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.
