, with a 
sinuous dorsal edge, and a long stake-like posterior process (Fig 3B). The symplectic is a 
 25 
fusiform bone as long as the quadrate. It articulates ventrally with the quadrate, dorsally with 
the metapterygoid, and posteriorly with the subopercle (Fig 3B). 
The hyomandibula has an axe-like shape with a sharp anteroventral process extended 
below the orbit. The dorsal border is nearly straight but vaguely inclined anteroventrally 
articulating with the sphenotic and the pterotic; the anterior surface is curved. In the inner view, 
the opercular process of this bone is located in the dorsal quarter of its posterior border, from 
which an articular head projects lightly. 
 
Opercular series 
The opercular series is complete and includes mostly thin, unornamented bones. The opercle 
is kidney-shaped, slightly higher than wide, and has a straight yet inclined anterior edge and a 
curved rear. In its inner view, the hyomandibular facet occupies the upper quarter of its anterior 
edge. The opercle overlaps the dorsal part of the subopercle, which is slice-shaped, dorsally 
straight, ventrally curved, and has a small spiny anterodorsal process. 
The suprapreopercle is almost always lost, but some fragments reveal its presence in 
the anterodorsal border of the opercle. The preopercle is an inverted L-shaped bone, in which 
the limbs are roughly equal in length and form a 90° anterior aperture angle. It has a 
conspicuous median ridge and a canal through most of both limbs. The ventral region of this 
bone covers most of the interopercle: an elongated triangular bone anteriorly tapered, with 
ventral and dorsal edges nearly straight, and a slightly curved rear. 
 
Hyoid and branchiostegal regions 
Most bones in this region are obscured. The hypohials are two conical and similarly sized bones 
(Fig 3B). The anterior ceratohyal is a rectangular and unpierced bone attached to both hypohials 
anteriorly and at least four branchiostegal rays ventrally. The posterior ceratohyal is unknown. 
 26 
The urohyal is triangular, elongated, and anteriorly tapered, it exhibits an anterior constriction 
that separates its small anterior articular head. 
 
Vertebral column and intramuscular bones 
The vertebral column consists of 35 to 42 vertebrae including the terminal complex. Among 
these, 16-20 are abdominal and 19-22 are caudal. The counts include the three anteriormost 
abdominal centra which often the opercle covers. The centra are well-ossified, cylindrically 
shaped, and slightly constricted in the middle. These have deep conical intervertebral surfaces 
and a couple of parallel lateral perforations separated by a longitudinal crest. The anterior five 
centra are slightly shorter than higher and their neural arches and respective spines are bigger 
than the rest, they abut without overlap (Fig 2B). 
All and only predorsal neural arches are autogenous, with inconspicuous anterodorsal 
processes and bifid neural spines. Both neural and haemal spines are thin, nearly straight, and 
tilted posteriorly, forming angles between 45 and 60° on the longitudinal axis of the vertebral 
column. The abdominal cavity is enclosed by 13 to 14 pairs of pleural ribs. In lateral view, 
these are curved and have small articular heads. The three anterior ribs are hook-shaped with a 
deep apicobasal groove, broad bases, and slender tips, while the subsequent ones are rather 
uniformly slender. The first two centra bear no ribs. About half of the ribs appear to articulate 
directly on the lateroventral surface of the respective centrum, while the posterior ribs attach 
with paraphophyses in the posterior half of the centra. 
A series of 10 to 13 supraneural bones occupy the interneural spaces of the predorsal 
region. The first four supraneurals are comparatively thicker and broader, and their dorsal 
halves bend backward, while the rest are rod-like. 
Intermuscular bones include epineural, epipleural, and epicentral bones. There are two 
sets of epineurals. The predorsal epineurals are more complex and situated closer to the neural 
 27 
arch. Four to five main arms extend from a central body, one posterodorsally and three or four 
anteroventrally; all arms distally have brush-like ends. Postdorsal and preanal epineurals are 
Y-shaped, anteriorly bifid, have no brush-like distal ends, and are fixed at the distal third of the 
respective neural spine. The epineurals placed behind the anal fin rest again closer to the 
vertebral centra. 
The epipleurals are elongated bones associated with haemal arches and spines of caudal 
centra between the pelvic girdle and the preural 5 or 6. The position and shape of these bones 
are symmetrically opposed to that of the epineurals placed above. These bones are Y-shaped 
and lie nearly horizontally below two or three centra. The proximal end is deeply bifurcated. 
In the middle abdominal region, these bones attach to the distal section of haemal spines. 
Subsequently, those epipleurals placed behind the anal fin tend to be fixed closer to the haemal 
arches. 
Epicentrals appear to be associated exclusively with abdominal vertebrae and are very 
small, especially compared to epineurals or epipleurals. They have a Y figure, with two small 
anterior branches and a long spiny posterior one; it is unclear exactly where they attach to the 
centra. 
 
Pectoral girdle and fin 
The supracleithrum is long, as wide as the first pleural rib, and somewhat curved. It articulates 
dorsally to the cleithrum, overlapping it. The cleithrum is a heavily ossified and curved bone; 
with ridges and arms from where more laminar bone extends, mainly dorsally and 
anteroventrally (Fig 2B). 
The coracoid is big, almost as long as the subopercle. It has an axe-like shape, an 
elongated narrow anterior half articulating medially with the cleithrum, and a deep posterior 
 28 
half articulating with the scapula. The scapular foramen is positioned in the ventral portion of 
the bone. Four prominent proximal radials and 13 to 16 unsegmented rays are present. 
 
Pelvic girdle and fin 
The pelvic fin is positioned opposite to the middle of the dorsal fin. The fin involves a 
conspicuous pelvic splint, a small ray, followed by a broad ray, then eight to 10 distally 
segmented and branched rays, which become smaller in lateromedial order (Fig 4B). The 
articular head of these rays is curved and sharp medially. The pelvic bones or basipterygium 
are triangular, about three times longer than wide, and are joined along their medial margin. In 
smaller specimens, the basipterygium consists only of a T-shaped keel, while in bigger 
specimens, this keel develops a broad lamellar medial wing. An indeterminate number of robust 
and short radials articulate to the posterior border of the pelvic bone and the head of the 
outermost pelvic rays. 
 
 29 
 
Fig 4. The pelvic fin of IGM 13984. (A) Photography under UV light. (B) Schematic drawing. 
Abbreviations: Bpt, basipterygium; Fr, fin rays; Ma, medial arm; Mw, medial wing; Pp, 
posterior process; Ps, pelvic splint. 
 
Dorsal fin 
The dorsal fin is single and falcate. It extends above the centra 14 to 25, with variations. This 
fin consists of up to three small and unsegmented rays and 10-14 distally segmented and 
 30 
branched rays. Among these, the most anterior segmented ray is the longest and broadest; 
beyond, the posterior dorsal rays become progressively shorter. 
The supporting pterygiophore series of this fin includes 11 to 13 long pterygiophores 
and small radials. In larger specimens, the first pterygiophore is large and has a complex shape 
with three thickened bars or arms anteroventrally projected (Fig 5B, D). The subsequent 
pterygiophores are rod-like bars that become smaller and thinner in anteroposterior order. The 
last one is a dorsal stay positioned horizontally. 
 
 31 
 
Fig 5. Dorsal fins. (A-B) Photography and schematic drawing of IGM 13987. (C-D) 
Photography and schematic drawing of IGM 13972. Abbreviations: Fr, fin rays; Pr, procurrent 
ray; Ptg, pterygiophore; Rd, radial. 
 32 
Anal fin 
The anal fin is single, falcate, and short (Fig 6A). It rises far in the back of the trunk, around 
the last fifth of the SL, below the centra 26 to 31, with variations. The fin consists of one to 
three rays and eight to 13 long, distally segmented, and branched rays (Fig 6A). Here, the 
second and third branched rays are the longest, and the others become progressively shorter. 
However, the length of the shortest is only slightly less than that of the longest. 
 
 
Fig 6. Anal fin. (A) Photography of IGM 13972. (B) Schematic drawing of IGM 13972. 
Abbreviations: Fr, fin rays; Pr, procurrent ray; Ptg, pterygiophore; Rd, radial. 
 
 33 
The anal fin is supported by nine to 10 pterygiophores along small radials (Fig 6B). The 
pterygiophores are bar-like slender structures laterally flattened with small articular heads. In 
larger specimens, the anteriormost pterygiophore shows a broad laminar wing anteriorly 
expanded at the base of its length. In the anteroposterior direction, they exhibit a slight decrease 
in size. From the third pterygiophore, they penetrate each interhaemal space of the spines 
placed above. The last one is a bifurcated and horizontal anal stay. Radials are present between 
the articular heads of proximal pterygiophores. The first pterygiophore supports the procurrent 
rays and the first segmented ray. 
 
Caudal skeleton 
The caudal fin has two symmetrical, triangular, and deeply forked lobes. The posterior height 
of this fin exceeds the maximum height of the body. The caudal formula is ix+I+8—8+I+viii 
[64]. The parahypural and the haemal spine of preural 2 are broad and autogenous. Uroneural 
1 is fused to preural centrum 1, and ural centra 1 and 2, forming a caudal complex or pleurostyle 
(Fig 7B). The uroneural 2 is located at the anterior edge of the pleurostyle, while an epural is 
situated at its posterior edge; both are elongated and autogenous (Fig 7B). Six autogenous 
hypurals decrease in length in ventrodorsal order, the first being the largest, triangular, and not 
fused (Fig 7B). The second one is significantly slenderer and rectangular. There is a diastema 
between the second and the third hypurals. From third to sixth, they are spatulate, slightly wider 
posteriorly. 
 
 34 
 
Fig 7. Caudal skeleton. (A) Photography of IGM 13984. (B) Schematic drawing of IGM 
13984. Abbreviations: Ep, epurals; H1-6, hypural plates; Hs, haemal spine; Ns, neural spine; 
Pcr, procurrent ray; Php, procurrent ray; Pu1-4, preural centra; U, ural centra, Un1-2, 
uroneurals. Black arrows highlight the first main ray. 
 
 35 
Scales 
The trunk is entirely covered by cycloid, heart-shaped scales. These are slightly longer than 
high and have two similar anterior lobes. These scales have a central focus, numerous circuli 
ornamenting the anterior part of their lateral surfaces, and straight radii horizontally projected 
on the posterior half. 
 
Phylogenetic analysis 
A single most parsimonious tree (an ACCTRAN optimization, Fig 8) was recovered with a 
length of 170 steps (CI: 0.741; RI: 0.758). We describe the topology and character mapping of 
the main diagnostic clades Gonorynchiformes, Chanidae, Chaninae, Chanini, and Chanos. 
 
 36 
 
Fig 8. The single most parsimonious tree obtained in TNT v. 1.6. Roman numbers on nodes 
indicate the main diagnostic clades mapped. Node Arabic numbers indicate >50 bootstrap 
values. 
 
Gonorynchiformes are monophyletic (node I) and supported in this hypothesis by seven 
characters: absence of orbitosphenoid (C: 1, 1); the presence of cephalic ribs articulating with 
the exoccipitals (C: 6, 1); reduced and flat blade-like parietals (C: 19, 1); absence of 
premaxillary ascending process (C: 28, 1); vomer extending anteriorly, beyond the level of the 
anterior margin of the mesethmoid (C: 45, 2); neural arch of first vertebrae in contact with 
 37 
exoccipital (C: 72, 1); and paired intermuscular bones consisting of three series (epipleurals, 
epicentrals, and epineurals) (C: 80, 1). 
Gonorynchiformes is then divided into two clades, one that includes Kneriidae and 
Gonorynchidae (node II). Sister group to the aforementioned clade is equal to the taxonomic 
family Chanidae (node III), a monophyletic group supported by 11 characters: a large 
premaxilla, very broad, concave-convex, with a long oral process (C: 27, 1); enlarged posterior 
region of the maxilla, swollen to a bulbous outline with a curved posterior border (C: 33, 1); 
presence of a notch in the anterodorsal border of the dentary (C: 38, 1); quadrate-mandibular 
articulation anterior to orbit, quadrate displaced but not elongated (C: 48, 1); very long 
symplectic, about twice the length of the ingroup (C: 49, 1); presence of an anterior 
metapterygoid process of the hyomandibular bone (C: 52, 1); expanded opercular bone, at least 
one third of the head length (C: 54, 1); preopercular expansion distal to the terminal openings 
of the preopercular canal branches present and restricted to the posteroventral corner (C: 61, 
1); first 5 to 10 neural arches autogenous in adults, at least laterally (C: 76, 1); extent of first 
uroneural to anterior end of preural centrum 2 (C: 95, 1); and autogenous haemal arch in preural 
centrum 2 (C: 103, 1). 
Chanidae is then divided into two sister clades. The one grouping †Rubiesichthys + 
†Gordichthys is equal to the subfamily †Rubiesychthynae (node IV), while the other 
corresponds to the taxonomic subfamily Chaninae (node V). The latter is supported by the 
presence of a maxillary process for articulation with the autopalatine (C: 32, 1); and a ridge 
present on the anteroventral limb of the preopercular bone (C: 60, 1). 
Node VI is equal to the taxonomic tribe Chanini, whose members share three characters 
in this hypothesis: exoccipital bones with a posterior concave-convex border and a projection 
above the basioccipital (C: 5, 1); a large mesethmoid with broad posterolateral wing-like 
 38 
expansions (C: 10, 2), and the neural arch and spine of preural centrum 1 well developed, with 
the spine about half as long as preceding ones (C: 92, 0). 
The last node (VII) is composed of the genus Chanos. The eight characters supporting 
this relationship are the presence of cephalic ribs articulating with both the exoccipitals and 
basioccipital (C: 6, 2); highly reduced parietals (C: 19, 2); the presence of caudal scutes (C: 90, 
1); the fusion of ural centra (U1, U2), preural centrum 1 (Pu1), and uroneural 1 (Un1) in a 
pleurostyle or caudal complex (C: 91, 1); open neural arch and no spine in preural centrum 1 
(C: 92, 2); uroneural 2 separated from ural centrum 2 (C: 96, 1); hypural 1 and terminal complex 
separated by a small hiatus (C: 100, 1), and the presence of a posterolateral process in the 
caudal endoskeleton (C: 104, 1). This hypothesis does not signal any autapomorphies for C. 
chanos or †Chanos chautus sp. nov. 
 
Geometric morphometrics 
We detected no outliers. The error was found not significant for either day (P= 0.5005) or 
specimens (P= 0.5005) (see Supporting information), indicating that the placement of 
landmarks was similar between replicates, and landmark estimation did not have any spurious 
effect. The first three principal components summarized 77.38% of shape variation on the 
unbent dataset. PC1 (64.56% of variance) describes relative head-size and body-depth ratios 
simultaneously. The PC1 vs PC2 scatter plot shows two groups: 23 specimens with more 
dorsoventrally compressed bodies and relatively smaller heads (morphotype 1) lie on negative 
values, while 17 specimens with deeper bodies and relatively bigger heads (morphotype 2) lie 
on the positive side of the cartesian plane (Fig 9A-B). 
 
 39 
 
Fig 9. PCA on unbent dataset. (A) PC 1 vs PC 2 scatterplot. ‘M1’ specimens are in black, 
and ‘M2’ specimens are in light red, with the mean shape of the groups; clouds are delineated 
by eye. (B-D) PC 1-3 shape variations as differences between maximum (black) and minimum 
(grey), effects magnified x1.5 for visualization purposes. 
 
PC2 (7.03% of variance) describes a subtle variation of anal-fin position, a higher 
positioned peduncle, and primarily related to pelvic fin depth, or a slightly more bulbous ‘belly’ 
for positive scoring specimens (Fig 9C). This variance does not seem as directly associated 
with arching as PC3, given that the exploratory PCA of the 15 not bent specimens also yielded 
a PC2 with the same pattern (Fig 10A-C). Nonetheless, it could still be related to preservation 
 40 
and or arching. PC3 (5.79% of variance) is correlated to residual upward bending on the 
positive scoring specimens (Fig 10D). In the exploratory PCA of the ‘total’ dataset, bending 
was the major axis of shape variation (PC1, 46.09%); meanwhile, the relative head size and 
body depth variation explained 35.96% of PC2 variance, still showing the morphotypes without 
overlap in PC1 vs PC2 scatter plot (Fig 10D-F). 
 
 
Fig 10. Exploratory PCAs. (A) Not bent dataset PC 1 vs PC 2 scatter plot. (B-C) Shape 
differences between maximum (black) and minimum (grey) for each PC. (D) ‘total’ dataset PC 
1 vs PC 2 scatter plot. (E-F) Shape differences between maximum (black) and minimum (grey) 
for each PC. Effects magnified x1.2 for visualization purposes. M1 specimens in black and M2 
specimens in light red 
 
For the ANOVA, differences between morphotype shapes were found significant 
(P<0.05), while the effects of size and its interaction with morphotypes were found not 
significant (Table 2); the morphotypes showed similar size ranges. The restricted size ranges 
of the sample do not signal marked body-shape changes, as seen in the gentle slopes for the 
fitted shape values and the comparison between small and large-sized specimens in Fig 11. 
 41 
 
Table 2. Results from exploratory permutational Procrustes ANOVA on morphotypes, size, 
and the interaction as factors.  
 Df SS Rsq F P P (99 
permutations) 
Morphotypes 1 0.11610 0.60127 56.9493 0.001 0.01 
Size 1 0.00151 0.00782 0.7404 0.7 0.68 
Morphotypes 
× size 
1 0.00209 0.01083 1.0258 0.422 0.39 
Residuals 36 0.00203 0.01083    
Total 39      
Significant effects are highlighted in boldface type. The last row shows P values from 99 
permutations instead of 999. 
 
 42 
 
Fig 11. Allometric relationship between shape and size among morphotypes. (A) 
Allometric trajectories are visualized as the predicted lines of the first PC of shape and CS as 
the predictor. M2 specimens are in red, and M1 specimens are in black. (B) A small M2, IGM 
14005. (C) A large M2, IGM 13978. (D) A small M1, IGM 14003. (E) A large M1, IGM 13974. 
Bars equal 10 mm. 
 
Head length was the only landmark-based linear morphometric trait to show an 
unambiguous bimodal distribution in frequency histograms (Fig 12A). Box plots show that 
head length and depth and body depth show no distribution overlap (Fig 12B-C).  
 
 43 
 
Fig 12. Landmark-based linear morphometrics graphics. (A) The head length frequency 
histogram of the total sample is expressed as SL%. (B-G) Mean deviations from within-group 
linear traits as SL%, error bar show 95% confidence intervals. M1 are in grey, while M2 are in 
light red. 
 
Discussion 
†Chanos chautus sp. nov. systematic position 
†Chanos chautus sp. nov. shows two of the morphological synapomorphies of 
Gonorynchiformes sensu Poyato-Ariza et al. [31]: the loss of the orbitosphenoid and the loss 
of the premaxillary ascending process. Other characters supporting the clade in our hypothesis 
include the presence of cephalic ribs, reduction of parietals, and paired intermuscular bones 
consisting of three series (epipleurals, epicentrals, and epineurals). 
 44 
†Chanos chautus sp. nov. has several of the synapomorphies of Chanidae sensu Poyato-
Ariza et al. [31]. These include a large, broad, concave-convex premaxilla with a spiny oral 
process; an enlarged maxilla, posteriorly bulbous and curved; the presence of a notch in the 
anterodorsal border of the dentary; a quadrate-mandibular articulation anteriorly displaced; a 
metapterygoid process in the hyomandibula, and an opercular bone expanded to at least one 
third the head length. Furthermore, the specimens possess other features that support the group 
in our hypothesis: a long symplectic (C: 49, 1); neural arches 5–10 autogenous, at least laterally 
(C: 76, 1); and haemal arch in preural centrum 2 autogenous (C: 103, 1). 
†Chanos chautus sp. nov. (at least M1) has one synapomorphy of Chaninae sensu 
Poyato-Ariza et al. [31], a maxillary notch and process for articulation with the autopalatine 
(C: 32, 1), which also supports the group in our hypothesis (node V). They also possess a 
synapomorphy that characterizes the tribe Chanini sensu Poyato-Ariza et al. [31]: mesethmoid 
large, with broad posterolateral wing-like expansions; and the homoplastic characters: 
symplectic and quadrate separated through cartilage; preopercular expansion in the 
posteroventral corner and part of the posterodorsal limb. From our hypothesis (node VI), they 
also show exoccipitals with a posterior concave-convex border and a projection above 
basioccipital (C: 5, 1). 
Regarding the genus Chanos (node VII), as defined in this hypothesis, †Chanos chautus 
sp. nov. shows cephalic ribs, but it is not clear where they articulate (C: 6, 2), caudal scutes are 
present in dorsal and ventral sections of the hypural plates and first rays at least in some 
specimens (e.g., IGM 13978, IGM 14022) (C: 90, 1); and hypural one and terminal centrum 
are at least not fused (C: 100, 1). Otherwise, †Chanos chautus sp. nov. unambiguously exhibit 
the remaining character states supporting the relationship: parietals are highly reduced (C: 19, 
2); ural centra (U1, U2), preural centrum 1 (Pu1), and uroneural 1 (Un1) are fused (C: 91, 1); 
it has no spine in preural centrum 1, only the inferred pleurostyle (C: 92, 2); uroneural 2 is 
 45 
autogenous to the centrum (C: 96, 1); and possesses a posterolateral process in the caudal 
endoskeleton (C: 104, 1). Reduced parietals (C: 19, 2) and the posterolateral process of the 
caudal endoskeleton (C: 104, 1) are synapomorphic, but the rest of these characters also appear 
in Kneria Steindachner, 1866 and Gonorynchus (Linnaeus, 1766). Thus, they manifest as 
homoplasies in the analysis. Convergent evolution of most of the caudal osteology can 
ontologically explain this in extant genera [30]; consequently, the characters are congruent, and 
the affinity of †Chanos chautus sp. nov. with extant C. chanos, is unmistakable. 
 
†Chanos chautus sp. nov. and other fossil Chanidae 
†Caeus Costa, 1857 from the Albian of Italy exhibits no fusion of caudal elements [12], and 
thus we unambiguously consider it a distinct taxon to †Chanos chautus sp. nov. Taverne and 
Capasso [12, p.13] suggested that †Prochanos Bassani, 1882 (Turonian-Maastrichtian, 
Croatia) might exhibit the fusion of caudal elements and be a Chanos, based on Bassani’s 
original illustration of the fossil [65, pl. 13]. However, the illustration of the caudal 
endoskeleton is not detailed. Moreover, Bassani [65, p. 218] erected a new genus based on four 
characters; one of them claims that the end of the vertebral column is more similar to that of 
†Leptolepis Agassiz, 1843 and †Tharsis Giebel, 1848 than to that of Chanos, likely referring 
to their not fused elements. The original description and diagnosis are general and superficial; 
therefore, the material needs re-description. Nonetheless, for the reasons outlined above, we 
consider †Prochanos tentatively distinct to Chanos (sensu C. chanos + †Chanos chautus sp. 
nov.). 
Most of the unarticulated elements of †Vangus fahiny Murray et al., 2023 
(Mastrichthian, Madagascar) bear a striking resemblance to that of Chanos. Nevertheless, 
Murray et al. [16] considered the hyomandibula as a diagnostic feature of a new genus. The 
hyomandibula of †Vangus is higher and narrower, the head is angled less steeply and has a 
 46 
curved dorsal surface, with a rounder condyle for articulation with the opercle and a deeper 
concavity dorsal to the condyle, and a less rounded concave anterior side than that of Chanos 
[16, fig 2 A-D]. 
†Cabindachanos dartevellei Taverne, De Putter, Mees, Smith, 2019 was based on a 
single partial specimen missing the caudal skeleton from Cabinda (Democratic Republic of 
Congo) [15]. It is coeval (Danian or Salendian) to †Chanos chautus sp. nov. (Danian) [15]. The 
close relationship with Chanos is evident given the overall resemblance; for example, the 
kidney-like opercle and the well-developed and long supraoccipital crest are similar in shape 
yet hypertrophied. The parietals are reduced, as in Chanos, but are slightly wider than long. 
Furthermore, relative to the posterodorsal limb, the anteroventral limb of the preopercle is 
considerably narrower than in other chanids, such as †Chanos chautus sp. nov. The scales, as 
illustrated, also vary; they are longer than deep, with no clear focus [15, fig 5]. The loss of 
subopercle was cited as the major reason for erecting a new genus [15]; however, as implied 
by various specimens of †Chanos chautus sp. nov., this is most likely an artifact of 
preservation, as the subopercle is often covered by the opercle in our sample. Thus, although 
the level of affinity to the genus Chanos is difficult to attain, it is distinct to C. chanos or 
†Chanos chautus sp. nov. 
Another Danian chanid found in literature is †C. torosus Danil’chenko, 1968 from 
Turkmenistan, but as for the Eocene and Oligocene of Italy, †C. brevis, †C. zignoi and †C. 
forcipatus, all need a modern description [2]. If these taxa were proven to be true Chanos 
species, the Chanos lineage ranged across the Tethys and the Proto-Caribbean earlier in the 
Cenozoic, at some point colonizing the Indo-Pacific Ocean before eventually going extinct in 
their former range. 
 
 
 47 
The morphotypes and Chanos chanos 
A direct anatomical comparison of †Chanos chautus sp. nov. with extant C. chanos reveals a 
remarkable structural similarity of almost every visible element, suggesting that Chanos might 
have undergone morphological stasis in the last 63 million years. Bones such as, but not 
restricted to, the dentary, urohyal, intermusculars, cleithrum, supracleithrum, the entire caudal 
endoskeleton, supraneurals, pterygiophores, and the scales are indistinguishable from that of 
C. chanos. They exhibit an identical interfrontal depression and an elongated supraoccipital 
crest with a bifid set of filaments. The pterotics are long and have branch-like extensions. The 
fused autopalatine-ectopterygoid-endopterygoid complex is also seen in C. chanos [e.g., 66, 
fig 10]. In all, the subopercle has an anterodorsal spiny process for articulation. 
Although it is only found in M1 specimens, we consider the mesethmoid shape as the 
only significant non-morphometric or meristic difference to distinguish between extant Chanos 
and †Chanos chautus sp. nov., and therefore the only osteological qualitative difference to be 
included in the diagnosis. The anterior notch seen in the mesethmoid of extant C. chanos is 
missing in †Chanos chautus sp. nov. Also, the lateral wings in †Chanos chautus sp. nov are 
shorter and undeveloped, the posterior wings are longer, and their aperture is narrower (Fig 
13). 
 
 48 
 
Fig 13. Chanos mesethmoids. (A) Schematic drawing and photography of IGM 13970 
mesethmoid. (B) Schematic drawing and photography under UV light of IGM 13973 
mesethmoid. (C) Schematic drawing and photography of C. chanos (CMR 1259 specimen) 
cleared and stained mesethmoid. All bars equal 3 mm. 
  
Another consistent but mild difference is the upper jaw of M2, a straight bate-like 
maxilla (Fig 14A-B). The bulbous and curved posterior section of the maxilla of M1 resembles 
more closely that of C. chanos (Fig 14C-F). The shallow facet and process for autopalatine 
immediately behind the anterior processes seen in M1 and C. chanos is apparently missing in 
M2. 
 
 49 
 
Fig 14. Chanos maxillae. (A) Schematic drawing and photography under UV light of IGM 
13979 maxilla. (B) Schematic drawing and photography under UV light of IGM 13985 maxilla. 
(C-D) Schematic drawing and photography of C. chanos (CMR 1259 specimen) cleared and 
stained left maxilla in lingual and lateral view, respectively. (E) Schematic drawing and 
photography under UV light of IGM 13974 maxilla. (F) Schematic drawing and photography 
of IGM 14015 maxilla. All bars equal 2.5 mm. 
 
Grande and Arratia [30] reported a predorsal length for C. chanos of 51% of SL and a 
preanal length of 83%. Hence, with 56.14-63.49% of SL, there is a significant difference to 
those of †Chanos chautus sp. nov. for the first trait (Table 1). Preservation did not allow the 
use of meristics in multivariate analyses. However, selected meristic counts on some specimens 
generally fall within those of C. chanos extant populations, except for vertebral count (Table 
3). There is no overlap between the vertebrae centra count of C. chanos (44-51) and †Chanos 
chautus sp. nov. (35-42), at least in the populations included. The number of centra slightly 
 50 
varies between morphotypes; there are 39 to 42 vertebrae, including the pleurostyle in M1 and 
35 to 38 in M2. This centra variation is related to the dorsal fin position, extending from 
vertebra 18 to 25 in M1 and 14 to 21 in M2. 
 
Table 3. Selected meristic counts. 
 
Vertebrae 
centra 
Pectoral 
rays 
Pelvic rays Anal rays Dorsal rays 
†Chanos chautus sp. 
nov. (n=18) 
35-42 13-16 i, 9-11 i-iii, 8-13 i-iii, 10-14 
†Chanos chautus sp. 
nov. M1 (n=9) 
38-42 15-16 i, 9-11 i-iii, 8-11 i-iii, 10-14 
†Chanos chautus sp. 
nov. M2 (n=9) 
35-38 13-14 i,9 i-ii, 10-13 ii, 10-14 
C. chanos Philipines 44-45 i, 13-17 i-ii, 7-12 i-iv, 6-9 ii-vi, 10-14 
C. chanos Indonesia 44-45 16-17 11-12 10-11 14-16 
C. chanos India 44 16 11 9-10 13-16 
C. chanos Hawaii 45 
  
ii, 9 ii, 12 
C. chanos Papua New 
Guinea 
 
16-17 11-12 9-11 13-17 
C. chanos not 
specified 
44-51 15-17 10-11 6-8 13-17 
Extant Chanos data from [5, 29]. Roman lowercase numbers refer to procurrent rays. 
 
Specimens as juveniles 
Chanos chanos spawns in shallow near-shore waters across the tropical/subtropical Indo-
Pacific [5]. The pelagic larvae (≈10mm) migrate to coastal transitional waters, e.g., estuaries 
or marshes that serve as nurseries [6]. Juvenile C. chanos feed on cyanobacteria, diatoms, algae, 
crustaceans, snails, worms, and zooplankton, seemingly preferring benthic items [5, 6]. After 
a few months and a growth of around 300 mm of SL, the juveniles reach their habitat capacity 
 51 
and then return to the sea, where they add pelagic fish to their diet and can reach a length of up 
to one meter [5, 6].  
We infer that both fossil morphotypes represent juveniles since, in smaller specimens, 
all unpair fin pterygiophores and supraneurals are rod-like, while, in bigger specimens, the 
anterior pterygiophores and supraneurals show broad lamellar wings. Similarly, the pelvic bone 
in smaller specimens is a T-shaped bone, while in bigger specimens, the keel develops a 
lamellar medial wing. In addition to the shape changes, the general degree of ossification is 
also size-related. We attribute these size-related trends to skeletal maturation [e.g., 67]. In 
combination with the size-related trends, we also consider the standard lengths of the sample, 
ranging from 35.8 to 147 mm, which are much smaller than those of mature extant milkfish [6, 
67]. 
Additionally, pending geological or geochemical evidence, we argue that the most 
parsimonious explanation is that the fossils were deposited in near-shore transitional waters. 
Our rationale follows: per our observations of the CNP material, only small-sized fish are found 
in both localities; the largest fossil fish specimen found in these localities is just 263.9 mm of 
SL, †Kelemejtubus castroi Cantalice and Alvarado-Ortega, 2017, a stem-percomorph [68]. 
Botanic material (plants and/or algae) is conspicuous and could have provided shelter for fish 
fry; however, these might not be necessarily autochthonous [22]. Most specimens are found 
well-preserved, semi- or articulated, semi- or complete, and in mass mortality events, 
indicating a low-energy and low-oxygen taphonomic scenario [69]. Some fishes reveal a strong 
marine influence, as some of the fossils come from generally reef-associated clades, such as 
the stem trumpetfish †Eekaulostomus cuevasae Cantalice and Alvarado-Ortega, 2016, the stem 
grouper †Paleoserranus lakamhae Cantalice, Alvarado-Ortega, Alaniz-Galvan, 2018, or the 
stem damselfish †Chaychanus gonzalezorum Cantalice, Alvarado-Ortega, Bellwood, 2020, as 
well as one undescribed taxon from the freshwater genus †Phareodus Leidy, 1873 [23, 26, 70, 
 52 
71]. Nonetheless, there are no fossil corals, and such fish taxa are rare, with only one or a 
handful of specimens. With dozens of juvenile specimens, by far the most common fossil fishes 
alongside chanids come from Anguilliformes and Clupeiformes, and several extant species of 
these groups are characterized by juveniles that migrate to transitional environments [e.g., 72, 
73]. Given these characteristics, in this scenario, it is likely that at least at times and at least 
parts of Belisario Dominguez and División del Norte depositional environments lost their 
connection to the shallow sea, and oxygen depletion influenced the death and state of 
preservation of entrapped organisms [e.g., 69, 74]. Therefore, presumably juvenile †Chanos 
chautus sp. nov. also migrated to transitional environments, as does extant C. chanos.  
 
The Chanos morphotypes from Palenque 
We discarded different scenarios that cannot account for the pattern of two morphotypes. Shape 
variation is not associated with a specific range of size, therefore, the differences in head size 
and body depth ratios are not products of allometric growth (Fig 11). Both morphotypes are 
found in both localities, hence, geographical variation of one taxon cannot account for them as 
suggested for †Dastilbe and other fossil species of Chanos [2, 17-21] as well as for extant 
populations of C. chanos [7-10]. Despite not being usually conserved near one another, 
specimens IGM 13977 (M1) and IGM 13984 (M2) are close to one another on the same slab 
(Fig 15), so they must have coexisted in time and space. Consequently, other related scenarios, 
such as seasonal variation or anagenetic change, cannot explain morphotype variation. 
 
 53 
 
Fig 15. IGM 13977 (M1) and IGM 13984 (M2) association. Bar equals 10mm. 
 
 54 
Sexual shape dimorphism [63] could explain the presence of two morphotypes. It is a 
feature that has scarcely been discussed in fossil fish [e.g., 22, 69, 75, 76]. Using meristics and 
linear morphometrics, Poyato-Ariza [22] found two coexisting morphotypes of the chanid 
†Rubiesichthtys gregalis, which only differed in body depth. He linked the broader-bellied 
morphotype with extra space for ovaries in females, as seen in some extant fish [e.g., 76, 77]. 
We are unaware of any morphometric works concerning sexual shape dimorphism in extant C. 
chanos. Discrimination between sexes typically requires manipulation, although the abdomen 
may appear distended in gravid females [5]. In some populations, males tend to be smaller than 
females, while in others, there is no indication of sexual size dimorphism [5]. The sex ratio 
varies widely, yet males tend to be more common, 56-69% [5]. It is not clear if sexual maturity 
is related to age or size [5]. It varies across localities and types of stocks yet estimates show 
that sexual maturation takes years (3-10) and SL >60 cm [5]. In Chanos morphotypes from 
Palenque, however, most of the shape variation is related to the head size, and only secondarily 
to body depth. Moreover, the maxilla presumably showed specific variation. The 
morphological traits perhaps associated with sexual dimorphism would have had to occur very 
early in development, and the trait would greatly diverge from the states seen in its close 
relatives C. chanos, †Francischanos, †Tharrhias or even †Rubiesichthys [14, 22]. Nonetheless, 
slender M1 specimens are more common (57.5%), and sexual shape dimorphism can reflect 
other requirements that support reproductive success, such as food supply, locomotion, or 
mating behaviors [69]. 
A taxon characterized by polymorphism could also account for the morphotypes. For 
example, there are numerous resource polymorphisms in the teleost Salvelinus alpinus 
(Linnaeus, 1758) (Salmoniformes) [79]. These arctic char morphotypes show a wide range of 
genetic divergence; some, such as the Lake Hazen sympatric morphs, do not signal genetic 
differentiation [45]. It has been proposed that heritable foraging behaviors and plastic 
 55 
morphology might promote assortative mating and eventually lead to reproductive isolation in 
the arctic char [45, 80]. Other S. alpinus morphs exhibit significant genetic differentiation, 
suggesting incipient speciation and the beginning of niche specialization [81]. 
The high morphometric differences and the slight osteological differences between 
morphotypes may signal early divergence or two well-established sympatric species 
(separately evolving metapopulation lineages, sensu DeQueiroz [82]). Chanids are 
morphologically conservative but characterized by high intraspecific and interspecific 
morphometric-meristic variability [14, 17-21]. Given that the morphotypes represent juveniles, 
type differentiation could have been driven by some underlying genetic constitution that had a 
phenotypic effect since early ontogenetic stages and not only by ambient-induced plasticity. 
Chanos chanos is characterized by physiologic and phenotypic plasticity [5, 10]. For example, 
abundance of food and current intensity were linked to differences in head, body, and caudal 
peduncle depths in juvenile milkfish along the coast of India [10]. However, these 
morphological stocks of C. chanos are not sympatric morphs but regional clusters [10, 83]. 
Moreover, morphological stocks (regionally ambient-induced) do not coincide with molecular 
populations (related to genetic flow and geographic/current barriers) [8, 10, 83, 84], which led 
Sri-Hari et al. [10, 83] to suggest that morphological and genetic evolution are decoupled in 
the milkfish. Consequently, phenotypic differentiation may not necessarily signal genetic 
divergence in this case. 
Our decision to erect a single new taxon instead of two was taxonomically pragmatic. 
†Chanos chautus sp. nov. may be a species complex, but we suggest a single species distinct 
to C. chanos as the null hypothesis for the presence of two Chanos morphotypes since any of 
the three scenarios, two species, a polymorphic, or a sexually dimorphic taxon is untestable 
with the available data. Hence, erecting a new taxon does not necessarily imply that †Chanos 
chautus sp. nov. was either a polymorphic or sexually dimorphic taxon. Despite the extreme 
 56 
osteological similarity between fossil and extant milkfishes, whether they were part of the 
direct ancestral lineage that led to the modern milkfish or whether they are C. chanos is also 
untestable; under a cladistic framework, fossil taxa are treated only as terminals and 
corroboration of the hypothesis would be based on the absence of characters [85, 86]. 
 
Head and depth variations 
Both fossil morphotypes show terminal mouths and fusiform bodies overall, such as C. chanos 
and other generalist teleosts not associated with a particular zone of the water column [5, 61]. 
However, it is still possible that they differed in some preference or performance related to 
their position in the water column. For example, teleosts characterized by polymorphism, such 
as Perca fluviatilis Linnaeus, 1758 (Perciformes) [87] and S. alpinus [88], show deeper bodies 
in benthic morphs and more streamlined bodies in pelagic morphs. Also, Chanos stocks of 
India with bigger heads, deeper bodies, and deeper caudal peduncles are probably the result of 
higher current intensity and high abundance of food [10]. 
Head and depth variation could also be linked to a differential foraging preference or 
performance [68, 87, 89], especially considering the morphotype-specific jaw variation. The 
elongated upper jaws and bigger heads in M2 could be adaptations for bigger food items or 
predatory adaptations for larger and slower prey when considering the bigger eyes and less 
streamlined shape in M2 [61]. A bigger head could also maximize buccal volume and suction 
velocity [90]. 
The distinct forms may have occupied different immediate ecological spaces due to 
resource competition [91, 92]. Such spaces might have been available (e.g., emptied or 
rearranged) in the aftermath of the K/Pg mass extinction event, early into the radiation of the 
megadiverse Acanthomorpha [e.g., 27, 93]. Species may not occupy the entire niche space 
because of various exogenous and endogenous factors, including chance and interactions with 
 57 
other organisms [85]. This hypothesis remains consistent with the overall conservativism 
tendencies of Chanos seen in fry life history and osteological stasis. The fish was likely still 
generalist in other parameters, for example, euryhaline, as extant C. chanos and probably most 
of Chanidae [4, 16]. Both morphotypes would still occupy the same fundamental niche, only 
varying or specializing [85]. Chanos may have an underlying genetic constraint that has 
prevented a higher osteological variation through protracted time. However, they can explore 
morphometric space through plasticity at least relatively more freely, and selection could have 
worked on the high-standing variation of those traits. 
Moreover, these substrate or foraging scenarios are not only consistent with 
polymorphism or diverging lineages but also for sexual dimorphism, as differential niche 
occupancy driven by competition between sexes is seen in teleosts, such as Jenynsia lineata 
(Jenyns, 1842) (Cyprinodontiformes) [91]. Otherwise, such as Decapterus macrosoma 
Bleeker, 1851 (Perciformes), bigger heads and deeper bodies were associated with higher 
nutritional needs for egg production in females [94]. Alternatively, bigger heads and deeper 
bodies could reflect traits under sexual selection of competing males instead of being selected 
through resource utilization [78, 94]. Another scenario that potentially explains the variation is 
perhaps different predation risks of adults [61, 95]. 
 
Conclusion 
The findings represent the first fossil record of the depauperate Chanidae in North America, 
contributing to our knowledge of chanid diversity and disparity following the mass extinction 
event K/Pg. It expands the range of Paleocene Chanos from the eastern Tethys Ocean of today's 
Turkmenistan to the western Proto-Caribbean of today's Chiapas, Mexico. We successfully 
maximized the sample size by dealing with post-mortem body torsion and estimating missing 
landmarks, both representing useful tools for paleontological studies with usually imperfect 
 58 
specimens. †Chanos chautus sp. nov. is described as an association of two morphotypes. Given 
the osteological variation in the upper jaw and morphometric variation in head size and body 
depth, they may have differed in some foraging preferences. However, it is impossible to test 
whether the pattern can be better explained by either sexual dimorphism, polymorphism of one 
taxon, or two sympatric lineages. The Chanos morphotypes from Palenque were juvenile fish 
and pending other geological sources of evidence, we argue that the localities were, at least at 
times, deposited in transitional environments. Therefore, Chanos appears to have conserved 
life history traits alongside its overall morphology throughout most of the Cenozoic. 
 
Acknowledgments 
This manuscript is a requirement for the first author (AG) to obtain the degree of Master in 
Biological Sciences (Systematics) within the Posgrado en Ciencias Biológicas at Universidad 
Nacional Autónoma de México. I (AG), extend my deepest gratitude to CONAHCYT and 
PAEP-UNAM, for the scholarships granted. We also deeply appreciate Alberto Montejo and 
his family for their permission and help during material collection (2008-2020). Thanks to 
Gabriela Bautista from ENALLT-UNAM for her notes on English grammar and scientific 
writing. Thanks to Susana Guzmán Gómez from Laboratorio de Microscopía y Fotografía de 
la Biodiversidad (II), IB-UNAM, who provided high-quality, high-resolution pictures of the 
small structures. Thanks to M.Sc. Violeta Amparo Romero Mayen for cataloging and storing 
the specimens in the type collection. Special thanks to Dr. Fernando del Moral from FESI-
UNAM, who donated the C. chanos specimen obtained from fishermen and gave some notes 
on extant fish biology. We are also grateful for Dr. César Campuzano insights into GM 
analyses, and for Dr. Paulo Brito who facilitated access to Brazilian chanids housed in the 
Universidade do Estado do Rio de Janeiro collection. We thank Dr. Jesús Alvarado for his 
 59 
various insights into fish anatomy; and finally, to the anonymous editor of the manuscript, 
whose comments improved this paper. 
 
Funding 
AG received financial support by the postgraduate scholarship of the Consejo Nacional de 
Humanidades, Ciencias y Tecnologías (CONAHCYT), with CVU number 1191092. 
Additional financial support for AG comes from PAEP-UNAM-2023 and DGAPA-PAPIIT 
project 206123. KMC received the grant from Dirección General de Asuntos del Personal 
Acádemico-Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica 
(DGAPA-PAPIIT) number 206123. The funders had no role in study design, data collection 
and analysis, decision to publish, or preparation of the manuscript 
 
Data Availability Statement 
All relevant data are within the paper and its Supporting information files. 
 
Author contributions 
Conceptualization, AG and KMC 
Data Curation, AG and KMC 
Funding Acquisition, AG and KMC 
Investigation, AG 
Methodology, AG 
Resources, KMC 
Validation, KMC 
Visualization, AG and KMC 
Writing – Original Draft Preparation, AG 
Writing – Review & Editing, AG and KMC 
 60 
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Discusión general y Conclusiones generales 
De acuerdo a la hipótesis filogenética, el material examinado pertenece al orden 
Gonorynchiformes y a la familia Chanidae. Su relación con el único género actual Chanos no 
es ambigua, varios caracteres soportan al clado, un complejo caudal; uroneural 2 separado al 
centro preural 2; un proceso posterolateral del esqueleto caudal; no hay espina en el en el centro 
preural 1; y los parietales son muy reducidos. Este es el primer registro de Chanidae en 
Norteamérica y extiende el rango geográfico del género Chanos durante el Daniano 
(Paleoceno, Cenozoico) del Tetis oriental de hoy Turkmenistán (Fara et al., 2010) al extremo 
occidental o proto Cáribe-Atlántico, hoy Palenque; por lo que los sábalotes antiguos eran Pan-
Tetianos desde inicios del Cenozoico.  
Los resultados de morfometría geométrica indican la presencia de dos morfotipos de 
Chanos fósiles en ambas localidades. El morfotipo 1 es largo y con una cabeza relativamente 
más pequeña que la del morfotipo 2; que tiene un cuerpo alto, con una cabeza más grande. Se 
logró maximizar el tamaño de la muestra al reducir la variación debida a la torsión post-mortem 
del 46.09% al 5.79% en los análisis de componentes principales. El error resultó no 
significativo, por lo que la estimación de landmarks no tuvo efecto alguno, incluso a partir de 
una muestra sin grupos a priori. Por lo tanto, la morfometría geométrica en general, el protocolo 
de unbending y la estimación de landmarks en particular, tienen un gran potencial para 
desarrollar estudios paleobiológicos de peces en diferentes contextos (p.ej., San Román et al., 
2024); pero están a expensas del tamaño de muestra y la calidad de la preservación.  
Más allá de las diferencias cuantitativas en proporciones derivadas de los análisis de 
morfometría es difícil establecer claras diferencias anatómicas cualitativas. La gran mayoría de 
los huesos son indistinguibles entre morfotipos de peces fósiles y actuales; los sabálotes 
muestran estasis morfológica desde inicios del Cenozoico. Entre morfotipos, la maxila tiende 
a ser alargada, bulbosa posteriormente pero recta en el morfotipo 2; en el morfotipo 1, la maxila 
 72 
es corta, posteriormente curva. Entre fósiles y actuales, al menos en el morfotipo 1, el 
mesetmoides tiene unas distintivas alas laterales cortas y alas posteriores largas, con un ángulo 
de apertura estrecho.  
 Al tener rangos de tamaño similares, los morfotipos no son un efecto del crecimiento 
alométrico. Al encontrarles en ambas localidades y en los mismos niveles, se puede asumir que 
coexistieron en tiempo y espacio, por lo que se descarta la diferenciación geográfica o cambio 
anagenético. El dimorfismo sexual podría explicar la presencia de dos morfotipos. El 
dimorfismo sexual de forma ya ha sido reportado para un género fósil de la familia Chanidae, 
†Rubiesicthys (Poyato-Ariza, 2005), donde como en muchos peces actuales, el abdomen se 
encuentra distendido por los ovarios en las hembras (p.ej., Molina et al., 2018). Sin embargo, 
en C. chanos no se ha reportado dimorfismo sexual. Más aún, en los morfotipos la variación 
está principalmente relacionada con la cabeza y los rasgos tendrían que expresarse muy 
tempranamente en el desarrollo. A pesar de esto, el dimorfismo sexual de forma puede estar 
relacionado de otras maneras con el éxito reproductivo, como en requerimientos nutrimentales 
o conductas diferenciales, por lo que no puede descartarse (Perazzo et al., 2019; Hernandez et 
al., 2022).  
Otro fenómeno visto en telósteos actuales que podría explicar la presencia de dos 
formas simpátricas es el polimorfismo (Alekseyev et al., 2014), o alternativamente dos especies 
(linajes evolutivos, sensu DeQueiroz, 2007). Por una parte, hay casos de polimorfismo con 
formas simpátricas con o sin divergencia genética (Malmquist et al., 1992; Gíslason et al., 
1999; Alekseyev et al., 2014; Hüne et al., 2023). Por otra parte, los stocks morfológicos de C. 
chanos actuales pueden o no tener divergencia genética; es decir, son clusters regionales 
inducidos por el ambiente y no poblaciones que son controladas por el flujo génico (Winans, 
80, 85; SriHari et al., 2019a,b). Por lo tanto, la diferenciación fenotípica no implica 
necesariamente divergencia genética en este caso. Más aún, la variación morfométrica y 
 73 
merística intraespecífica en Chanidae es alta (Arambourg y Schneegans, 1935; Taverne, 1981; 
Davis y Martill, 1999; Dietze, 2007; Brito y Amaral, 2008; Fara et al., 2010). Ninguna de estas 
tres explicaciones, dimorfismo sexual, un taxón polimórfico o dos especies simpátricas, puede 
ser puesta a prueba con los datos disponibles, por lo que se optó por una especie independiente 
a C. chanos como hipótesis nula.  
En cualquiera de los tres escenarios, las diferencias en la altura del cuerpo, cabeza y 
maxila entre morfotipos podrían ser resultado de variaciones en la dieta o su posición en la 
columna de agua, como en visto en teleostéos actuales con formas (y/o especies) simpátricas 
(Hjelm et al., 2001; Bower y Piller, 2015; SriHari et al., 2019a; Hüne et al., 2023). Incluso, la 
forma menos fusiforme, la cabeza, maxila, y ojos más grandes en el morfotipo 2 podrían ser 
adaptaciones para presas más grandes y lentas (Caldecutt y Adams 1998; Bower y Piller, 2015). 
Esta inusual diversidad de formas vista temprano en el Cenozoico en Chanos podría ser 
resultado del rearreglo de las comunidades marinas que siguieron a la extinción masiva 
Cretácico-Paleógeno (Alfaro et al., 2018; Cantalice et al., 2022), permitiendo a Chanos 
explorar el espacio morfométrico y ecológico. Tal vez a partir de la plasticidad fenotípica, como 
en el sábalote actual las condiciones hidrológicas y la abundancia de comida influencian la 
altura del cuerpo y el tamaño de la cabeza (SriHari et al., 2019a).  
Los Chanos de Palenque fueron ejemplares juveniles, dada la laminirización de 
basipterigios, los primeros pterigióforos y supraneurales en ejemplares más grandes que se 
atribuyen a la maduración esqueletal. Además, aunque se necesita de otras fuentes de evidencia 
geológica, la hipótesis que necesita menos suposiciones ad hoc es que BD y DN fueron 
depositadas en ambientes transicionales y que Chanos ha conservado el rasgo de historia de 
vida de migrar a dichos ambientes como juveniles por alrededor de 63 millones de años. Las 
características paleontológicas y tafonómicas de BD y DN que apoyan esta hipótesis son la 
presencia de plantas (aunque el material no es necesariamente autóctono); la presencia de 
 74 
†Phaerodus, un grupo fósil de agua dulce (Alvarado-Ortega et al., 2015); solo se encuentran 
peces de una longitud <260 mm de longitud estándar; finalmente, para taxones generalmente 
asociados a arrecifes solo hay uno o un par de ejemplares, mientras que los especímenes de 
anguilas, sardinas y Chanos son los más comunes, grupos donde los juveniles de ciertas 
especies actuales migran a ambientes transicionales (p.ej., Hare et al., 2021; Wright et al., 
2022).  
 
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 80 
Anexo (Suppporting Information) 
 
S1 PDF. Supplementary information. A pdf containing: a list of the referred material, 
including locality and the precise missing landmarks of each specimen from the GM sample. 
Table S1a. Landmark system; Fig S1a. Landmark system visualized; Table S1b. Linear 
measurements of each specimen from the unbent dataset; Fig S1b. The summary of the Error 
ANOVA. Character list. 
 
S1 PDF 
 
Referred material 
Housed in Colección Nacional de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de 
México. Mexico City, Mexico. Accession numbers were created and updated in the manuscript after reviewing, 
in brackets are the reference tags used during our research for each specimen, all GM datasets still have a CH tag. 
 
 
Total geometric morphometric sample.  
 
● IGM 13970 (CH01). DN. 3 missing landmarks: 5, 6, 7. 
● IGM 13971 (CH02). BD. 1 missing landmark: 14 
● IGM 13972 (CH03). BD. 
● IGM 13973 (CH04). DN. 4 missing landmarks: 5, 6, 7, 11. 
● IGM 13974 (CH05). DN. 
● IGM 13975 (CH06). DN. 1 missing landmark: 2. 
● IGM 13976 (CH07). BD. 3 missing landmarks: 3, 4, 8. 
● IGM 13977 (CH09). BD. 3 missing landmarks: 7, 8, 9. 
● IGM 13978 (CH10). DN. 1 missing landmark: 1.  
● IGM 13979 (CH11). BD. 1 missing landmark: 11. 
● IGM 13980 (CH12). BD. 
● IGM 13981 (CH13). BD. 
● IGM 13982 (CH14). BD. 
● IGM 13983 (CH15). BD. 
● IGM 13984 (CH16). BD. 
● IGM 13985 (CH17). DN. 2 missing landmarks: 8, 9. 
● IGM 13986 (CH19). DN. 
● IGM 13987 (CH20). DN. 
● IGM 13988 (CH21). BD. 1 missing landmark: 10. 
● IGM 13989 (CH22). BD. 
● IGM 13990 (CH25). BD. 
 81 
● IGM 13991 (CH26). BD. 1 missing landmark: 14.  
● IGM 13992 (CH27). BD. 
● IGM 13993 (CH28). BD. 1 missing landmark: 11 
● IGM 13994 (CH29). DN. 1 missing landmark: 14 
● IGM 13995 (CH30). BD. 
● IGM 13996 (CH31). BD. 
● IGM 13997 (CH32). BD. 
● IGM 13998 (CH33). BD. 
● IGM 13999 (CH34). BD. 
● IGM 14000 (CH36). BD. 
● IGM 14001 (CH37). BD. 
● IGM 14002 (CH38). BD. 
● IGM 14003 (CH39). BD. 
● IGM 14004 (CH40). BD. 1 missing landmark: 2. 
● IGM 14005 (CH41). BD. 
● IGM 14006 (CH43). BD. 
● IGM 14007 (CH44). BD. 
● IGM 14008 (CH46). BD. 
● IGM 14009 (CH47). BD. 
● IGM 14010 (CH48). BD. 
● IGM 14011 (CH50). DN. 1 missing landmark: 1.  
● IGM 14012 (CH52). BD. 
● IGM 14013 (CH53). BD. 
 
Extra specimens used only as comparative material 
 
● IGM 14014 (CH18). DN. An impression. Around 80 mm of SL. Lacks the 
dorsoposterior edge of the body outline.  
● IGM 14015 (CH23). BD. Complete. Associated to a large juvenile eel. around 40 mm 
of SL. 
● IGM 14016 (CH24). BD. Complete. Tail bent. Around 65 mm of SL. M1. 
● IGM 14017 (CH35). DN. The largest specimen, around 180 mm of SL. The skull is 
heavily fragmented, missing the belly. By eye, a M1. 
● IGM 14018 (CH42). BD. Complete. An impression, badly preserved. Around 80mm of 
SL. By eye, a M1. 
● IGM 14019 (CH45). BD. Complete. A faint impression of 55 mm of SL. 
● IGM 14020 (CH49). BD. Complete but distortion is highly sigmoidal, skull in dorsal 
view. Around 48 mm of SL. 
● IGM 14021 (CH51). DN. By eye, a large M2, >150mm of SL. It is missing most of the 
head. 
● IGM 14022 (CH54). DN. Only a well-preserved tail, maybe a M2. 
● IGM 14023 (CH08). DN. Around 130 mm of SL. Big M2. Incomplete. 
 
 
 
 82 
 
Table S1. Landmark system. 
Num. Type Definition Notes 
1 ii Snout tip  
2 ii End of frontals 
3 i Origin of dorsal fin Positioned between first pterygiophore and first ray 
4 i Insertion (end) of dorsal fin Positioned between last pterygiophore and last ray 
5 ii 
Origin of anteriormost dorsal procurrent 
caudal-fin ray At the distal tip of the ray 
6 i End of the vertebral column Between pleurostyle and hypural plate 1 
7 ii 
Origin of anteriormost ventral 
procurrent caudal-fin ray At the distal tip of the ray 
8 i Insertion (end) of anal fin Between last pterygiophore and last ray 
9 i Origin of anal fin Between first pterygiophore and first ray 
10 i Origin of pelvic fin Between basipterygium and first lateral ray  
11 i Origin of pectoral fin rays Between basipterygium and first medial ray  
12 i or ii Cleithrum-supracleithrum articulation 
In some specimens, it must be digitized as the posteriormost edge of 
the opercle 
13 i or ii Interopercle-subopercle articulation 
In some specimens, it must be digitized as the anteroventralmost 
edge of opercle 
14 ii Angle between the arms of the preopercle’s ridge 
15 ii Ascending process of parasphenoid or posteroventral margin of the orbit 
 
 
 
 83 
  
 
Fig S1. Landmark system visualized. A, landmarks digitized over CH12. B, landmark system over the 
mean shape of Chanos chautus sp nov.  
 
 
 
 
 
 
 
 
 
 
 
Table S2. Linear measurements of each specimen from the unbent and estimated dataset. 
 SL(mm) HL(mm) HL(SL%) HD(mm) HD(SL%) BD(mm) BD(SL%) PD(mm) PD(SL%) PA(mm) PA(SL%) CPD(mm) 
CPD(SL%
) 
CH01 (M1) DN 147.87 40.82 27.59 24.65 16.67 21.7 14.67 85.53 57.84 125.65 84.97 7.79 5.26 
CH10 (M2) DN 115.22 47.91 41.58 27.56 23.91 25.58 22.2 68.91 59.8 94.76 82.24 11.24 9.75 
CH43 (M2) BD 103.48 40.48 39.11 24.39 23.56 25.2 24.35 60.81 58.76 84.96 84.96 10.54 10.18 
CH19 (M1) DN 88.36 26.22 29.67 14.89 16.85 15.35 17.37 50.62 57.28 69.53 78.68 7.29 8.25 
CH06 (M1) DN 83.49 24.69 29.57 15.33 18.36 16.94 20.28 47.26 56.6 67.81 81.21 7.57 9.06 
CH11 (M2) BD 83.44 33.88 40.6 19.32 23.15 22.05 26.05 48.94 58.65 70.88 84.94 10.4 12.42 
CH20 (M1) DN 82.7 26.04 31.48 15.22 18.4 17.33 20.95 48.62 58.79 70.41 85.14 7.22 8.73 
CH05 (M1) DN 82.32 23.58 28.64 14.38 17.46 15.65 19.01 48.2 58.55 68 82.6 5.4 6.55 
CH04 (M1) DN 78.02 21.63 27.72 13.31 17.05 15.99 20.49 45.3 58 65.54 84 6.9 8.84 
 84 
CH30 (M2) BD 72.82 25.94 35.62 15.08 20.7 19.34 26.55 42.45 58.29 61.14 83.96 8.45 11.6 
CH50 (M2) DN 70.51 30.73 43.58 17.25 24.46 17.98 25.49 44.77 63.49 60.42 85.68 6.25 8.86 
CH31 (M1) BD 69.66 18.52 26.58 10.21 14.65 12.8 18.37 40.71 58.44 58.56 84.06 5.53 7.93 
CH25 (M2) BD 69.11 29.28 42.36 16.27 23.54 16.3 23.58 42.11 60.93 56.59 81.88 5.84 8.45 
CH44 (M1) BD 67.98 19.78 29.09 10.34 15.21 13.36 19.65 39.75 58.47 58.82 86.52 5.23 7.69 
CH21 (M1) BD 61.05 17.36 28.43 10.19 16.69 11.48 18.8 35.46 58.1 48.79 79.91 4.48 7.33 
CH53 (M2) BD 60.77 25.21 41.48 12.82 21.09 14.31 23.54 37.34 61.44 55.11 90.68 5.69 9.36 
CH12 (M2) BD 59.5 22.1 37.14 12.09 20.32 14.86 24.97 36.32 61.04 50.09 84.18 6.96 11.69 
CH40 (M2) BD 59.48 22.66 38.09 14.46 24.31 15.88 26.69 33.94 57.06 48.19 81.01 5.57 9.36 
CH29 (M1) DN 58.14 16 27.51 8.76 15.06 11.83 20.34 32.64 56.14 46.9 80.66 5.34 9.18 
CH13 (M2) BD 56.59 22.53 39.81 12.6 22.26 15.16 26.79 33.15 58.57 46.2 81.66 5.46 9.64 
CH27 (M2) BD 56.56 23.39 41.35 12.63 22.33 13.67 24.17 33.18 58.66 46.54 82.28 5.13 9.07 
CH04 (M1) BD 55.39 20.76 37.47 11.99 21.64 12.99 23.45 32.84 59.28 44.09 79.59 4.34 7.83 
CH09 (M1) BD 55.03 16.34 29.69 9.54 17.33 8.08 14.68 33.86 61.53 45.74 83.11 3.94 7.15 
CH16 (M2) BD 53.99 22.77 42.17 11.71 21.68 11.93 22.09 32.37 59.95 44.31 82.07 5.44 10.07 
CH41 (M2) BD 53.64 20.73 38.64 11.01 20.52 12.73 23.73 30.74 57.3 43.74 81.54 4.66 8.68 
CH17 (M2) DN 50.81 20.64 40.62 11.85 23.32 11.8 23.22 30.76 60.61 43.35 85.31 3.7 7.28 
CH33 (M1) BD 48.26 14.14 29.29 7.66 15.87 8.22 17.03 27.42 56.81 39.88 82.63 3.64 7.54 
CH52 (M1) BD 46.71 13.74 29.41 7.79 16.67 7.87 16.49 26.92 57.63 38.88 83.23 3.91 8.37 
CH38 (M1) BD 46.23 14.05 30.39 8.64 18.68 8.88 19.21 26.83 58.03 39 84.36 4.46 9.64 
CH46 (M1) BD 46.15 12.61 27.32 7.33 15.88 7.88 17.07 28.19 61.08 38.45 83.31 3.81 8.255 
CH02 (M1) BD 45.96 12.61 27.43 7.22 15.7 6.39 13.9 26.45 57.55 36.94 80.37 3.8 8.26 
CH03 (M1) BD 44.53 14.14 31.75 7.84 17.6 8.87 19.91 25.23 56.65 37.53 84.28 4.51 10.12 
CH15 (M2) BD 43.3 17.09 39.46 10.08 23.27 11.2 25.86 25.4 58.66 36.51 84.31 4.12 9.51 
CH39 (M1) BD 42.53 13.21 31.06 7.16 16.83 6.62 15.56 24.93 58.61 34.73 81.66 2.99 7.03 
CH22 (M2) BD 42.11 16.55 39.3 9.48 22.51 9.77 23.2 24.91 59.15 35.45 84.18 3.68 8.73 
 85 
CH26 (M1) BD 41.83 12.6 30.12 6.19 14.79 7.1 16.97 24.24 57.94 33.43 79.91 3.4 8.12 
CH37 (M1) BD 40.42 12.23 30.25 7.6 18.8 7.8 19.29 23.17 57.32 33.51 82.9 3.6 8.9 
CH47 (M1) BD 38.61 11.08 28.69 6.81 17.68 6.77 17.56 22.21 57.52 32.07 83.06 2.78 7.2 
CH34 (M1) BD 36.8 9.84 26.73 6.59 17.9 6.37 17.31 20.9 56.79 30 81.52 2.92 7.93 
CH48 (M1) BD 35.89 10.56 29.42 6.45 17.97 7.06 19.67 20.49 57.09 30.23 84.22 3.35 9.33 
Mean   33.655  19.266  20.762  58.61  83.069  8.728 
SL, standard length (interlandmark distance 1-6); HL, head length (interlandmark distance 1-12); HD, head depth (interlandmark distance 2-
13); BD, body depth (interlandmark distance 3-10); PD, predorsal length (interlandmark distance 1-3); PA, preanal length (interlandmark 
distance 1-9); CPD, caudal peduncle depth (interlandmark distance 5-7).  
 
Error 
FigS2. Screenshot of the ANOVA’s summary. 
 
Character list from Ribeiro et. al. (2018). 
Cranium 
1. Orbitosphenoid: present [0], absent [1]. 
2. Basisphenoid: present [0], absent [1]. 
3. Pterosphenoids: well developed and articulating with each other [0], slightly reduced, 
not  articulating anteroventrally but approaching each other anterodorsally [1], greatly 
reduced and  broadly separated both anteroventrally and anterodorsally [2].  
4. Posterolateral expansion of exoccipitals: absent [0], present [1]. 
5. Exoccipitals: posteriorly smooth with no projection above the basioccipital [0], with 
a  posterior concave-convex border, and a projection above basioccipital [1]. 
6. Cephalic ribs: absent [0], present and all articulating with the exoccipitals [1], present 
and  articulating with both the exoccipitals and basioccipital [2]. 
 86 
7. Supraoccipital crest: small, short in lateral view [0]; long and enlarged, projecting 
above  occipital region and first vertebrae, forming a vertical, posteriorly deeply pectinated 
blade [1]. 
8. Foramen magnum: dorsally bounded by exoccipitals [0]; enlarged and dorsally bounded 
by  supraoccipital [1].  
9. Brush-like cranial intermuscular bones (sensu Patterson, Johnson, 1995): absent [0], 
present  [1]. 
10. Mesethmoid: wide and short [0]; long and slender, with anterior elongate lateral extensions 
[1]; large, with broad posterolateral wing-like expansions [2]. 
11. Wings (extensions) of lateral ethmoids: absent [0]; present [1].  
12. Nasal bone: small but flat [0]; just a tubular ossification around the canal [1].  
13. Frontals: wide through most of their length, narrowing anteriorly to form a triangular 
anterior border [0]; elongate and narrow except in postorbital region [1]; wide, anteriorly 
shortened, anterior border roughly straight [2].  
14. Interfrontal fontanelle: absent [0]; present [1].  
15. Frontal bones: paired in adult [0]; co-ossified, with no median suture [1].  
16. Foramen for olfactory nerve in frontal bones: absent [0]; present [1]. 
17. Relative position of the parietals: medioparietal (in full contact with each other along their 
midline) [0]; mesoparietal (sensu Poyato-Ariza, 1994); partly separated by the 
supraoccipital, posteriorly, and partly in contact with each other, anteriorly) [1]; 
lateroparietal (completely  separated from each other by the supraoccipital) [2]. 
18. Parietal portion of the supraorbital canal: absent [0]; present [1].  
19. Parietals: large [0]; reduced but flat and blade-like in shape [1]; highly reduced [2]; 
absent  as independent ossifications [3]. 
Orbital region  
20. Number of infraorbitals: five or more [0]; four [1]; three or fewer [2].  
21. Infraorbital bones not including lacrimal: well developed [0]; reduced to small, 
tubular  ossifications [1].  
22. Lacrimal: flat and comparable in length to subsequent infraorbitals [0]; flat, long and 
large,  with keel near lower edge [1].  
23. Supraorbital: present [0]; absent [1]. 
Jaws 
24. Teeth in premaxilla, maxilla, and dentary: present [0]; absent [1].  
25. Premaxilla: consisting of one solid element [0]; premaxilla consisting of two 
distinct  elements, with a shorter, non-osseous element lying ventral to a much longer 
osseous portion,  which in turn articulates with the maxilla [1].  
26. Premaxillary “gingival teeth”: absent [0]; present [1].  
 87 
27. Premaxilla: small, flat and roughly triangular [0]; large, very broad, concave-convex, 
with  long oral process [1]; narrow and elongated, its length more than one half of the length 
of the  maxilla [2].  
28. Premaxillary ascending process: present [0]; absent [1].   
29. Morphology of maxillary articular process: thin and pointed [0]; robust and bulky [1]; flat 
and  hypertrophied, higher than the main body of the bone [2].  
30. Length of maxillary articular process: short, less than 30% of the total maxillary length 
[0];  long, 30%-40% of the total maxillary length [1]; very long, about 50% of the total 
maxillary  length [2].  
31. Dorsal and ventral borders of the maxillary articular process: straight or slightly curved 
[0];  very curved, almost describing an angle [1].  
32. Maxillary process for articulation with autopalatine: absent [0]; present [1]. 
33. Posterior region of the maxilla: slightly and progressively expanded to form a thin blade, 
with roughly straight posterior border [0]; very enlarged, swollen to a bulbous outline, 
with  curved posterior border [1]. 
34. Supramaxilla(e): present [0]; absent [1]. 
35. Symphysis: low, pointed [0]; higher than immediately posterior part of the dentary, robust 
[1].   
36. Notch between the dentary and the anguloarticular bones: absent [0]; present [1].  
37. Articulation between dentary and angulo-articular: strong, dentary not V-shaped 
posteriorly  [0]; loose, with a posteriorly V- shaped dentary [1].  
38. Notch in the anterodorsal border of the dentary (“leptolepid” notch): absent [0]; present [1]. 
39. Mandibular sensory canal: present [0]; absent [1]. 
Palate and suspensorium 
40. Dermopalatine: present [0], absent [1].  
41. A patch of about 20 conical teeth on endopterygoid and basibranchial 2: absent [0]; 
present  [1].  
42. Ectopterygoid: well developed, ectopterygoid overlapping with the ventral surface of 
the  autopalatine by at least 50% [0]; reduced, articulating with the ventral surface of the 
autopalatine by at most 10% through cartilage, resulting in a loosely articulated 
suspensorium [1]. 
43. Teeth on vomer and parasphenoid: absent [0]; present [1].   
44. Anterior portion of vomer: horizontal [0]; anteroventrally inclined, nearly vertical 
[1];  dorsally curved [2].   
45. Spatial relationship between vomer and mesethmoid anteriorly: vomer and 
mesethmoid  ending at about the same anterior level [0]; mesethmoid extending anteriorly 
beyond the level of  anterior margin of vomer [1]; vomer extending anteriorly beyond the 
level of anterior margin of  mesethmoid [2]. 
46. Metapterygoid: large, broad and in contact with quadrate and symplectic through 
cartilage  [0]; reduced to a thin rod [1]. 
47. Quadrate: with posterior margin smooth [0]; elongated forked posterior process [1].   
 88 
48. Quadrate-mandibular articulation: below or posterior to orbit, no elongation or 
displacement  of quadrate [0]; anterior to orbit, quadrate displaced but not elongate [1]. 
49. Symplectic: elongated in shape but relatively short [0]; very long, about twice the length 
of  the ingroup [1]. 
50. Symplectic and quadrate: articulating directly with each other [0]; separated 
through  cartilage [1]. 
51. Articular head of hyomandibular bone: double, with both articular surfaces placed on 
the  dorsal border of the main body of the bone [0]; double, with the anterior articular 
surface  forming a separate head from the posterior articular surface [1].   
52. Metapterygoid process of hyomandibular bone: absent [0], present, anterior [1]; 
present,  ventral [2].   
53. Ossified interhyal: present [0]; absent as an independent ossification [1].  
 
Opercular series  
54. Size of opercular bone: normal, about one quarter of the head length [0]; expanded, at 
least  one third of the head length [1].  
55. Shape of opercular bone in lateral view: rounded/oval [0]; triangular [1]. 
56. Opercular apparatus on external surface of opercle: absent [0]; present [1]. 
57. Opercular borders: free from side of head [0]; partly or almost completely connected to 
side  of head with skin [1].  
58. Angle formed by preopercular limbs: obtuse [0]; approximately straight [1]; acute [2].  
59. Posterodorsal limb of preopercular bone: well developed [0]; reduced, correlated 
with  expansion of anteroventral limb that meets its fellow along the ventral midline [1].  
60. Ridge on anteroventral limb of preopercular bone: absent [0]; present [1]. 
61. Preopercular expansion distal to the terminal openings of the preopercular canal 
branches:  absent, preopercular bone not enlarged [0]; present, restricted to the 
posteroventral corner [1];  present in posteroventral corner and part of the posterodorsal 
limb [2].  
62. Suprapreopercular bone: absent [0]; present as a relatively large, flat bone [1]; present 
as  tubular ossification(s) [2].  
63. Major axis of subopercular bone in lateral view: inclined [0]; subhorizontal 
[1]. 
64. Subopercular clefts: absent [0]; present [1]. 
65. Posterodorsal ascending process of interopercular bone: absent [0]; present 
[1]. 
 
Branchial arches 
66. Teeth on fifth ceratobranchial: present [0]; absent [1].  
67. First basibranchial in adult specimens: ossified [0]; unossified [1]. 
68. Fifth basibranchial in adult specimens: cartilaginous [0]; ossified [1].  
69. First pharyngobranchial in adult specimens: ossified [0]; unossified 
[1]. 
 89 
Vertebrae  
70. Two anteriormost vertebrae: as long as posterior ones [0]; shorter than posterior ones [1]. 
71. Autogenous neural arch anterior to first vertebra: present [0]; absent [1]. 
72. Neural arch of first vertebra and exoccipitals: separate [0]; in contact [1]. 
73. Neural arch of first vertebra and supraoccipital: separate [0]; in contact [1]. 
74. Spine on neural arch of first vertebra: present, well developed [0]; present but reduced [1]. 
75. Anterior neural arches: no contact with adjoining arches [0]; abutting contact laterally with 
adjoining arches, no overlapping [1]. 
76. Neural arches 5–10 in adults: fused to centra [0]; autogenous, at least laterally [1]. 
77. Neural arches to vertebrae posterior to the dorsal fin in adults: fused to centrum 
[0];  autogenous, at least laterally [1]. 
78. First two anterior parapophyses: autogenous [0]; fused to centra [1]. 
79. Rib on third vertebral centrum: similar in size and shape to posterior ones [0]; widened 
and  shortened [1]; modified into Weberian apparatus [2]. 
Intermuscular bones  
80. Paired intermuscular bones consisting of three series: epipleurals, epicentrals, 
and  epineurals: absent (at least one complete series is absent) [0]; present (three series) [1]. 
81. Anterior (first six) epicentral bones: unmodified, no differences in size from others 
[0];  highly modified, much larger than posterior ones [1]. 
82. Shape of anterior supraneurals 1 and 2: narrow and separated [0]; large and in contact [1].  
83. Posterior process on the posterior border of first supraneural: absent [0]; present [1]. 
84. Number of supraneurals: several supraneurals in a long series [0]; two or fewer 
supraneurals  [1]. 
Girdles and fins  
85. Postcleithra: present [0]; absent [1]. 
86. Lateral line and supracleithrum: supracleithrum pierced through dorsal region 
[0]; supracleithrum pierced all through its length [1]; lateral line does not pierce 
supracleithrum [2].   
87. Fleshy lobe of paired fins: absent [0]; present [1]. 
88. Caudal fin morphology: elongated, posteriorly forked [0]; higher than long, slightly 
incurved posteriorly [1]. 
89. Fringing fulcra in dorsal lobe of caudal fin: present [0]; absent [1].   
90. Caudal scutes: absent [0]; present [1]. 
Caudal endoskeleton  
91. Ural centra (u1, u2), preural centrum one (pu1), and uroneural one (un1): autogenous 
[0];  fused [1].  
 90 
92. Neural arch and spine of preural centrum one: both well developed, spine about half as 
long  as preceding ones [0]; arch complete and closed, spine rudimentary [1]; arch open, no 
spine [2]. 
93. Uroneurals (regardless of number): arranged in a linear series [0]; arranged in a 
double  series [1].  
94. Total number of uroneurals regardless of their fusion to other elements of the 
caudal  endoskeleton (dealt with in character 92 above): three [0]; two [1]; one [2].  
95. Anterior extent of first uroneural: to anterior end of first preural [0]; to anterior end of 
second preural [1]; to anterior end of third preural [2]; uroneural fused to caudal fin 
complex [3].  
96. Uroneural two and second ural centrum: in contact [0]; separated [1]; uroneural two 
absent  as an autogenous ossification [2].  
97. Parhypural and preural centrum 1: independent in adults [0]; fused only in large adults 
[1];  fused since early ontogenetic stages [2]. 
98. Reduction in the number of hypurals: six [0]; fewer than six [1]. 
99. Hypurals 1 and 2: autogenous [0]; partly fused to each other [1]. 
100. Hypural 1 and terminal centrum: articulating [0]; separated by a hiatus [1]; fused 
[2]. 
101. Hypural 2 and centrum: fused [0]; autogenous [1]. 
102. Hypural 5 and second ural centrum: separate [0]; articulating [1].  
103. Hemal arch in preural centrum 2: fused to the centrum [0]; autogenous 
[1].  
104. Posterolateral process of caudal endoskeleton: absent [0]; present [1].  
Scales and lateral line  
105. Type of scales: cycloid [0]; modified ctenoid [1].  
106. Lateral line: not extending to posterior margin of hypurals [0]; extending to 
posterior  margin of hypurals [1].