Background. Condorchelys antiqua is a stem turtle previously described from the Queso Rallado Locality, Cañadón Asfalto Formation (Early-Middle Jurassic), Chubut, Argentina. In 2014 a skull (MPEF-PV 10900) and an articulated shell with associated postcranial remains (MPEF-PV 10884) of C. antiqua were discovered in the mentioned locality.
Methods. The new specimens have been prepared by L. Reiner (technician at the Museo Egidio Feruglio) and then studied using the regular paleontological methodology (e.g., direct observations and under the microscope, photography, and drawings). The new data provided by these two specimens, plus some other isolated remains that were re-prepared, allowed us to score a 25% more of the characters of C. antiqua in the most complete up to date matrix for Mesozoic turtles. The matrix was build in Mesquite and analyzed under maximum parsimony in TNT. We performed two rounds of Tree Bisection Reconnection and calculated a strict consensus. Branch supports were provided using bootstrap, jackknife, and Bremer support.
Results. We regard the specimen MPEF-PV 10900 as represented by the basicranium and some previously unknown skull bones like the maxilla, frontal, parietal, and postorbital. This specimen also shows that there were no teeth in the pterygoid of C. antiqua. On the other hand, specimen MPEF-PV 10884 is an almost complete carapace and plastron with 4 thoracic vertebrae and ribs, scapular and pelvic girdles, humerus, left radius, femur, tibia, fibula, 2 metatarsal/carpals, 2 phalanges, 1 ungual phalanx and 1 chevron bone. The preliminary cladistic analysis resulted in 750 most parsimonious trees of 893 steps. The result of the analysis corroborates the position of C. antiqua as a stem turtle more derived than the Triassic and Australochelys africanus. Condorchelys antiqua is recovered in a polytomy together with other early-middle Jurassic turtles (e.g., Kayentachelys aprix and Indochelys spatulata) and the clade formed by the remaining turtles.
Discussion. Although many new characters have been scored for C. antiqua in the phylogenetic analysis, this new information did not resolved the phylogenetic position for this taxon. We consider that more information about I. spatulata and K. aprix could help to better understand the relationships among these basal taxa. On the other hand, these new specimens of C. antiqua bring valuable information to decipher the anatomy of the poorly known non-shell postcranial elements of these Early to Middle Jurassic stem turtles. In light of the recently suggested aquatic preferences of C. antiqua, the next step on this research would be to explore other adaptations to the aquatic environment using postcranial elements, particularly the limb bones.