ANGUILLIFORMES Goodrich, 1909

Alvarado-Ortega, Jesús, Cuevas-García, Martha, Melgarejo-Damián, Pilar, Cantalice, Kleyton Magno, Alaniz-Galvan, Abril, Solano-Templos, Gisel & Than-Marchese, Bruno Andrés, 2015, Paleocene fishes from Palenque, Chiapas, southeastern Mexico, Palaeontologia Electronica 10 (14), pp. 1-22 : 6-7

Order ANGUILLIFORMES Goodrich, 1909 Family indeterminate

Genus and species indeterminate

Referred Material. IGM 4547 (Figure 3.1-3.3), complete specimen lacking the posterior part of the tail, and with an estimated total length (TL) of 118.37 mm. IHNFG-4891, complete specimen with an estimated TL of 135 mm; IHNFG-4893 and IHNFG-4894, two complete specimens preserved in the same flagstone and both measuring 135 mm of TL. All specimens were recovered from the Belisario Domínguez quarry (i.e., Locality-IGM 3870).

Remarks and comparison. Elopomorpha is a natural clade that has been divided into three to five orders, including Albuliformes, Anguilliformes, Elopiformes, Notacanthiformes, and Saccopharyngiformes (e.g., Greenwood et al., 1966; Inoue et al., 2001; Nelson; 2006; Wiley and Johnson, 2009). The morphological specializations of the most derived of the elopomorph orders, Anguilliformes and Saccopharyngiformes, have led to controversial interpretations that have therefore hindered the establishment of a robust and consensual classification. Forey et al. (1996) were the first authors to recognize both orders as part of a monophyletic group, a proposal supported by recent studies ( Santini et al., 2013; Chen et al., 2014).

The specimens referred in the present work are recognized as true Anguilliformes , commonly known as eels, because their long and cylindrical snake-like bodies, their absence of a pelvic fin, and their pectoral fin located in the middle of the trunk far from the ventral border and behind the fourth vertebra (Figure 3.1) are distinctive characters of this order ( Nelson, 2006; Wiley and Johnson, 2009; Johnson et al., 2011; Chen et al., 2014). Additionally, the Belisario Domínguez eels cannot belong in the Saccopharyngiformes since the latter is a group of highly specialized fishes lacking their opercular bones and branchiostegal rays, whereas in the Mexican specimens such bones are present (Figure 3.3).

Although the origin of eels dates back to the Cenomanian ( Forey et al., 1996), their fossil record is scarce and the knowledge on the morphology of a large number of living eels is limited. Therefore, it is not presently feasible to classify Cretaceous eels within families solely created to include their Cenozoic and extant relatives. At the beginning of the present decade, Johnson et al. (2011) described Protoanguilla palau , a species which was placed in its own family, Protoanguillidae , as a putative living sister group of both Cenozoic and extant eels. However, succeeding studies ( Santini et al., 2013; Chen et al., 2014) identified this species at the base of the living eels as the sister group of the family Synaphobranchidae . In this paper, we follow the comparative exercise of Johnson et al. (2011) as a template to analyze the Paleocene eels recovered from the Belisario Domínguez quarry.

The Belisario Domínguez specimens, like most eels (with the exception of the “saccopharyngiforms”), present a rostrocaudally elongated opercle with a bottle-neck articular condyle. Moreover, these Mexican fossils share the following characteristics with all extant eel taxa (Figure 3): 1) scaleless bodies and, when present, scales are unimbricated; 2) absence of endopterygoid (not illustrated in this work); 3) long dorsal and anal fins confluent with caudal fin; and 4) less than eight rays in each caudal fin lobe. In contrast, Cretaceous eels exhibit scales and an endopterygoid, and their dorsal and anal fins are clearly separated from a caudal fin with at least eight rays in each lobe (see Johnson et al., 2011, p. 6). In addition, the number of total vertebrae in extant and most Cretaceous eels ranges between 98 and over 300, in Protoanguilla it varies between 79 and 87, and in some saccopharyngiform eels this number is reduced to 70. The number of total vertebrae in these Mexican fossils ranges between 92 and 93, which lies in the 90 to 95 range of Luenchelys Belouze et al., 2003 , a primitive anguillifrom of uncertain affinities from the Cenomanian marine deposits of Namoura, Lebanon. This characteristic makes Luenchelys and these Mexican fossils eels different from practically all the other eels.

As mentioned above, the fossil record of eels is scarce and rarely includes complete skeletons ( Daniltshenko, 1964; Wiley and Stewart, 1981; Forey et al., 2003). The Cenozoic specimens are extremely scarce; these are known from the Upper Paleocene strata of the Danata Formation in Turkmenistan; the renowned Eocene marine quarries of Monte Bolca, Italy ( Blot, 1984; Sytchevskaya and Prokofiev, 2004); as well as the Middle Eocene freshwater strata of Messel, Germany, where the oldest representative of the genus Anguilla Shaw, 1803 , was collected and named as Anguilla ignota by Micklich, 1985.

In this matter, the Palenque eels collected from the Belisario Domínguez quarry present a major opportunity to extensively study this group, since they are the fourth Paleogene record of eels worldwide and the first Paleocene eel of America, and their intermediate affinities between Cretaceous and Modern eels are hitherto unique.