Brontotheriidae is a family of extinct mammals belonging to the order Perissodactyla, the order that includes , , and . 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 of several tonnes. They lived around 56–34 million years ago, until the very close of the Eocene. Brontotheres had a Holarctic realm 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.
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. Titanotherium The earliest stem-brontotheres, had an estimated body mass of only
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, with body masses of over a tonne, perhaps exceeding , in large individuals of Megacerops, although some small species such as Nanotitanops did persist through the Eocene. Some genera, such as Dolichorhinus, Evolution highly elongated skulls. Some later brontotheres developed horn-like bony projections of the skull. The 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 , 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.
+ Classification for Brontotheriidae after Mihlbachler 2008 and Mader 2010 |
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.
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