Brontotheriidae

Brontotheriidae
Brontotheres
Temporal range: 56–34 Ma
Eocene
Brontotherium hatcheri skeleton at the
National Museum of Natural History, Washington, DC
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Perissodactyla
Suborder: Hippomorpha
Family: Brontotheriidae
Marsh, 1873
Genera
See Text

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 not true rhinos and are probably most closely related to horses. They lived around 56–34 million years ago, until the very close of the Eocene.

Contents

Characteristics and evolution

Skull of Rhinotitan

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.[1] 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 2.5 m 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.[1]

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is based on a 1997 publication by McKenna and Bell, it summarizes research that was conducted before 1920 and is badly outdated.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 the 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.

Old Classification (summarized by McKenna and Bell, 1997) New classification (Mihlbachler et al., 2004a, 2004b; Mihlbachler, 2005)
  • Family Brontotheriidae
    • Pakotitanops incertae sedis, from Pakistan
    • Nanotitanops incertae sedis, from Asia
      • Subfamily Lambdotheriinae
        • Lambdotherium, from North America
        • Xenicohippus, from North America
      • Subfamily Palaeosyopinae
      • Subfamily Dolichorhininae
      • Subfamily Brontotheriinae
      • 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
  • 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 Kazakhstan, 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
        • Subtribe 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.; Lucas, S.G.; Emry, R.J.; Bayshashov, B. (2004b). "A new brontothere (Brontotheriidae, Perissodactla, Mammalia) from the Eocene of the Ily Basin of Kazakhstan and a phylogeny of Asian 'horned' brontotheres". American Museum Novitates 3439: 1–43. doi:10.1206/0003-0082(2004)439<0001:ANBBPM>2.0.CO;2. 

Notes

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