Northwest Nebraska during Peak Mammal

A Paleocastor walks into a phone booth … (already heard this one?)

Take Nebraska Highway 29 from Mitchell north to Harrison and you will drive through miles and miles of rolling grasslands beneath skies that seem to go on forever. You might see occasional cattle, or pronghorn antelope. Make the same trip twenty million years ago and you would drive for miles and miles through rolling savannas beneath skies that seem to go on forever but … instead of the occasional animal you’d see multitudes of mammalian creatures—some huge, some kind of familiar, others quite strange. This was northwest Nebraska during Peak Mammal, an American Serengeti.

Answer to recent geo-challenge: Agate Fossil Beds National Monument 20 million years ago (early Miocene). Diorama by Jay Matternes; source & source.

Prehistoric mammals have been largely upstaged by the dinosaurs with their over-the-top charisma. This is naive, and a real shame. After all, it was mammals that underwent “one of the most spectacular evolutionary radiations ever documented” (Prothero 2006). With the extinction of non-avian dinosaurs (birds are dinosaurs) 65 million years ago, small mammals came out of the shadows and begin to diversify—a diversification that accelerated into an explosion. We know of only only eight species of placental mammals from 65 million years ago, mostly tiny shrew-like creatures. But just ten million years later, there were on the order of 100 groups of species (genera), including all major orders in existence today—“from shrews and rodents to giant whales and flying bats” (Prothero 2006).

The best record of these exciting evolutionary times is contained in 60-million-years-worth of sediments eroded from the Rocky Mountains and carried east to form the Great Plains. A good amount of wind-borne volcanic ash from Utah and Nevada ended up in the mix, and lots of mammalian carcasses were buried.

Three major pulses of deposition created the Great Plains from Rocky Mountain debris. Source.

Then about 130 years ago, a man was visiting his sweetheart on the upper Niobrara River when he spotted a fossilized leg bone sticking out of a rock. James Cook soon married his sweetheart, bought the Agate Springs Ranch from his father-in-law, and continued to find bone fragments and curious inexplicable features.

A Devil’s corkscrew or daemonhelix.

Cook contacted paleontologist Olaf A. Peterson of the Carnegie Museum in Pittsburgh, Pennsylvania. Peterson visited the ranch in 1904, starting paleontological studies that have continued off and on to the present. The ranch (now a National Monument) turned out to be a key site in the amazing record of prehistoric life on the Great Plains. “The great bonebed at the Monument remains one of the most impressive and scientifically interesting paleontological sites in North America” (Graham 2009).

Entrance to Agate Fossil Beds National Monument in northwest Nebraska. NPS photo.

Why so many bones in one place? Paleontologists think that during an extended drought, thirsty starving dying animals gathered at the muddy remains of waterholes. Carcasses accumulated, were torn apart by scavengers, and trampled by the still-living. Then the river flowed again, burying dismembered skeletons in sediment. Eventually the sand, silt, volcanic ash, and bones became rock. When the Niobrara River carved its valley, it exposed bone-filled sandstone and siltstone.

Bone slab from Carnegie Hill—part of a diorama in the Visitor Center.

University Hill in the distance, so-named because the University of Nebraska had a fossil quarry there.

Even though Agate Fossil Beds National Monument is a key site for the best record of one of the most spectacular evolutionary radiations ever documented, visitation is light. I met only one person on the trails, the Artist-in-Residence. It was a cool calm spring day, brilliant in the sunshine. I walked through rolling grasslands beneath blue sky that seemed to go on forever. Sometimes I could just make out a ghostly multitude of mammalian creatures—some huge, some kind of familiar, others quite strange—chalicotheres, rhinoceroses, entelodonts, beardogs, … oh my!

Devil’s corkscrews turned out to be fossilized burrows of the dry-land beaver, Paleocastor.

Tracks of ancient mammals in ashy lime mud, dog tail for scale.

Rhinos were really common. Diceratherium niobrarense on left, and the smaller Menoceras.

The dreaded beardog, Daphoenodon, was the largest carnivore in the area, about the size of today's wolf. It was neither a bear nor a dog, but a member of the extinct family Amphicyonidae.

Scavenger Dinohyus, the “terrible pig." It was not a pig but an entelodont. Some stood six feet tall at the shoulder, about the size of a male American bison (our “buffalo”).

Dinohyus today.

In the distance, a herd of Morupus approaches the muddy remains of a water hole, filled with carcasses and bones. Moropus was a chalicothere, an extinct group related to horses and rhinos.

Three chalicotheres stand in air-conditioned comfort.

A beardog, two entelodonts, and three chalicotheres meet at the waterhole … (you supply the punchline).

Sources (in addition to links in post)

The beautiful scenes and portraits of Miocene fauna come from the Monument’s trail signs and Visitor Center.

Graham, J. 2009. Agate Fossil Beds National Monument Geologic Resources Inventory Report. Natural Resource Report NPS/NRPC/GRD/NRR—2009/080. National Park Service, Denver, Colorado. PDF here.

Maher, HD, Jr., Egelmann, GF, and Shuster, RD. 2003. Roadside geology of Nebraska. Mountain Press Publishing Company.

Prothero, DR. 2006. After the Dinosaurs, the Age of Mammals. Indiana University Press.

Trimble, DE. 1980. The geologic story of the Great Plains. U.S. Geological Survey Bulletin 1493. Available online.