New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

New specimens, including a complete skull and limb material, from the Paleocene Nacimiento Formation, San Juan Basin, New Mexico, extend our knowledge of the morphology of the platysternid turtle Cardichelyon. This skull is very similar to that of extant Platysternon megacephalum though smaller relative to the size of the carapace. Similarities to Platysternon include: skull is about as long as wide with a pronounced hook, forward placed orbits and broad and thick parietals. The broad triturating surfaces may indicate a relatively durophagus diet, which, combined with the hook, leads to the inference that Cardichelyon consumed motile invertebrates. This skull adds to the evidence that a platysternid turtle distinct from emydids and other testudinoids existed in North America in the early Paleocene. This suggests that the split of tesugirids from emydids and platysternids must have happened during or before the early Paleocene, perhaps as far back as the Cretaceous. Combined with another new specimen that appears to be emydid like from the Late Cretaceous (Campanian) Fruitland Formation, we suggest that emydids and platysternids were present in North America relatively early compared to the testugurid clade, which is not known before the Eocene. We speculate that this may indicate that the emydid-platysternid from testugurid split was the result of a vicariant split of testudinoids between North America and Asia, with each group originating on the respective landmass.