New material of Emydoidea hutchisoni from Nebraska shows the species lived in the North American Miocene from medial Barstovian (approximately 14.5–13 million BP) to Clarendonian (approximately 11.5–9 million BP) times. Previously unknown plastra, limb girdle elements, and limbs of E. hutchisoni have provided new characters that have led to a revised diagnosis of the species. Derived characters shared by E. hutchisoni and Emydoidea blandingii include an elongated nuchal bone, ovoid plastral shape, short and broad femoral scute overlap, very wide xiphiplastral shelf, xiphiplastral striations, and an angular coracoidal border. But E. hutchisoni is distinct from E. blandingii in being smaller and having an akinetic shell with a shorter nuchal area, and it probably was more aquatic. The plastral lobes of a complete E. hutchisoni plastron are firmly fused together by dovetail sutures and a second, less complete plastron, has dovetail sutures on the anterior surface of the hypoplastron. Moreover, the pectoral abdominal scute sulcus occurs well anterior to the hyo-hypoplastral suture and expanded plastral buttresses are present on both plastra, furthur demonstrating a completely akinetic shell in E. hutchisoni. Two Terrapene sp. plastra from the same late Barstovian horizon in Nebraska as E. hutchisoni, however, are as kinetic as recent species of the genus, thus making the often proposed “Emydoidea-Terrapene clade” questionable. But an Emydoidea sp. indet. hyoplastran from a temporally equivalent Miocene (late Barstovian site) in Nebraska has a weak hinge line, indicating that it may be an advanced form, ancestral to E. blandingii and that E. hutchisoni is primitive. This suggests the possibility that E. hutchisoni may be a sister taxon to Emys, Emydoidea, and Terrapene.
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