SUDDEN VS. DRAWN-OUT PLEISTOCENE EXTINCTION: PRELIMINARY RESULTS FROM THE AMERICAN SOUTHWEST

This study is a review of previously published data by others, focusing on Pleistocene vertebrates and their last appearance dates (LADs) in the American Southwest. The Pleistocene extinction in North America is one of the more hotly debated subjects in archaeology and paleontology. Hypotheses for this event range from overkill to climate change to other fringe hypotheses (comet impact, disease, solar radiation, etc.). Many vertebrates went extinct during the Pleistocene Epoch (2.6-0.011 Ma). Extinct megafauna (mammoths, bison, camels, horses, tapirs, etc.) are present at archaeological and paleontological sites across the Southwest through the Pleistocene. Few megafauna (e.g., mammoths and bison) coexisted or were associated with humans. Small extinct mammals (rabbits, rodents, shrews, etc.) and various birds, reptiles, amphibians, and fish are also present in the Pleistocene fossil record in the Southwest. Only a few small taxa (e.g., Aztlan rabbit and Stock’s vampire bat) lived before or during human presence, with limited direct associations.

Based on LADs, most small mammals (especially rodents and rabbits) and fish disappeared from the Southwest during the Late Blancan LMA (before 1.8 Ma) and Irvingtonian LMA (1.8-0.26 Ma). Many large mammals lived through the Rancholabrean LMA (260-11 ka), but few coexisted with humans (e.g., Clovis hunters at ~13.4 ka, based on calibrated radiocarbon dates). LADs of birds, amphibians, and reptiles (with few from the latter classes) are later in the Pleistocene, regardless of size. Most LADs are from sites in or near southern deserts (Sonoran and Chihuahuan Deserts) as opposed to northern deserts (Colorado Plateau, southern Great Basin, and Mojave Desert). Conditions in southern deserts might have been favorable to more taxa than in other areas in the Late Pleistocene. Overall, the Pleistocene extinction in the Southwest is more temporally dispersed than expected for a sudden event. These patterns could be from preservation or recovery biases. More detailed research on extinct taxa (as well as extant taxa no longer in the Southwest) is recommended.