Brontotheres are perissodactyl land mammals, a large group that includes tapirs, rhino-like animals, rhinoceroses, and horses. According to Mihlbachler and Prothero (2021), bronthotheres were the dominant herbivore land mammals of the middle and late Eocene of North America (South Dakota, Nebraska, Wyoming, Colorado) and a few places in Asia.
Brontotheres [“bront” is Greek, meaning thunder, and “ther” is Greek, meaning beast]. Brontotheres have been referred to, therefore, as “thunder beasts” and less commonly as “thunder horses.” They have also been referred to as “titanotheres” (also derived from Greek, meaning giant beasts).
The rhino-like brontotheres first “appeared” during the early Eocene in both western North America and Pakistan (Asia) during the Eocene. Contrary to what early workers believed, brontotheres did not live during the Oligocene. It has been difficult to determine which country they actually originated in, but the general consensus is that they likely originated in North America. Brontotheres then migrated via the Bergingia1 land bridge (see one of my recent previous posts) to western Asia and eventually to eastern Europe. There were, however, some migrations brontotheres from Asia into North America (Mihlbachler, 2008).
The early Eocene brontotheres were small size, but by middle to late Eocene, they reached their largest size. Most of the Eocene genera and species were hornless, but horns appeared during the late Eocene. Horns were highly variable in size and shape within single species of brontotheres, thereby confusing early workers who overnamed the species of these fossils.
Fig. 1. The late Eocene Megacerops gigas had two large nasal horns.
Fig. 2. The skeleton of the skull of Megacerops gigas a late Eocene brontothere from Colorado. This species, which is endemic to western North America, was 8 feet tall at the shoulder and 15’ in length, which would have been larger than any living rhinoceros. This exhibit is in the Mammal Hall of the Los Angeles County Museum of Natural History.
Figure 3: Fragments of late Eocene brontothere teeth from the nonmarine Sespe Formation in Simi Valley, southern California. The largest fragment is 2 mm in height. Brontotheres have been found also elsewhere in Southern California Eocene deposits (e.g., San Diego area).
The worldwide cooling event that began near the end of the Eocene (and continued during the Oligocene) was responsible for grasslands replacing the forests. This dramatic change in vegetation was undoubtedly responsible for the demise of the brontotheres, which were browsing herbivores that lived in forests.
Fig. 4. A modern-day African rhinoceras (5 feet tall at the shoulder = substantially smaller than Megacerops) housed at the Los Angeles County Museum many years ago. True rhinoceroses first evolved during Miocene time. They occurred in both North America and Asia. They went extinct in North America during the Pliocene, but they continued to thrive in Africa, Asia, and Europe. Today, only five species are left and they live in tropical climates. Two species live in Africa, and three are found variously in India, Sumatra, and Java.
Further information:
Prothero, D.R. 1994. The Eocene-Oligocene transition: Paradise lost. Cambridge University Press, New York, 291 pp. (paperback). See pp. 34, 153–164.
An excellent overview of the subject.
References Cited:
Mihlbachler, M.C. 2008. Species taxonomy, phylogeny, and biogeography of the Brontotheridae (Mammalia: Perissodactyla). Bulletin of The American Museum of Natural History 311:1–145.
Mihlbachler, M.C. and D. Prothero. 2021. Eocene (Duchesnean and earliest Chadronian brontotheres (Brontotheriidae), Protitanops curryi and cf. Parvicornus occidentalis, from west Texas and Mexico. Palaeontologia Electronica. DOI: 10.26879/944
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