Brontotheriidae

From Paleos

Brontotheriidae, also called Titanotheriidae, was a family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinos, and tapirs. Although brontotheres were probably most closely related to horses, they bore a strong, yet superficial resemblence to rhinoceroses, with which they are often confused, even though the brontotheres were not true rhinos. They lived around 58-30 million years ago, from the early to late Eocene. In their time, the brontotheres were highly successful, radiating out from their origins within North America into Asia, then Europe, before they became extinct at the end of the Eocene period.

Brontotheres retained 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

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on a recent publication (McKenna and Bell, 1997), it summaries research that was conducted before 1920 and is badly outdated.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):

• Family Brontotheriidae
  • Pakotitanops incertae sedis, from Pakistan
  • Nanotitanops 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
      • 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

New classification (Mihlbachler et al., 2004a, 2004b; Mihlbachler, 2005):

• Family Brontotheriidae
  • Pakotitanops incertae sedis, from Pakistan
  • Mulkrajanops incertae sedis, from Pakistan, 1.25 m tall
  • Eotitanops, from North America, 0.5 m tall
  • Palaeosyops, from North America, 1 m tall
  • Subfamily Brontotheriinae
    • Bunobrontops, from Asia
    • Mesatirhinus, from North America, 1 m tall
    • Dolichorhinus, from North America, 1.25 m tall
    • Sphenocoelus, from North America, 1.25 m tall
    • Desmatotitan, from Inner Mongolia, 1.25 m tall
    • Fossendorhinus, from North America
    • Metarhinus, from North America, 1 m tall
    • Microtitan, from Inner Mongolia, 0.75 m tall
    • Sthenodectes, from North America, 1.25 m tall
    • Telmatherium, from North America, 1.25 m tall
    • Metatelmatherium, from North America and Inner Mongolia, 1.25 m tall
    • Epimanteoceras, from Inner Mongolia, 2 m tall
    • Hyotitan incertae sedis, from Inner Mongolia, 2.2 m tall
    • Nanotitanops incertae sedis, from Asia
    • Pygmaetitan incertae sedis, from China, 0.5 m tall
    • Acrotitan incertae sedis, from Inner Mongolia, 0.3 m tall
    • Arctotitan incertae sedis, from China
    • Qufutitan incertae sedis, from China
    • Tribe Brontotheriini
      • Protitan, from Inner Mongolia, 2 m tall
      • Protitanotherium, from North America, 2 m tall
      • Rhinotitan, from Inner Mongolia, 2.5 m tall
      • Diplacodon (including Eotitanotherium), from North America, 2 m tall
      • Pachytitan, from Inner Mongolia, 2 m tall
      • Brachydiastematherium, from Eastern Europe, 2 m tall
      • Sivatitanops, from Asia and Europe
      • Subtribe Embolotheriina
        • Gnathotitan, from Inner Mongolia, 2.5 m tall
        • Aktautitan, from Kazakstan, 2.5 m tall
        • Metatitan, from Mongolia, 1.5 m tall
        • Nasamplus, from Inner Mongolia
        • Protembolotherium, from Outer Mongolia, 2 m tall
        • Embolotherium (including Titanodectes), from Mongolia, 2.5 m tall
      • Subfamily Brontotheriina
        • Parabrontops, from Mongolia, 2 m tall
        • Protitanops, from North America, 2 m tall
        • Notiotitanops, from North America, 2 m tall
        • Dianotitan, from China, 2 m tall
        • Duchesneodus, from North America
        • Megacerops (including Menodus, Brontotherium, Brontops, Menops, Ateleodon, and Oreinotherium), from North America, 2.5 m tall

References

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

• Mihlbachler, M.C. 2004. Phylogenetic Systematics of the Brontotheriidae (Mammalia, Perissodactyla). Ph.D. Dissertation, Columbia University. 757 pp.

• Mihlbachler, M.C. , S.G. Lucas, and R.J, Emry. 2004a. The holotype specimen of Menodus giganteus, and the “insoluble” problem of Chadronian brontothere taxonomy. In S.G. Lucas, K. Zeigler, and P. E. Kondrashov (eds.), Paleogene Mammals. Bulletin of the New Mexico Museum of Natural History 26: 129-136.

• Mihlbachler, M.C., S.G. Lucas, R.J. Emry, and B. Bayshashov. 2004b. A new brontothere (Brontotheriidae, Perissodactla, Mammalia) from the Eocene of the Ily Basin of Kazakstan and a phylogeny of Asian "horned" brontotheres. American Museum Novitates 3439: 1-43.