Dinohyus (Dinohyus hollandi)

Order: Artiodactyla

Family: Entelodontidae

Dinohyus hollandi ('terrible hog of Holland') was a large bison-sized member of an ancient group of mammals known as entelodonts. Fossil evidence indicates that the Dinohyus first arrived during the Oligocene of North American about 29 million years ago and is believed to have gone extinct during the Miocene around 16 million years ago. Its emergence parallels the rise of the vast North American grasslands that dominated the landscape of that time. Standing as much as 7 feet at the shoulder with a body length spanning 10 feet, Dinohyus was a large animal by any standards. Its limbs were long and gracile ending in two small-hoofed toes. The relatively small proportions of the legs seem to be in contrast with its large robust head and body and probably gave it the ability to effectively move across its prairie grassland habitat.

Fossil skulls of Dinohyus measure more than a meter long. The large jaws were lined with an arsenal of teeth, possessing large upper and lower canine tusks and an assortment of premolars and incisors. A pronounced sagittal crest ran the length of the top of the skull providing attachment for the huge jaw muscles. The most distinct features of the skull were the large jugals (cheekbones), which jutted downward and out from the orbits. The jugals may have acted as facial protection during battle with sexual rivals or simply used for display to show sexual dominance. Healed punctures and scrapes found on Dinohyus skulls as well as other bones indicate that they probably did fight against themselves. The lower jaw also possessed a unique set of bony knobs know as "mandibular tubercles," which may have also served as a sexual feature or for recognition in a group hierarchy.

The Dinohyus has sometimes been described as being a carnivore, but examination of its dentition and the dietary habits of its modern relatives indicate that it was probably an omnivore. An omnivorous life-style would have allowed for a more diverse menu during times of drought or other times of hardship. Dinohyus was probably an opportunistic feeder and may have spent most of its time scavenging the kills of predators or rooting in the earth for plants and roots. Its large size and ferocious gape would have easily deterred predators away from their prey as the Dinohyus moved in to feed. Although it was not an active hunter, it may have dispensed of young, weak, or dying animals. Fossilized Moropus (a mammal contemporary with Dinohyus) bones have been found possessing tooth marks that accurately fit those of Dinohyus and indicate that it was at least feeding on carrion.

Some of the most remarkably preserved Dinohyus specimens have been found at the Agate Fossil Beds National Monument located on the Niobrara River in northwestern Nebraska. The fossil site is believed to have resulted from a deadly drought in which animals began to gather around increasingly shrinking water holes that pitted the stream bed. The scene would have been reminiscent of present-day water-hole congregations that occur during droughts in Africa. The majority of the fossilized bones belong to the ancient rhinoceros called Menoceras, whereas Dinohyus bones are rare in comparison. Other finds at the Agate Fossil Beds include the remains of the chalicothere Moropus, the camel Stenomylus, the prehistoric beaver Paleocastor, and the bear-dog Daphoenodon.

Mounted skeletons of Dinohyus other than those at the Agate Fossil Beds can be found at the University of Nebraska State Museum in Lincoln, Nebraska; the Carnegie Museum in Pittsburgh, Pennsylvania; and the Museum of Comparative Zoology at Harvard University in Cambridge, Massachusetts.