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Brontotheriidae, also called Titanotheriidae, 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, although they were actually more closely related to horses. They lived around 56–34 million years ago, until the very close of the Eocene. ==Characteristics and evolution== 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.〔(Titanotherium )〕 The earliest brontotheres, such as ''Eotitanops'', were rather small, no more than a meter in height, and were hornless. Brontotheres, over time, evolved massive body sizes, 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 in size, up to in height with bizarre hornlike skull 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Brontotheriidae」の詳細全文を読む スポンサード リンク
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