Late Cretaceous mammal horizons from the San Juan Basin, New Mexico. American Museum novitates ; no. 2845

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New York, N.Y. : American Museum of Natural History
"Small mammals from Upper Cretaceous rocks exposed in Hunter Wash and higher in Ojo Alamo and Barrel Spring Arroyos, and near Burnham Trading Post in the San Juan Basin are described and compared to Late Cretaceous mammals from Wyoming, Montana, and Alberta. Superposed assemblages from the Hunter Wash-Ojo Alamo area cluster around two levels that are separated by about 200 m of strata and at least 1 million years of time. Small samples from the younger localities are comparable to Lance and Hell Creek mammals of the latest Cretaceous (late Maastrichtian). The older localities yield richer assemblages that display affinity with faunas from the Judith River, Oldman, and Milk River formations, generally thought to be Campanian in age. Present paleomagnetic reversal correlations suggest a middle to late Maastrichtian age for the entire terrestrial sequence in the San Juan Basin. This correlation agrees with the approximate ages of Lance and Hell Creek faunas, but is about 6 million years younger than ages presently estimated for known Judith River and Oldman assemblages. Accepting the hypothesized paleomagnetic age for the San Juan Basin terrestrial rocks, this suggests that the Judithian land mammal 'age' defined by Lillegraven and McKenna (1986) persisted through most of the Maastrichtian and was replaced by Lancian-like assemblages after 68 million years ago. Further, the fact that the older Hunter Wash assemblage includes elements occurring in both the Milk River and the Judith River formations may suggest that differences between Milk River and Judith River are mainly ecological. Alternatively, Milk River and Judith River elements may have survived anomalously late in the San Juan Basin as a consequence of faunal provincialism. This view suggests that the San Juan Basin was part of a biogeographic province separate from northern faunas. As a third alternative, if the paleomagnetic correlation for the San Juan Basin is rejected, the age of the Hunter Wash assemblage can be estimated on biochronological arguments as intermediate between the Milk River and Judith River faunas; i.e., near the Campanian/Maastrichtian boundary. Of course, further paleontological and chronological work throughout North America will help to clarify this situation by strengthening the biostratigraphic basis for the definition of mammalian biochrons"--P. 2.
30 p. : ill.. map ; 26 cm.
Includes bibliographical references (p. 28-30).