Paleontological Study of Hemphillian (Late Miocene to Early Pliocene) Vertebrate Localities in Central Mexico

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Known late Hemphillian (Early Pliocene) terrestrial vertebrate sites are rare in Mexico, and apparently, unequivocal early Hemphillian fossils (Late Miocene) have not yet been reported from anywhere in the country (fossils of these ages are extremely rare in all Central America). Outside the area of the presently proposed study only three localities of late Hemphillian age have been reported: Tehuichila in the state of Hidalgo; La Goleta in the state of Michoacan (which probably contains more Blancan than Hemphillian age fossils); and Yepomera in the northern state of Chihuahua. The first two sites have not been well documented stratigraphically, although the third, and most productive site, has. It is, however, only about 150 miles south of the U.S.-Mexican border. Inside the presently proposed study area, which lies within the east- central part of the state of Guanajuato in central Mexico, are two reported Hemphillian vertebrate sites. One, El Ocote, has been discussed in several reports; the other, Coecillo, has not been well collected or studied. A third, Rinconada, is newly discovered. The last site contains very abundant, well preserved fossils which appear to represent early as well as late Hemphillian forms. Knowledge of early Hemphillian vertebrates from Central America would be important for several reasons. These include a better understanding of the earliest times of dispersals in the Late Cenozoic between North and South America, determination whether dispersals were episodic rather than continual, comparison of relatively poorly known Hemphillian faunas in Central America with well known ones in the United States, and valuable information concerning the origin and diversification of a number of mammalian genera and species which appeared suddenly in the United States' Great Plains area that probably evolved on the Mexican Plateau. These genera and species include such important groups as horses and cricetid rodents. Additionally, data gathered during the proposed research would provide valuable information on changing climatic and ecologic conditions during an important time interval. The major objective of this proposed research is to carefully collect fossils and data in a stratigraphic context at the Rinconada and Coecillo sites. Screen washing for fossils would be part of the operation. Fossils would then be prepared and studied in order to obtain knowledge relevant to the items stated above.