A new chelid turtle (Testudines, Pleurodira) from the Cretaceous of Patagonia, Argentina

A new species of chelid turtle (Calvarichelys coloniensis gen. et sp. nov.) from the Upper Cretaceous La Colonia Formation (Chubut Province, Argentina) is described here. This species is represented by an almost complete skull and a complete lower jaw. Detailed description and comparison of the species enables differentiation of this species from the rest of stem and crown group Chelidae. In particular, this new chelid is characterized by an almost non-existent temporal emargination with a lateral emargination that reaches the parietals but leaves an extended skull roof with almost parallel margins. Also, this new species is similar to some Australasian (e.g. Emydura macquarii, Elseya dentata) and South American (Platemys platycephala) short-necked chelids and some extinct chelids (e.g. Mendozachelys wichmanni) in the presence of a fused and hooked mandibular symphysis. Three types of total evidence analysis (implied and equal weights using maximum parsimony, and Bayesian inference) were carried out to evaluate the phylogenetic relationships of the new species of turtle presented here. In all analyses, C. coloniensis is recovered inside a clade with other Cretaceous South American species such as Mendozachelys wichmanni, Palaeophrynops patagonicus, Lomalatachelys neuquina and Bonapartemys bajobarrealis. However, in the cladistic analyses the mentioned clade is recovered inside crown-Chelidae as the sister group of the short-necked Australasian chelids, while in the Bayesian inference it is located outside crown-Chelidae, being the sister group of Prochelidella spp. + crown Chelidae. These differences highlight the impact of the methodology used (e.g. parsimony, Bayesian inference) on the results.