Brontotheriidae, also called Titanotheriidae, is a family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinos, and tapirs. Although brontotheres are probably most closely related to horses, superficially they looked rather like rhinoceroses, although they were not true rhinos. They lived around 56-34 million years ago, from the early to late Eocene.
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. The earliest brontotheres, such as Eotitanops, were rather small, no more than a meter in height, and were hornless. Later brontotheres evolved massive body sizes, although some small species, such as Nanotitanops did persist through the Eocene. Some genera, such as Dolichorhinus, evolved highly elongate skulls. Later brontotheres were massive in size, up to 2.5 m in height, and had evolved bizarre hornlike appendages. For instance the North American brontothere Megacerops 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. However, unlike rhinos, 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 due to an inability to adapt to drier conditions and tougher vegetation (such as grasses) that spread during the Oligocene.
Two classification systems for the Brontotheriidae are presented below. The first contains 43 genera and 8 subfamilies and although it is based on a recent publication (McKenna and Bell, 1997), it summarizes research that was conducted before 1920 and is badly outdated. The second classification is based on very recent research (Mihlbachler et al., 2004a, 2004b; Mihlbachler, 2005). It indicates that many of the previous subfamily names are invalid. Also several recently discovered brontotheres are included in the newer classification. Note that although Lambdotherium and Xenicohippus were previously included in the Brontotheriidae, they are no longer considered to be 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.
|Old Classification (summarized by McKenna and Bell, 1997)||New classification (Mihlbachler et al., 2004a, 2004b; Mihlbachler, 2005)|
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