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Rapid body size decline in Alaskan Pleistocene horses before extinction


About 70% of North American large mammal species were lost at the end of the Pleistocene epoch1. The causes of this extinction—the role of humans versus that of climate—have been the focus of much controversy1,2,3,4,5,6. Horses have figured centrally in that debate, because equid species dominated North American late Pleistocene faunas in terms of abundance, geographical distribution, and species variety, yet none survived into the Holocene epoch. The timing of these equid regional extinctions and accompanying evolutionary changes are poorly known. In an attempt to document better the decline and demise of two Alaskan Pleistocene equids, I selected a large number of fossils from the latest Pleistocene for radiocarbon dating. Here I show that horses underwent a rapid decline in body size before extinction, and I propose that the size decline and subsequent regional extinction at 12,500 radiocarbon years before present are best attributed to a coincident climatic/vegetational shift. The present data do not support human overkill1 and several other proposed extinction causes2,3, and also show that large mammal species responded somewhat individualistically to climate changes4,5,6 at the end of the Pleistocene.

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Figure 1: Length of Alaskan Pleistocene horse metacarpals plotted against AMS radiocarbon dates, with polynomial trendline.
Figure 2: Smoothed plot of new Alaskan dates for horses (n = 73) and mammoths (n = 72) in radiocarbon years.
Figure 3: Smoothed plot of numbers of Alaskan horse metacarpals available in collections against their lengths summed in 5-mm increments.


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I am grateful to R. Tedford for sampling access to these Alaskan equid and mammoth fossils. Similar thanks go to R. Gangloff and R. Harington for dating access to mammoth fossils from Alaska and the Yukon Territory. M. L. Guthrie, A. Lister and P. Matheus made suggestions that improved the manuscript. AMS dates were done at the AMS Laboratory, University of Arizona. This work was supported by the National Science Foundation.

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Correspondence to R. Dale Guthrie.

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Dale Guthrie, R. Rapid body size decline in Alaskan Pleistocene horses before extinction. Nature 426, 169–171 (2003).

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