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The alpha taxonomy and paleogeography of testudinid and geoemydid turtles of the early Eocene (Wasatchian-Bridgerian) of western North America have received little attention since their original description. We reassess the alpha taxonomy of six species of the genera Echmatemys and Hadrianus. Additionally, we analyze new material, including juvenile specimens of Hadrianus corsoni, H. majusculus, E. haydeni and E. naomi. Proceeding taxonomic assessment we also examined a large sample of extant Gopherus agassizii to help us understand what characters were and were not consistent between individuals. This led us to use different characters than other recent assessments of fossil testudinoid morphology. A phylogenetic analysis reveals that the four nominal species of Echmatemys examined form a polyphyletic group, with E. lativertabralis as the outlier. In examining the global stratigraphic distribution of early Testudinidae we conclude that the oldest and most basal tortoises are from the western United States. From this we conclude that the Testudinidae likely evolved in North America from one of the geoemydid-like forms lumped in the genus Echmatemys, which have their lowest stratigraphic occurrence in the earliest Wasatchian North American land-mammal “age” (early Eocene, Ypresian). From here we suggest a bi-directional migration of testudinids to Europe and Asia took place. The Asian migration gave rise to the extant Manouria lineage and the European migration spawned all extant tortoises more derived than Manouria and Gopherus. |
