|The entelodont Daeodon scavenging the corpse of the chalicothere Moropus|
Agate Fossil Beds National Monument
Formation: Harrison & Anderson Ranch Formations
Age: Miocene (23-19 Ma)
While global temperature rebounded somewhat at the end of the Oligocene and at the beginning of the following Miocene Epoch, the global jungles and forests of the Eocene were gone. In place of the trees that had blanketed landscapes from Scandinavia to the Australia, grass was beginning to take over. Grasslands were a novelty at the beginning of the Eocene, increasingly common by the end of the Miocene, and are among the most widespread biomes on Earth today. They are affected by climate, tending to grow in cooler, drier environments than forests, but they also play a major role in shaping climate as well as the organisms that occupy them. One of the best places to observe these effects is in a place still known for its plains today: the panhandle of Nebraska. Erosion by the Niobrara River has exposed a number of mammal fossil-bearing localities in the area, including some konzentrat-lagerstätten in the form of spectacularly productive bone beds. Animals found here include relatives of familiar plains-dwellers such as horses, camels, and rhinos, as well as extinct taxa such as pig-like (but probably predatory) entelodonts, bear-dogs, and, strangest of all, chalicotheres (which I can only describe as horse-rhino-panda hybrids). Many of these animals show what clearly seem to be adaptations to living in grasslands: long legs reflecting greater running ability, larger size, and defensive structures, all necessitated by the absence of trees in which to hide or from which to ambush prey. Even among small mammals, adaptations to this new ecosystem are apparent, most notably in the beaver Paleocastor which, millions of years before its relatives became aquatic, was tunneling beneath the Nebraska plains, leaving behind the burrows known to early fossil hunters as "Devil's Corkscrews." Other characteristics of Agate mammals may or may not reflect the pressures of grassland life. The higher crowned teeth that were beginning to evolve in taxa such as camels and horses were long explained as adaptations to a diet of abrasive grass, but many recent studies have suggested that the connection between tooth height and grass abundance is not as close as once thought. The bone beds themselves were long interpreted as evidence of herd behavior and living in large groups would certainly be useful in an open landscape, as it provides safety in numbers for herbivores and allows pack hunting in carnivores. In some cases, the bone beds may indeed represent the death of several members of a herd around a watering hole during a drought, but it's also possible that the bones accumulated over a long period of time or were concentrated in particular locations by rivers.
Visit: Agate Fossil Beds National Monument, south of Harrison, preserves the most important historic bone beds as well as several Paleocastor burrows.
Fossils: The national monument visitor center has some nice fossil displays, but much of the best Agate material can be seen at the American Museum of Natural History, the Carnegie Museum, and the University of Nebraska State Museum, though there's hardly a major museum in the country without a specimen or two from the area.
Is there a relevant book full of gorgeous fossil photos that I can gift to a paleontologically-minded friend?: The Park Service publishes a nice, if somewhat dated, overview of the science and history of Agate.
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.