Thalattosuchia Fraas, 1901

Alvarado-Ortega, Jesús, Barrientos-Lara, Jair Israel, Espinosa-Arrubarrena, Luis & Melgarejo-Damián, Pilar, 2013, Late Jurassic marine vertebrates from Tlaxiaco, Oaxaca State, southern Mexico, Palaeontologia Electronica (24 A) 17 (2), pp. 1-25 : 15-16

Infraorder Thalattosuchia Fraas, 1901

Family, Genus, and species undetermined

Figure 10.1-3

Referred material. IGM 9324, fragments of two dorsal vertebrae ( Figure 10.1-3 ) from Yosobé. This material was originally preserved within a nodule from the shale layer between limestone strata L4 and L5 ( Figure 4 ).

Occurrence. Yosobé, Tlaxiaco, Oaxaca. Kimmeridgian-Tithonian shales of the Sabinal formation.

Description. Although incomplete, these bones are recognizable as middle dorsal vertebrae of a thalattosuchian crocodyliform. Whereas the posterior vertebra only preserves the anterior half, the anterior is almost complete. The centra are stout and long cylindrical structures, 1.3 times longer than high, and strongly constricted in the middle (in lateral view the ventral and lateral surfaces are concave in a longitudinal direction). In an anterior-posterior view, both intervertebral articular surfaces are ovoid, slightly wider than high (34 versus 32 mm. respectively), and somewhat flattened along the neural canal floor. These vertebrae are weakly amphicelic and both intervertebral articular surfaces are slightly concave.

The neurocentral suture is an irregular and particularly sinuous line located in the upper third of the centrum. The neural arch in the most complete vertebra bears a neural spine that is as long as the centra. The ventral posterior area of the neural spine is projected posteriorly beyond the centrum posterior edge; additionally, this area is wider and laterally carries the flat articular surfaces of the posterior zygapophysis. The anterior zygapophysis is a small process separated from the neural spine and anteriorly projected up and forward. There is no hypapophysis.

Very close to the base of the neural arch and below the anterior zygapophysis is a stout diapophysial process projected laterally from the centrum, whose end must bear the parapophysial process anteriorly and distal process posteriorly. This process is bi-lobed in cross section ( Figure 10.3 ).

Remarks. IGM 9324 is referred to the thalattosuchia crocodyliformes based on its weak amphicelic condition, relatively long neural arch, strong middle constriction, and the up and forward projection of the anterior zygapophysis processes (e.g., Ferrusquía-Villafranca and Comas, 1988; Krebs, 1962). Andrews (1913, p. 161) regarded that the dorsal vertebrae are those in which the parapophysis is wholly or partly borne on the neural arch. Following this criterion, the vertebrae described here is considered dorsal because its paraphophyses are on the base of the neural arches.

The Thalattosuchia are grouped in two monophyletic families, Teleosauridae and Metriorhynchidae (e.g., Mueller-Töwe, 2005). Because of the few differences between the dorsal vertebrae of both families (e.g., Buffetaut and Thierry, 1977, p. 169) and the incompleteness of the vertebrae preserved in IGM 9324, it is not possible to have a more accurate taxonomical determination of this specimen.

Ferrusquía-Villafranca and Comas (1988) described the first thalattosuchian remains found in Mexico, some isolated vertebrae from alleged Neocomian marine strata uncovered in the Papalutla region, Oaxaca. Later, Tithonian metriorhynchids from Puebla and Coahuila, Mexico were described as Geosaurus vignaudi by Frey et al. (2002) and G. saltillense by Buchy et al. (2006a). Young and Andrade (2009) and Young et al. (2012) placed these species in the genus Cricosaurus Wagner, 1858 , as C. vignaudi and C. saltillense , respectively. More recently, Buchy et al. (2013) amended the name of the latter, renaming it Cricosaurus saltillensis . Additionally, Buchy et al. (2006b) noted the occurrence of Kimmeridgian and Tithonian teleosaurids and metrorhynchids in localities of Coahuila and Nuevo León, northeastern Mexico. Although the revision of the holotype of Plesiosaurus mexicanus suggests the presence of a metriorhynchid ( Buchy, 2008) or else, of Cricosaurus saltillensis ( Buchy et al., 2013) ; additional and comparable fossils from the Tlaxiaco Basin are required to reach a better taxonomical conclusion. Finally, Buchy (2008) reported the finding of skull remains from the Kimmeridgian strata of the La Casita Formation in Nuevo León, Mexico, which she conferred to the genus Dakosaurus Quenstedt, 1856 . In the present context, IGM 9324 is the southernmost thalattosuchian fossil known in North America. The possible accurate taxonomical identification of this specimen will be possible after the preparation of other numerous crocodylomorph fossil remains already collected in Yosobé.

Figures

FIGURE 4. Geologic map and lithostratigraphical column of Yosobé. M, Marl stratum; L1-L8, limestone strata; the intermediate strata (L1-L2, L2-L3, ... L7-L8) are the bituminous shales layers that carry the vertebrate assemblage of Yosobé.

FIGURE 10. Vertebrates of marine reptiles from Yosobé. 1-3, Specimen IGM 9324, undetermined thalattosuchian crocodyliform in anterior (1), dorsal (2), and lateral (3) views. 4-6, Specimen IGM 9325, undetermined ophtalmosaurid ichthyosaur in anterior (4), dorsal (5), and ventral (6) views. In both specimens the arrows show the anterior direction. Abbreviations: dp, diapophysial process; fna, facet for the neural arch; ias, intervertebral articular surfaces; ncf, neural canal floor; ncs, neurocentral suture; nf, nutritive foramen; ns, neural spine; pz, posterior zygapophysis.