Mammal Fossils


What Animals Roamed Here?

While some animals whose fossil remains were found at Agate Fossil Beds are now extinct, others are represented by a few modern relatives or descendants.

Paleocastor had powerful clawed forelimbs for digging and long, curved teeth like modern beavers. Herds of Stenomylus, gazelle-camels about two feet tall, grazed grasslands beside the three-toed, pony sized rhinoceroses Menoceras. The most common mammal in the bonebed, Monoceras may have roamed these plains in large herds. Only a few oreodonts, about the size of a sheep, have been found here. They are most common in the carnivore dens nearby, where they were the prey of beardogs.

Fossil remains of the ancestors of the modern horse, Parahippus, also have been found in the waterhole but are rare. Horses became extinct in North America millions of years after the die-off event at Agate, not to return until brought back by the Spaniards. Moropus was quite fantastic. Related to both the horse and the rhinoceros, it was large, had back legs shorter than the front, with great clawlike hooves. It probably browsed leaves of bushes and small trees.

Another large animal, Dinohyus, was a giant entelodont related more closely to cows and pigs than to carnivores. Tracks of this huge scavenger have been found in the waterhole mud. It broke bones with its teeth (bite marks show on chalicothere limb bone). Discoveries in the 1980s included fossil remains of beardogs and other carnivores and their dens -- one of the few paleontological sites of this type in the world.

Man in cowboy hat kneels alongside a rock cliff, looking at a stone
James and Kate Cook originally discovered the bone bed while out on a horseback ride through their cattle range.

NPS Photo / Cook Collection archives

Discovery of the Fossils

Most of the land that is now Agate Fossil Beds National Monunent was once part of the Agate Springs Ranch, owned by James and Kate Cook. They bought the ranch from her parents in 1887, a few years after they found what they called "a beautifully petrified piece of the shaft of some creature's leg bone."

In 1892, Erwin H. Barbour of University of Nebraska was the first scientist to examine the strange "Devil's Corkscrews of Agate." These were later recognized as the fossilized burrows of Paleocastor.

In 1904, Olaf Peterson of the Carnegie Museum in Pittsburg was the first professional paleontologist to excavate in the 'great bonebed' in the Fossil Hills. The American Museum of Natural History, Yale University, and other institutions also worked here between 1904 and 1923. The competition between these groups to find the best specimens were -- in a word -- spirited.

Specimens from the bone bed are now the basis of many amazing collections in museums around the world.


The Species of Miocene Agate

Beardogs had heavy heads and strong jaws.
Finding fossils of predators such as the beardog was unusual.


In animal populations, far fewer carnivores exist than herbivores, making their fossil remains relatively rare. When paleontologists from Carnegie Museum unearthed a number of carnivore remains at Agate Fossil Beds in 1905, it was a major event.

The most significant skeletons found were those of the beardog Daphoenodon Bones found in their dens showed the wolf- and hyena-like beardogs were the largest carnivores alive 22-23 million years ago in North America. They preyed on juvenile rhinos, camels, and small sheep-like oreodonts. The beardogs of Agate lived here during and after the droughts represented in the bone bed.

The story might have ended there, but in 1981, University of Nebraska scientists, while searching for the site of the original discovery, found underground dens containing fossil of beardogs and other carnivores. These burrows are the oldest known record of denning behavior in large mammal carnivores.

Other beardogs like the Temnocyon were larger, closer in size to wolves. They were characterized by a heavy head and strong jaws.

Dinohyus also known
Although not related to a pig, this mammal's discoverer Olaf A. Peterson named it Dinohyus, which means "Terrible Pig."


The rarest of all the mammals found in the Agate Springs quarries, the Dinohyus (now called a Daeodon) had bone-crushing teeth. These let them scavenge the remains of other grassland animals here.

Imagine a bison sized pig. Six to eight feet tall at the shoulder. They had a three-foot long head, and a massive jaw with bone-crushing teeth… some of them as large around as a man’s wrist. Even though Dinohyus looked and acted like the “Terrible Pig” its name translates to, they were not related to modern pigs.

However, they served a similar purpose in their ecosystem like hogs today. They scavenge as an omnivore, eating leaves, fruits, and carcasses of dead animals. Menoceras and Moropus fossils in the bone bed show many signs of being processed by these large beasts.

Menoceras, rhino that reached 3 ft. tall at maturity.
During the Miocene Epoch, herds of the pony-sized Menoceras lived on the future High Plains.


Rhinos were varied and abundant during most of the Cenozoic Era around the world. They ranged in size from the three-foot tall Menoceras of North America, to a giant Asian species. Another rhino species called Diceratherium also lived in this area on the grasslands, from 17 to 32 million years ago. Numerous bones of these animals were found in Carnegie's Quarry A, but very few were found in the waterhole bonebed. This suggests that this species may not have been trapped by the drought at the water source.

On the other hand, Menoceras was found by the thousands in the bonebed. About the size of a pony, Menoceras, which was at first confused with Diceratherium, was much smaller than any modern rhino. Unlike modern rhinos which have two horns in a line (one in front of the other), a Menoceras male had two horns side-by-side at the end of its nose. Females did not have horns at all.

While some of the other mammal species were found with nearly complete skeletons, almost all the Menoceras bones were completely disjointed, scattered, and jumbled. This is probably because of aggressive scavanging of their carcases by Dinohyus, and because of ligaments decaying and allowing their bones to move freely in the water and mud of the drying waterhole.

A very tall, horse-like creature resting on the ground next to a river with another horse-like maternal creature grooming her young baby.
These Moropus share characteristics with several animals, including the modern horse and rhinoceros.


Moropus was a distant relative of the horse. It’s one of the more puzzling mammals here at Agate. For many years, paleontologists thought its feet had claws rather than hooves. In fact, before they discovered its skull, they thought the foot structures meant it was the remains of a very large ground sloth. That original theory is still shown in the name, which translates to “sloth foot.”

Adults stood about 7-8 feet tall, with a slim neck, long front legs and short hind legs. They even had a small switch-style tail that resembled that of a horse. Moropus browsed shrubs and the low-hanging branches of trees. Occasionally, they used their claw-like hooves to dig up roots and tubers to eat.

Paleontologists found a herd of about twenty individuals in the bonebed.

A beaver-like creature standing over a corkscrew-shaped hole leading to an underground burrow.
Dry-land beavers called Palaeocastors dug corkscrew-shaped entryways to their burrows.


The spiral burrows are now a popular attraction here at Agate, but scientists struggled at first to determine how they were made. When Erwin H. Barbour first saw the corkscrews, he concluded they were fossils of enormous taproots. He named them “Daemonelix,” or “Devil’s Corkscrew.” He spent 1891 and 1892 excavating more of the mysterious corkscrews here at Agate.

In 1904, Olaf Peterson of the Carnegie Museum found a small rodent skeleton inside one of the corkscrews and offered a new scientific theory. After finding other skeletons in similar placements, Peterson theorized that they weren’t taproots, but the remains of spiral burrows dug by prehistoric beavers, later filled in with roots, sand and silt.

This ancient beaver Paleocastor didn’t build dams like its modern cousin. Instead, these dry-land beavers dug 6 to 8 foot tall spiral burrows. This style of burrow helped keep flood waters out of their nesting chambers at the bottom, and helped maintain a more consistsent temperature and humidity. That was important, since they occupied grasslands with broad, sandy streams bordered by sand dunes. A seasonally dry climate supported a rich environment of grasses, shrubs, and woody vegetation. The Miocene grassland was also home to many species of camels, small rhinos, horses and oreodonts along with Paleocastor.

The small, agile beaver used its powerful jaws and front incisors to dig its burrow, and its strong front limbs to then push the loose dirt out behind it. It lived in communities of these burrows, much like modern prairie dogs.

Camel-like creatures with one adult and 2 smaller ones running in the grass.
Two feet tall at the shoulder, the camel called Stenomylus traveled in herds. It's one of the two camel species found at Agate Fossil Beds National Monument.


The tiny gazelle-camel Stenomylus probably grazed in large herds for protection from predators. Researchers from Amherst College, including Professor Frederick B. Loomis, discovered their scattered skeletons in 1907, in a bluff face about two miles southeast of the Fossil Hills.

Stenomylus resembled a smaller version of the modern African gazelle. It stood only about two feet high at the shoulder. The name means “narrow grinding tooth,” and used its high-crowned teeth to graze and chew grasses. The ‘stenomylus quarry’ rock is about three million years older than the main Agate bone bed.

Much like the Menoceras, the Stenomylus herds died in large numbers relatively “at once” during a period of severe droughts and occasional flash floods. Many of them were found as intact skeletons, which implies they were quickly buried before scavengers were able to process them.

Last updated: March 18, 2024

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