New Mexico Geological Society Annual Spring Meeting — Abstracts

A partial shell of the well-known Upper Cretaceous turtle Adocus was collected in the Crevasse Canyon Formation, east of the Caballo Mountains in Sierra County, New Mexico. It was found stratigraphically low in the Crevasse Canyon Formation, so its age is close to the Coniacian-Santonian boundary (approximately 86 Ma). This is the southernmost known occurrence of an Adocus in North America and may be the oldest North American record of the genus. A fragmentary specimen was previously reported from Santonian strata in Utah and tentatively assigned to the genus based on surface sculpture.

The specimen from the Crevasse Canyon Formation is the anterior half of the plastron as well as most of the free edge of the anterior half of the carapace. This includes a partial nuchal bone missing a portion of its left half. In addition to this, are many isolated shell fragments of indeterminate placement. The Crevasse Canyon specimen has several features that justify assigning it to the genus Adocus: (1) the characteristic surface sculpture of pits oriented in lines across the shell; and (2) a mosaic of characteristics seen in Campanian A. bossi and A. kirtlandicus. These include the structure of the bridge, which is similar to that observed in A. bossi from the upper Campanian Kirtland Formation of northwestern New Mexico. The cervical scute is trapezoidal, narrowing posteriorly, as in A. bossi. The bridge of this specimen displays a longitudinal sulcus, as in A. kirtlandicus. Unlike A. bossi, the free edge of the carapace of the Crevasse Canyon Adocus ends in a thick, rounded shape rather than coming to a pointed edge. The gular scute meets the humeral-pectoral sulcus just ahead of the posterior end of the gular scutes. We conclude that the Crevasse Canyon specimen is likely a new species of Adocus, possibly ancestral to the Campanian-age Adocus of northwestern New Mexico.