Re-evaluation of the taxonomy of Compsemys: an enigmatic turtle from around the Cretaceous–Paleogene boundary

Compsemys is an enigmatic turtle first described over 150 years ago. Its taxonomic placement has been in flux, with many of its referred species being removed. For the last three decades or so, Compsemys victa has been the only accepted North American species. New fossil specimens collected from the Paleocene of North America suggest that a re-evaluation of its taxonomy is in order, calling into question the long-held belief that only one species of Compsemys is valid in North America. Paleocene specimens are far more complete than the holotype of C. victa, but have all been referred to this species based on similar surface sculpturing. Differences between Cretaceous and Paleocene fossil material suggest multiple species of Compsemys are valid. In addition to the genotypic species C. victa, two Paleocene species (Compsemys puercensis and C. torrejonensis) are valid based on morphological differences of the carapace and plastron, particularly aspects of scute sulci (such as those of the humerals, pectorals, femorals, and vertebrals), the neurals, and some aspects of the shapes of the plastral lobes. North American Compsemys survived the Cretaceous–Paleogene mass extinction. Rather than being a monospecific genus, this taxonomic re-evaluation of Compsemys shows how it became more diverse during the early Paleocene period, at least in the American southwest, perhaps exploiting the loss of numerous other taxa during the K-Pg mass extinction event. Not only does this re-interpretation of Compsemys' taxonomy suggest an increase in diversity of North American compsemydids, but it also leads to the possibility of anagenesis within the genus during the Late Cretaceous and Paleocene periods, at least in the American southwest.