22 December 2015

22. The Mammoth Site

The Mammoth Site
Location: South Dakota, USA
Age: Pleistocene (26,000 years ago)

The Waco Mammoth Site, profiled a couple of days ago, is a moment (or, to be precise, three moments) frozen in time.  Catastrophic events that entomb numerous animals simultaneously, as happened at Ashfall as well as at Waco, are one way to get a konzentrat lagerstätte.  Rancho La Brea illustrates the other way to generate a bone bed: have some kind of natural trap in which specimens build up over time.  This method is also nicely illustrated by a second mammoth-rich locality a few states north of Waco.  On the southern edge of the Black Hills, a sinkhole opened up roughly 26,000 years ago.  A lake formed in this sinkhole that served as a home to invertebrates and fish and as a pitfall trap to terrestrial organisms.  While these range from rodents to bears (including some of the best remains of the giant short-faced bear Arctodus ever found outside of Rancho La Brea), the majority of the animals preserved here are mammoths.  Delightfully to those of us who enjoy exposing the "everything's bigger in Texas" mantra for the lie it is, more than twice as many individuals have been uncovered from the South Dakota site, representing both Columbian and woolly mammoths.  As has been clearly established by the last couple of posts, Columbian mammoths are widespread across North America, but the presence of their cold-adapted relative in the Black Hills illustrates an important trend in Pleistocene climate.  While the "global hothouse" of the Eocene is a bit of an oversimplification (see the earlier posts on the Okanogan and Florissant), it is true that the difference in climate between the poles and the equator was smaller early in the Cenozoic than it is today.  However, the presence of a species usually associated with tundra and other cold environments - and its absence from a site less than a thousand miles to the south - demonstrates that the strong latitudinal gradients associated with temperature today were solidly in place by the Pleistocene (though with the caveat that, because it sits at a relatively high altitude, the Mammoth Site is not a perfect analog for lower-lying localities at the same latitude).  This increase in the difference in climate between more polar and more equatorial ecosystems may seem somewhat trivial, but it's long been suggested that this difference plays a major role in driving several ecological and evolutionary trends.  Not only was climate in the Pleistocene abnormally cold and variable, then, but life on land was affected by huge climatic extremes even within the confines of a single continent.  Lagerstätte such as the Mammoth Site and Rancho La Brea show how North American organisms and ecosystems responded to this new climatic regime, but things looked quite different one continent to the south.

Visit: If you live in the US, at some point in your life you'll make the requisite trip to Mt. Rushmore.  Spend as little time as possible there and head south to the sites that really should be headlining your Black Hills itinerary: Custer State Park, Wind Cave, and Hot Springs, where the Mammoth Site has been developed into an excellent on-site museum.
Fossils: Most of the mammoths from the Mammoth Site remain in situ, and anything that's been excavated has remained on-site at the museum.
Is there a relevant book full of gorgeous fossil photos that I can gift to a paleontologically-minded friend?: There is, published by the museum in Hot Springs, whose web site seems to be temporarily down, meaning I can't link to it.

This post is part of my 2015 Paleontology Advent Calendar, a series of vignettes on lagerstätten - sites of exceptional fossil preservation - that document changes in climate and environments through the Cenozoic.  You can see the other posts here.

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