Anatomy and Relationships of Palaeochersis talampayensis, a Late Triassic Turtle from Argentina

Abstract

A complete cranial and postcranial anatomy and a phylogenetic analysis of Palaeochersis talampayensis, the oldest turtle from South America, is presented here. Palaeochersis talampayensis was found in Los Colorados Formation (Norian-Rhaetian) of La Rioja Province, Argentina. The Late Triassic turtle record is scarce worldwide, being represented by Proganochelys quenstedti and Proterochersis robusta from Germany, Proganochelys ruchae from Thailand and some fragments described as Proganocelyidae indet. from Greenland. For this reason the anatomical description of Palaeochersis talampayensis gives new information about the anatomy of the first representatives of Testudinata. The cranial description was based mainly in Palaeochersis holotype (PULR 68), but an additional specimen PULR 72 brought complementary information to skull description. The skull anatomy of Palaeochersis was compared with Proganochelys, Australochelys, some panpleurodiran and pancrytodiran turtles. Palaeochersis talampayensis has primitive character states in the skull like the presence of lacrimal and supratemporal bones, the presence of a quadrate pocket, a foramen trigemini partially enclosed, middle ear limits partially developed, presence of an interpterygoid vacuity, presence of a cultriform process and a high dorsum sellae. However, Palaeochersis talampayensis also has derived character states like the absence of vomerine teeth, basipterygoid articulation sutured, processus paraoccipitalis tightly articulated to quadrate and squamosal, and cranioquadrate space partially enclosed in a canal. The postcranial description was based on Palaeochersis holotype (PULR 68) and on specimen (PULR 69) represented by an almost complete skeleton and a hindfoot, respectively. The phylogenetic hypothesis proposed in this paper, based in the examination of ninety cranial and postcranial characters scored for fifteen taxa, suggests that Palaeochersis together with Australochelys africanus (from the Lower Jurassic of South Africa) form a monophyletic assemblage (family Australochelyidae). This family remains as the sister group of the Panpleurodira plus Pancryptodira and it is characterized by the presence of a wide transverse occipital plane with depressions for the nuchal musculature and by a roofing of the posterodorsal margin of the temporal fossa by an overhanging process of the skull roof. As a result of this analysis, the characters related with the pelvic girdle (e.g. morphology of the pelvis and relationship between the pelvis and the shell) are homoplastic, appearing twice during turtle evolution, one time in Palaeochersis (and might be in Australochelys) and other time in Panpleurodira.