The dramatic size changes recorded just before extinction were most unexpected (Fig. Explanations for these apparent size changes include the possibility that my sampling methods may somehow have affected this plot and artificially created a drop in apparent metacarpal size. The apparent high metacarpal variability during the LGM (Fig.
What, on the other hand, do we know about environmental changes in Alaska at the time of these extinctions?
Broadly speaking, the last glacial was a time in which the cold/arid northern Mammoth Steppe was most extreme, though still capable of supporting a rich diversity of large mammals.
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. In contrast, fossils of the other equid Alaskan species have unusually long gracile metapodials, making them similar to the living Equus hemionus from central Asia, and also similar to fossil New World hemione-like forms.
It is possible that the disappearance of this small-bodied hemionid species somehow affected selection pressures on later Alaskan caballoid horses, promoting reductions in body size and increased numbers of the latter during the LGM, around 18–20 kyr , but at present this seems unlikely given the implied time lag between the hemionid extinction and caballoid response.
Compared with the hemionids, caballoid horse fossils from Alaska are much more numerous and much better studied.The graph of size/time does not show two overlapping clusters, as one would expect with species replacement.Instead, smaller variants continue to emerge on the graph over time, and larger variants continue to fall out.This would necessitate multiple species replacements. In contrast, the explanation of a simple body-size decline in one widespread population seems much more parsimonious.Comparable size declines at the end of the Pleistocene are not unique to horses but at a later date.Note that this new array of Alaskan dates shows a gap of 1,000 radiocarbon years (close to 1,300 calendar years) between the last dated horse and mammoth fossils (Fig. There is accumulating archaeological evidence of human presence in Alaska beginning around 12 kyr Plot covers the past 24,000 years (dates summed within 1,000-year intervals).