New Mexico Geological Society Annual Spring Meeting — Abstracts

Turtles of the genus Echmatemys have long been ignored in phylogenetic analysis, so we analyze the phylogenetic placement of seven species of these turtles. We find that the genus is diphyletic. The species from the lower Eocene San Jose Formation in the San Juan Basin, New Mexico, are stem Testudinidae rather than geoemydids as previously hypothesized. Furthermore, Hadrianus majusculus is actually the most primitive tortoise known, lacking common, more derived tortoise traits such as costal wedging. Given the primitive state in North American tortoises, we suggest that Testudinidae (tortoises) originated in southern North America from one of the geoemydid-like forms lumped in the genus Echmatemys, which first appear in the earliest Wasatchian. This is contrary to the conventional wisdom that tortoises originated in Asia, where their most basal member (based on genetic studies) lives today. We suggest an alternative interpretation that tortoises arose in North America and subsequently emigrated to Asia and Europe during the second thermal maximum in the later part of the Wasatchian land-mammal “age.” This warm period slightly preceding the deposition of the Bridgerian-equivalent units that yield the oldest tortoises in Europe and Asia. From Europe, immigration to Africa and from Africa to South America would follow in the Oligocene and later. Reports of unpublished Paleocene material in Asia may indicate that tortoises originated in Asia and then, together with some members of their stem lineage, immigrated to North America and then onward to Europe. This explains members of its stem-lineage co-occurring with tortoises in the San Jose Formation. In short, our understanding of tortoise origins is still limited but is improving as older material is being reanalyzed in the light of more recent discoveries.