New Mexico Geological Society Annual Spring Meeting — Abstracts

A recently named species of the baenid turtle Neurankylus from the Torrejonian interval of the Paleocene Nacimiento Formation of New Mexico shows some significant new morphological features. The skull exhibits larger exoccipitals than are seen in either N. eximius or N. lithographicus. In addition, the vomer is narrow and elongate, similar to Trinitichelys, one of the oldest known baenids. It is also worth mentioning that the jaw structure of this turtle is similar to that of Baena arenosa, with a near identical ramii angle and a similar position of the posterior end of the triturating surfaces. Further, in the skull, the jugal takes up a similarly wide portion of the posterior rim of the orbit. The broad, triangular shape of the jugal is similar to that in more basal eubaenids, supporting the basal placement among baenid turtles of the recently named Neurankylus species. An associated distal phalanx has a prominent ventral process for muscle attachment. It is similar to that seen in Terrapene, Chelydra, Compsemys, and Hoplochelys. We suggest this relates to a bottom walking mode of locomotion, as this process is absent in swimming turtles such as Trachemys. Indeed, these muscles would be useful for holding the digits in place, rather than their extension, which would be useful for active swimming. This, as well as the resemblance of the jaw to that of modern chelydrid turtles, suggest a similar habitus as a river-dwelling predator. Additionally, a second posterior lobe of the plastron is included in the original collection, which appears to scale directly from that of the holotype, indicating the presence of a second, similarly-sized individual of the species. The vertebrae are similar to those of N. baueri from the older, Upper Cretaceous Fruitland Formation. These similar vertebrae, as well as similarities in the plastral sulci, lead us to suggest a possible anagenetic descent of Neurankylus n. sp. from N. baueri in northwestern New Mexico.