Brontotheriidae

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. Brontotheres had a Holarctic distribution, with the exception of Western Europe: they occupied North America, Asia, and Eastern Europe. They were the first fossilized mammals to be discovered west of the Mississippi, and were first discovered in South Dakota.

Characteristics and evolution
This group has also been referred to as "Titanotheres." "Titan" refers to the mythological Greek gods who were symbols of strength and large size, and "theros" is Greek for "wild animal." "Bronto" is Greek for "thunder," which may be how this group got the nickname "thunder beasts."

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 wear patterns observed on Brontothere teeth suggests a folivorous diet. Early Brontotheres had brachydont teeth with thick enamel, while later forms evolved a more hypsodont style tooth with thinner enamel.

Brontotheres also shared an elongated postorbital cranium, meaning that their skulls are lengthened between their eyes and ears. They also had anteroposteriorly abbreviated (shortened) faces.

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 hornless.

Brontotheres evolved massive bodies, with some species standing over 2.5 meters (7 feet) tall, although some small species such as Nanotitanops did persist through the Eocene. Some genera, such as Dolichorhinus, evolved highly elongated skulls. Later brontotheres were massive, up to 2.5 m 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, along with highly developed neck musculature, suggest that brontotheres were highly gregarious (social) and males may have performed some sort of head-clashing behavior in competition for mates. Females had smaller appendages, which may have been used to ward off predators and protect young. In Asia, another species of brontothere, Embolotherium, evolved a similarly gigantic body size; however, instead of the slingshot-like horns of the Megacerops, they evolved a single elongated bony process that was composed of both nasal and frontal bones. Embolotherium may have used its large nasal cavity to make vocalizations in order to communicate with others of its species. 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. Similarly to Giraffes, their horns were covered in skin and did not have grooves for nutrient blood vessels. There is some evidence of secondary bone growth, likely due to impact from head clashing.

Brontotheres had likely adapted to the warmer and more humid climates of the Eocene, and probably became extinct because they could not adapt to the drier conditions and more open landscapes of the Oligocene.

Discovery
Brontotheres were one of the first fossilized mammals to be discovered west of the Mississippi, with the first fossil being found in 1846 in the Badlands, South Dakota. Joseph Leidy was the first researcher to scientifically describe brontothere fossils, followed by Cope and Marsh, who studied skulls and entire skeletons. Marsh came up with the term "Brontotheridae," identified them as odd-toed ungulates, and identified distinguishing characteristics of the group. Brontotheriidae fossils have been found in eastern Europe, eastern Russia, Kazakstan, Pakistan, southeast Asia, Korea, Japan, the southeastern U.S., and Canada.

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