Megacerops

Research history

Fossils of Megacerops were among the first mammal fossils from the American West to receive scientific attention. Before scientific study, remains exposed by rainstorms were found by Lakota Sioux and Pawnee peoples, who linked them to legends of wakíŋyaŋ, or thunder beasts. The first scientifically described brontothere fossil was a fragment of a right jaw from the White River badlands of South Dakota, described by Hiram A. Prout in 1847. Prout identified it as a large perissodactyl but thought it belonged to a giant Palaeotherium.

In 1849 Auguste Pomel designated it the type specimen of Menodus giganteus. In 1850 David Dale Owen, Joseph Granville Norwood, and John Evans described additional brontothere teeth and jaws, naming them Palaeotherium? proutii. Joseph Leidy acquired the Evans fossils and in 1852–1853 suggested the name Titanotherium. In 1870 Leidy described the first brontothere fossil preserving most of the distinctive horns, found in Colorado, and provisionally referred it to a new genus and species, Megacerops coloradensis.

Description

Megacerops was a huge, robust animal built for strength rather than speed. It resembled modern rhinoceroses through convergent evolution but differed in several features: it had four toes on the front feet and three on the hind feet, well-developed canines, and molars similar to those of chalicotheres. Its horn cores were composed of bone, unlike rhinoceros horns made of keratin. The largest Megacerops specimens reached about 2.5 m at the shoulder and roughly 5 m in length.

Weight estimates range from 2–3 t up to 5 t. The skull was massive, long, broad, and vaguely saddle-shaped, with coossified nasal bones, greatly developed zygomatic arches, and a high, broad occiput. These cranial features were correlated with the development of the horns.

Paleobiology

The horns of Megacerops were the most developed among North American brontotheres, forming two bony protuberances above the nose reminiscent of a slingshot. Horn shape, size, and orientation varied greatly between individuals, and in rare specimens the two horns fused into a plate-like structure across the muzzle. This variability may have resulted from an indeterminate growth system where minor genetic differences or developmental accidents produced large variation. Larger individuals with more elaborate horns are traditionally assumed to be males.

The horns were composed of non-deciduous bone with large air cavities at the base. Anatomically they most closely resemble giraffe ossicones. The horns were likely covered entirely in skin, not keratin, as they lack the vascular impressions seen in keratin-sheathed horn cores.

Paleoecology

Megacerops lived in warm temperate to subtropical forests and open woodlands. Oxygen isotope analyses of tooth enamel from the White River Group indicate low δ18O and δ13C values, suggesting it preferred wetter, denser parts of the ecosystem such as riparian areas. It was by far the largest animal in its environment, with an estimated home range of about 20 km². The White River badlands preserve a detailed record of contemporary fauna, including perissodactyls such as Mesohippus, Metamynodon, Hyracodon, Trigonias, and Subhyracodon; artiodactyls including Archaeotherium, Merycoidodon, and Poebrotherium; and small mammals like rabbits, beavers, gophers, mice, and squirrels.

Did You Know?

• Horned brontotheres (subtribe Brontotheriina) originated in Central Asia.
• Megacerops was the last brontothere in North America and the last living member of the Brontotheriita.