Pycnodus sp.

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 : 14-16

Pycnodus sp.

Referred Material. IGM 4551, almost complete specimen from the División del Norte quarry (i.e., Locality-IGM 3869) preserved in part (Figure 8) and counterpart. This specimen lacks the tips of the lower and upper jaws; its estimated standard length is 79 mm. IHNFG-4890, complete specimen from the Belisario Domínguez quarry; its estimated standard length is 90 mm.

Remarks and comparison. These specimens display an assortment of diagnostic characteristics for the order Pycnodontiformes as previously recorded by numerous authors (e.g., Nursall, 1996; Machado and Brito, 2006) including the presence of molariform teeth arranged in rows on the vomer and prearticular bones and differentiated cloacal and contour ridge scales (here named dorsal ridge scales and ventral keel scales) (Figure 8). Additionally, the referred specimens show a parietal peniculus-like process, an unusual structure and a diagnostic characteristic of the family Pycnodontidae sensu Poyato-Ariza and Wenz (2002) .

Poyato-Ariza and Wenz (2002) erected the Pycnodontinae subfamily, a group within the Pycnodontidae family, to include the Pycnodus and Oropycnodus genera. Although Kriwet (2005) suggested and alternative hypothesis in which these fishes are not sister taxa, new discoveries support the idea that this is a natural and diverse group which also includes three Italian monospecific genera: Tergestinia Capasso, 2000 , from the Paleocene deposit of Trebiciano; Pseudopycnodus Taverne, 2003 , from Campanian-Maastrichtian strata of Nardo; and Polazzodus Poyato-Ariza, 2010 , collected in Turonian-early Senonian outcrops at Polazzo. In addition, the Portuguese fish Silvienodus Poyato-Ariza, 2013, from Cenomanian deposits at Laveiras, might also belong in this subfamily.

Among Pycnodontiformes , the subfamilies Pycnodontinae and Nursallinae share the occurrence of a bifid scale above the cloacal opening ( Poyato-Ariza and Wenz, 2002, characteristic 104(1)); this peculiar scale is present in specimens IGM 4551 and IHNFG-4890. Pycnodontines also are distinctive and well characterized by the presence of postcephalic lacuna that leave the endocranium posteriorly exposed, incompletely ossified or reduced to bar-like structures scales in their dorsal lateral abdominal region, and a number of 10 to 15 ventral keel scales ( Taverne and Capasso, 2012; Poyato-Ariza, 2013, p. 99). The specimens referred here are considered to be members of Pycnodontinae because, in addition to the occurrence of the bifid scale mentioned above, they show the typical bar-like scales in the anterior dorsal area of the trunk and approximately 12 ventral keel scales (10 in front of the cloaca and 2 behind).

At present, it is difficult to support the inclusion of Silvienodus in the Pycnodontinae subfamily because of ambiguous data published; its author suggested the lack of postcephalic lacuna in this fish but later he confirmed its presence ( Poyato-Ariza, 2013, p. 94, 99). Oropycnodus and Pycnodus share the presence of a well-defined dermocranial fenestra, a lateral opening in the dermocranium bordered by the frontal, parietal, dermal supraoccipital bones (referred to as temporal fenestra by Nursall (1996) and Taverne and Capasso (2012)). Although this fenestra is also present in other pycnodontiforms, among Pycnodontines it is exclusively present in Oropycnodus and Pycnodus , as well as in the División del Norte and Belisario Domínguez specimens referred here.

Oropycnodus differs from Pycnodus and Mexican specimens IGM 4551 and IHNFG -4890 because the bifid scale in its cloaca is bordered by modified comma-shaped scales, its caudal peduncle is not differentiated (there are less than three neural and haemal spines between the beginning of caudal endoskeleton and the end of anal and dorsal fins), and it exhibits a post-cloacal notch (formed between the two postcloacal ventral keel scales). On the other hand, the genus Pycnodus and the specimens referred here do not have such comma-shaped scales in the cloaca, their caudal peduncle is well differentiated, and they do not exhibit any post-cloacal notch (see Poyato-Ariza and Wenz, 2002, characters 7, 71, 104 and 105). Hence, we conclude that the División del Norte and Belisario Domínguez specimens referred here belong to the genus Py cnodus.

Numerous Cretaceous-Eocene nominal species of Pycnodus have been described based mainly on extremely incomplete specimens. However, Poyato-Ariza (2013) has restricted the genus to only one species from the Eocene deposits of Monte Bolca, Pycnodus apodus . The Mexican specimens described above show characteristics that suggest they are representatives of a new species of this genus. In comparison to previous anatomical data of Pycnodus apodus ( Poyato-Ariza and Wenz, 2002) , these Mexican fishes show a greater number of total and post-cloacal (10) and ventral keel scales (2) versus 7 and 1 scales, respectively, found in P. apodus . Also, the División del Norte and Belisario Domínguez specimens show less dorsal and anal pterygiophores or axonosts (4142 and 32, respectively) than in P. apodus (58 and 45) and a large or regular-sized last neural spine that clearly differs from the vestigial condition of this structure present in P. apodus . Regardless the above observation, a more complete anatomical analysis of these Mexican fishes is required before naming a possibly new species.

The occurrence of Pycnodus in Mexico confirms the presence of Pycnodontinae fishes in North America, evidencing a wide marine distribution of this subfamily (see Alvarado-Ortega et al., 2006). The body shape of pycnodontiforms suggests that these fishes were not competent pelagic swimmers. This observation, along with a restricted temporal occurrence of the known members of this group: División del Norte and Belisario Domínguez Pycnodus sp. (Paleocene, North America, Mexico), Pycnodus (Eocene, Europe, Italy), and Oropycnodus (Paleocene, Europe, France), suggest that this crown group of the subfamily Pycnodontinae has a long evolutionary history that could be traced back to at least the middle Cretaceous, when continents derived from Pangea were not yet significantly apart from each other.

Other Fishes

The present paper is a mere overview of the fish diversity so far collected from the Paleocene marine strata of the División del Norte and Belisario Dominguez quarries. Some other fishes not presented in this work include specimens under current preparation, awaiting taxonomic identification, or part of other descriptive studies still in progress. Although the fish diversity from these localities has not been completely determined yet, the authors believe that there may be other possible pycnodontiforms, at least another clupeid fish, another possible serranid perciform, characiforms that may belong to the family Erythrinidae , a specimen related to Syngnathiformes, as well as fishes of both major groups of the orders Gonorynchiformes , Gonorynchidae and Chanidae .