ORIGIN AND EVOLUTION OF THE NORTH AMERICAN GLYPTODONT, GLYPTOTHERIUM (XENARTHRA, CINGULATA), IN NORTH AMERICA

North American glyptodonts were tropical and sub-tropical members of the late Neogene fauna of Central America, Mexico, and southern United States. The earliest record of glyptodonts in North America is from sediments dated at 3.9 mya in central Mexico, indicating they arrived from South America during early stages of the Great American Biotic Interchange. Glyptodonts were widespread and diverse in South America, with dozens of genera that span at least 35 million years, but were limited to only one genus in North America. The South American genus Glyptodon appears to be the closest relative to glyptodonts in North America. Glyptotherium, the North American genus, differs from Glyptodon in dentition, cranial anatomy, thickness of carapacial osteoderms, sculpturing on the external surface of the osteoderms, overall architecture of the carapace, and anatomy of the caudal armor. Glyptotherium populations reached as far north as Oklahoma and northwestern New Mexico on the eastern margin of the Colorado Plateau, and eastward to the Atlantic seaboard of southeastern U.S. Five previously recognized species have been recently synonymized as an anagenetic pair of species: G. texanum (oldest) to G. cylindricum (youngest) with only minor change in morphology over at least four million years. This situation contrasts with the South American genus Glyptodon, with earliest occurrence dated at 1.07mya, and greater species diversity. Recently discovered specimens of adult individuals from central Mexico (Early Blancan Land Mammal Age) and the 111 Ranch fauna of southern Arizona (late Blancan Land Mammal Age) indicate that Glyptotherium was quite large early in its occupation of North America, nearly as large as Glyptodon, in contrast with previous interpretations. Associated faunas containing Glyptotherium include capybaras, another immigrant from South America, indicating close association with aquatic habitats.