Brontotheriidae

Brontotheres Temporal range: 56–34 Ma PreЄ Є O S D C P T J K Pg N 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.

Characteristics and evolution

Megacerops

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is based on a 1997 publication by McKenna and Bell, it summarizes research that was conducted before 1920 and is badly outdated.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

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

1. ^ ^{a b} Titanotherium

External links

• Brontotheroidea at Mikko's Phylogeny Archive