15 December 2015

15. Ashfall

The Ashfall Bone Bed
Location: Nebraska, USA
Formation: Ash Hollow Formation
Age: Miocene (12 Ma)

We've seen several lagerstätten that preserve gorgeous fossils over the course of the month, but most of these are from sites in which remains of organisms accumulated over the course of long periods of time.  Moments of time are only rarely preserved in the fossil record, and Ashfall is among the very few sites to do so.  Twelve million years ago, the Great Plains had begun to take on their modern appearance, while to the west, what would become the Boise area sat atop the volcano that today fuels the geysers and hot springs of Yellowstone.  Eruptions of this titanic volcano produced immense clouds of ash, one of which settled onto a watering hole in northeast Nebraska, starving and suffocating the local wildlife and creating Ashfall.  Small animals died first, probably within hours, as shown by their abundance at the bottom of the ash layer.  Larger animals took up to a few weeks to die, and are consequently more common at the top of the bone bed.  The resulting deposit is not only full of complete skeletons, even of such fragile animals as birds and amphibians, but provides a snapshot of life on the nascent Great Plains that provides endless opportunities for research.  Interested in evolution and anatomy?  Nowhere will you find as many complete and articulated skeletons of everything from camels to snakes as at Ashfall.  Interested in the behavior of extinct mammals?  A whole herd of rhinos is entombed, providing even better evidence than the Agate bone beds of the evolution of herding behavior in plains animals.  Development?  Some taxa, particularly the horse Pseudhipparion, are represented by everything from newborns to full-grown adults.  Paleoecology?  An entire ecosystem preserved in the blink of a geological eye is present here.  Paleopathology?  Larger animals show signs of abnormal bone growth associated with high rates of volcanic particulate in the atmosphere.  Taphonomy?  Not only was the speed of burial unusually fast, but preservation in a layer of volcanic ash is also rare in the fossil record.  In short, Ashfall is more than simply a treasure trove of fossils: more than any other lagerstätte featured this month, it is a frozen moment in time and a font of information about the evolution of life on the Great Plains.

Visit: The "Rhino Barn" at Ashfall Fossil Beds State Historical Park should be at or near the top of everyone's paleontology life list.  It's a bit out of the way from pretty much anywhere, but well worth the detour.
Fossils: Most of the really spectacular fossils from Ashfall remain in situ, but the University of Nebraska State Museum in Lincoln houses and displays specimens excavated from the site.
Is there a relevant book full of gorgeous fossil photos that I can gift to a paleontologically-minded friend?: There is, and it's hot off the presses!  Information about it is available in the link to the State Historical Park above.

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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