Brontotheriidae is a family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinoceroses, and tapirs. Superficially, they looked rather like rhinos with some developing bony nose horns, and were some of the earliest mammals to have evolved large body sizes. They lived around 56–34 million years ago, until the very close of the Eocene.

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Quick Facts Scientific classification, Genera ...

Brontotheres Temporal range: 56–34 Ma PreꞒ Ꞓ O S D C P T J K Pg N Eocene
Megacerops skeleton at the National Museum of Natural History, Washington, DC
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Perissodactyla
Family:	†Brontotheriidae Marsh, 1873
Genera
See text
Synonyms
Menodontidae Titanotheriidae

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Brontotheres retain four toes on their front feet and three toes on their hind feet. Their teeth are adapted to shearing (cutting) relatively nonabrasive vegetation. Their molars have a characteristic W-shaped ectoloph (outer shearing blade).

The evolutionary history of this group is well known due to an excellent fossil record in North America.^[1] The earliest brontotheres, such as Eotitanops, were rather small, no more than a meter in height, and hornless.

Brontotheres evolved massive bodies, although some small species such as Nanotitanops did persist through the Eocene.^[2] Some genera, such as Dolichorhinus, evolved highly elongated skulls. Later brontotheres were massive, up to 2.5 m (8.2 ft) tall with horn-like skull appendages. The North American brontothere Megacerops, for example, evolved large sexually dimorphic paired horns above their noses. The sexually dimorphic horns suggest that brontotheres were highly gregarious (social) and males may have performed some sort of head-clashing behavior in competition for mates. Unlike rhinoceros, in which the horns are made of keratin, however, the horns of brontotheres are composed of bone (the frontal bone and nasal bone) and were placed side-to-side rather than front-to-back.

Brontotheres probably became extinct because they could not adapt to drier conditions and more open landscapes of the Oligocene.^[3]

Brontotheres are an early diverging clade within Perissodactyla. Although historically suggested to be closely related to horses,^[4] phylogenetic analyses have recovered them to lie outside the clade containing chalicotheres, rhinoceroses, tapirs and horses,^[5] or more closely related to chalicotheres, rhinoceroses and tapirs than to horses.^[6]

Two classification systems for Brontotheriidae are presented below. The first contains 43 genera and 8 subfamilies, and although it is based on a 1997 publication by McKenna and Bell, it summarizes research that was conducted before 1920 and is badly outdated. The second classification is based on 2004 and 2005 research by Mihlbachler et al., which indicates that many of the previous subfamily names are invalid. Several more recently discovered brontotheres are included in the newer classification.

Although Lambdotherium and Xenicohippus were previously included in Brontotheriidae, they are no longer considered members of this family. Lambdotherium, though excluded, may be the closest known relative to brontotheres. Xenicohippus is now thought to be an early member of the horse family, Equidae.