Late Cenozoic mammalian remains have been collected from New Mexico for more than a century. The most important nineteenth-century collections came from the Española Basin. Work in this century has continued to focus mainly on the Miocene faunas in that basin and the northern part of the adjoining Albuquerque basin. The lithostratigraphy of these basins has recently been reviewed and revised, but the description of the large collections of fossil remains from the basinal deposits is largely incomplete. Pliocene and early Pleistocene faunas have been found in deposits that mark the development of external drainage of the major rivers of the state. Study of these assemblages has been cursory, although the lithostratigraphy of these deposits has received considerable attention in recent years.

The Miocene basinal deposits accumulated under conditions of internal drainage in local structural environments that led to the development of partly synchronous depositional records. Biostratigraphies worked out in each basin overlap sufficiently so that the composite succession forms an important biochronological standard for southwestern North America.

The Española and Albuquerque-Belen basins contain the only Miocene faunas of importance presently known in New Mexico. Their fossil records complement one another in that the northern Albuquerque basin has the only adequate representation of the early Miocene and the Española basin a more complete record of the medial and late Miocene. Synchronous faunas in these basins show a high degree of taxonomic resemblance extending to elements that represent endemic taxa or forms rare elsewhere. Compared with coevel assemblages in the Great Plains, there is considerable generic and specific similarity. This allows easy reference of the New Mexican faunal succession to the sequence of North American mammal ages that are essentially typified by the Great Plains biochronology. In addition to these taxonomic relationships, the New Mexican faunas retain throughout the Miocene characters that may typify a southwestern faunal province. The general features of the latter would include diverse and abundant artiodactyls, especially Camelidae, including a late occurrence of endemic stenomyline genera, and less diverse and abundant perissodactyls, especially horses, when compared with the Great Plains.

The fossil vertebrate record of Pliocene and early Pleistocene time in New Mexico is largely confined to the basins of the Rio Grande and the Gila River and is derived from deposits of the axial rivers that were the percursors of the modern streams and from the piedmont slope facies of the basin margins. Sites scattered along the length of the Rio Grande rift from the Santo Domingo basin near Santa Fe south to the international border have yielded Blancan and early Irvington faunas. These assemblages can be arranged in temporal order from their position in local stratigraphic columns, from their biological affinities, and with respect to dated basalts and ashes interbedded in the sediments containing the fossil remains. Important reference sequences are available in adjacent Arizona and Texas, where mammalian faunas and calibrated biochronologies compare closely with those in New Mexico. The New Mexican record supports the southwestern biochronology for Pliocene and early Pleistocene time, including the following events and their calibration: the last appearance of hipparionine horses (Nannippus) and the first appearance of the South American immigrants, the glyptodont Glyptotherium and probably the mylodontid sloth genus Glossotherium, were nearly synchronous events close to the Gauss-Matuyama boundary (2.5 m.y. B.P.); at least three species Equus were present in the Southwest by the time of the Nannippus extinction; the first appearance of the Asiatic immigrant Mammuthus in the southwest between 1.2 and 1.4 m.y., postdating the Olduvai Event and, by definition, indicating that the Blancan-Irvingtonian boundary, at least as determined for the Southwest, lies close to the beginning of medial Pleistocene time.

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