A new large tortoise from the early Oligocene (Arikareean NALMA) of Oaxaca, southern Mexico and its phylogenetic position within Pan-Testudinidae

Pan-Testudinidae is the total clade of extant terrestrial tortoises, which includes extinct fossil members of their stem lineage. Members of this clade have a rather scarce fossil record in Mexico, and the few specimens known in scientific collections are poorly studied. Here, we described a new species of basal testudinid turtle, based on a single specimen from the early Oligocene deposits exposed in the marginal facies of the Chilapa Formation in Oaxaca, southern Mexico. The new taxon exhibits osteological characteristics that support its insertion as a basal Testudinidae. The phylogenetic relationships of the new turtle were assessed using a total evidence approach (morphological + molecular) in a global Pan-Testudinidae context using Implied Weighted Maximum Parsimony (IWMP), Standard Maximum Parsimony (SMP) and Bayesian Inference (BI). Although the BI consensus tree is not well resolved, the results obtained by IWMP and SMP retrieved its branching close to the root of Testudinidae. The differences between the topologies of the three phylogenetic analyses show that the position of several taxa within Testudinidae is affected by the phylogenetic analyses performed. The new taxon from Oaxaca, here reported, represents the first Palaeogene and the southernmost tortoise described from Mexico and the oldest Testudinidae known in the country.