• The Agate Fossil Hills where mammal fossils were excavated in the early 1900's

    Agate Fossil Beds

    National Monument Nebraska

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Mammal Fossils


The Miocene Monsters

The animals found at Agate Fossil Beds National Monument come from the early Miocene Epoch some 19-21 million years ago. Scientists describe the Miocene Epoch as a time period from 5-23 million years ago when the land mass was drying out, flowering plants were proliferating, and abundant animals responded to a new food source, as grasslands replaced forest and jungle. Some of these creatures, though slightly different anatomically, resemble those of today. Others came in bizarre shapes and sizes, long extinct, and truly reflect the monsters of yesteryear, whether real or imagined. These are the animals that replaced the dinosaurs.

Three main animals were associated with the death event that became “The Great Bonebed Of Agate.” These were the small rhino Menoceras, the large Moropus, and fearsome Dinohyus. Another quarry site is comprised almost entirely of the small gazelle camel, Stenomylus, in considerable abundance. Certain other deposits nearby reveal the land burrowing beaver, Palaeocastor, with its curious spiral home. A final animal found less frequently is the predator, Daphoenodon, of the beardog family.

Menoceras, rhino that reached 3 ft. tall at maturity.

Herds of the pony-sized Menoceras lived on the Miocene plains.


The North American rhinoceroses became extinct 5 million years ago. Menoceras, the rhino that lived where Agate Fossil Beds is today, was a smaller version of the modern rhino. The animals reached three feet in height, about the size of a large dog. Modern rhinos have horns growing one in front of the other, while the Menoceras had two horns growing side by side on the end of the nose. These animals ate leaves and stems of plants near the river. They spent much of their day lying in the shallows of the water hole to drink, escape bugs and stay cool. When the multi-year drought occurred and the food supply disappeared, the Menoceras remained at the water hole where they died of malnutrition, and scavengers devoured their bodies. When water flowed again, the river washed the bones into a crook or oxbow in the river. The piling up of these bones created the Great Bonebed of Agate.

Moropus-mystery animal

These beasts looked like a composite of several animals.


In North America, from the late Eocene to the middle Miocene, lived the chalicothere Moropus or “sloth foot.” When the feet were first found, it was believed that they belonged to a ground sloth. Then they found the skull much like a large horse, slim neck, long front legs sloping back, short hind legs and a little switch tail. Together with those three curved claws on each foot this skeleton could have belonged to some mythical beast. Standing 7-8 feet tall with heavy legs and a stilted walk, it would be easy to see why other animals would move out of his way. He was a browser of shrubs and low branches, occasionally ate grass and he even dug easily accessible roots and tubers. Only a couple dozen skeletons of the Moropus were found in the Agate quarries.

Dinohyus also known

Dinohyus was not related to the pig but his nickname was "Terrible Pig".


The rarest of all the animals in the Agate quarries was the Dinohyus, known as the “terrible pig,” a beast that would have given the bravest of the Miocene animals pause. This animal, a cross between a bison and a pig with a whole lot of mean thrown in, could stand, at maturity, six foot tall at the shoulders and possessed a three foot long head filled with massive, crushing teeth. Even though he looked and acted like his nickname, he is not related to the modern pig, but was an omnivore, not choosy about what he ate. Leaves, fruits or the carcass of a dead animal could make a meal. Only two skeletons of the Dinohyus were found by the early paleontologists, one on each of the Fossil Hills.

The tiny Stenomylus camel

Two foot tall Stenomylus camels traveled in herds also.


Likened more to a tiny antelope than a modern camel, the Stenomylus camel was a small (only 24” high at maturity), delicate-looking creature. A herd animal like the Menoceras, they lived in, and ate the abundant grasses of the area. Also like the Menoceras, they suffered mass deaths at a gradually shrinking waterhole during a severe drought. Many skeletons were found with the head pulled back in an unnatural position, caused by the tightening of the muscles in the backs of their necks. The Stenomylus quarry was excavated two miles south and east of the main quarries. Professor Frederick B. Loomis of Amherst College and his field crew discovered the scattered skeletons and bones in 1907.

Ancient beavers called Paleocastors dug cork-screw shaped burrows.

Palaeocastors dug cork-screw shaped burrows to access their lodges.


Palaeocastor, “ancient beaver” did not build dams; in fact, they chose to live not at the water’s edge but in well-drained ground. They dug deep, 6.5 to 8 foot spiral burrows which led to their lodges. Palaeocastors dug their lodges at an upward angle from the bottom of the burrow so rain water would not settle in the lodge itself. These burrows were called Daemonelix, Devil’s Corkscrews, and were originally thought to be taproots of some ancient giant plant. The Palaeocastor was about the size of a large modern day prairie dog and used its teeth as well as its claws to dig the burrows.

Beardogs had heavy heads and strong jaws.

Finding fossils of predators such as the Beardog was unusual.


Bigger that a coyote and smaller than a wolf, the beardog Daphaenodon was characterized by a heavy head and strong jaws. They are credited with disarticulating many of the skeletons that constitute the Agate bonebed. Fossils of carnivores are usually harder to find than those of herbivores, which makes the discovery of several of these animals especially interesting. After the original find of the fossils in 1905, another amazing discovery occurred in 1981 when scientists found the dens that the beardogs had dug to live in. A mature animal along with a juvenile were found in a den, as well as bones from what they had eaten.

Did You Know?

Picture of the Agate Springs Ranch taken in 1898.

The name, Agate Fossil Beds National Monument, came from the name of James Cook’s Agate Springs Ranch. Travelers to the ranch would say that they were going to Agate. The agate in the area is of a moss agate type, but is not a reason why the park was established. More...