Late Cenozoic mammals from central Mexico

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Abstract
Historically, the knowledge of late Cenozoic mammals from central Mexico has been slow in developing. Most early studies did not include stratigraphic controls or meaningful correlations. At present the geology is incompletely known at most fossil-yielding localities. However, research in the past several years, especially in the State of Guanajuato, has provided much needed stratigraphic control relevant to collected fossils. Of the eleven faunas identified in this paper, only the Cedazo (Pleistocene), Las Tunas, Rancho El Ocote, Coecillo, and Rancho Viejo (Pliocene) have this control. These faunas, then, with several taxa reported here for the first time, are considered most important. They provide significant information about changing climates, a trend from humid, warm conditions in the Pliocene to drier and cooler conditions in the Pleistocene, and mammalian dispersals. Most fossils can be derived from North American ancestors; however, South American constituents can be identified beginning in Hemphillian time with the occurrence of sloths, Megalonyx and Glossotherium (Rancho El Ocote). By the late Pleistocene, ten South American genera occur in central Mexico.