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. They lived around 58-30 million years ago, from the late 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 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.

Classification of Brontotheres

There are 43 genera of brontotheres in 8 subfamilies:

• Family Brontotheriidae
  • Pakotitanops incertae sedis, from Pakistan
  • Nanotitan incertae sedis, from Asia
    • Subfamily Lambdotheriinae
      • Lambdotherium, from North America
      • Xenicohippus, from North America
    • Subfamily Palaeosyopinae
      • Palaeosyops (including Eotitanops), from North America, 0.5 m tall
      • Mulkrajanops, from Pakistan, 1.25 m tall
    • Subfamily Dolichorhininae
      • Metarhinus, from North America, 1 m tall
      • Sphenocoelus, from North America, 1.25 m tall
      • Mesatirhinus, from North America, 1 m tall
    • Subfamily Brontotheriinae
      • Duchesneodus, from North America
      • Brontotherium, from North America
      • Megacerops, from North America, 2.5 m tall
    • Subfamily Embolotheriinae
      • Titanodectes, from Asia
      • Embolotherium, from Mongolia, 2.5 m tall
      • Protembolotherium, from Outer Mongolia, 2 m tall
    • Subfamily Brontopinae
      • Brachydiastematherium, from Eastern Europe, 2 m tall
      • Pachytitan, from Inner Mongolia, 2 m tall
      • Dianotitan, from China, 2 m tall
      • Gnathotitan, from Inner Mongolia, 2.5 m tall
      • Microtitan, from Inner Mongolia, 0.75 m tall
      • Epimanteoceras, from Inner Mongolia, 2 m tall
      • Protitan, from Inner Mongolia, 2 m tall
      • Rhinotitan, from Inner Mongolia, 2.5 m tall
      • Metatitan, from Mongolia, 1.5 m tall
      • Dolichorhinus, from North America, 1.25 m tall
      • Protitanotherium, from North America, 2 m tall
      • Parabrontops, from Mongolia, 2 m tall
      • Oreinotherium, from North America
      • Brontops, from North America
      • Protitanops, from North America, 2 m tall
      • Pygmaetitan, from China, 0.5 m tall
    • Subfamily Telmatheriinae
      • Acrotitan, from Inner Mongolia, 0.3 m tall
      • Desmatotitan, from Inner Mongolia, 1.25 m tall
      • Arctotitan, from China
      • Hyotitan, from Inner Mongolia, 2.2 m tall
      • Sthenodectes, from North America, 1.25 m tall
      • Telmatherium (including Metatelmatherium), from North America and Inner Mongolia, 1.5 m tall
      • Sivatitanops, from Asia and Europe
    • Subfamily Menodontinae
      • Diplacodon, from North America, 2 m tall
      • Eotitanotherium, from North America
      • Notiotitanops, from North America, 2 m tall
      • Menodus, from Europe and North America
      • Ateleodon, from North America

References

• McKenna, M. C, and S. K. Bell. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp.