Brontotheriidae

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; Equidae and Brontotheriidae make up the suborder Hippomorpha. They lived around 56–34 million years ago, until the very close of the Eocene.

Brontotheres Temporal range: 56–34 Ma PreꞒ Ꞓ 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
Family:	†Brontotheriidae Marsh, 1873
Genera
See text
Synonyms
Menodontidae Titanotheriidae

Characteristics and evolution

Life reconstruction of Megacerops

Life reconstruction of Protitanops curryi

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 hornless.

Brontotheres evolved massive bodies, 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, up to 2.5 m (8.2 ft) tall with horn-like skull appendages. The North American brontothere Megacerops, for example, 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. Unlike rhinoceros, in which the horns are made of keratin, however, 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 because they could not adapt to drier conditions and tougher vegetation (such as grasses) that spread during the Oligocene.[1]

Classification of Brontotheres

Classification for Brontotheriidae after Mihlbachler 2008[2] and Mader 2010[3]

Brontotheriidae     Eotitanopinae        Palaeosyopinae    Brontotheriinae  Brontotheriini    Rhadinorhinina        Telmatheriina    Brontotheriina    Brontotheriita      Embolotheriita

Two classification systems for 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 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, Titanotherium, Ateleodon, and Oreinotherium), from North America, 2.5 m tall

Notes

1. Titanotherium
2. Mihlbachler, Matthew C. "Species taxonomy, phylogeny and biogeography of the Brontotheriidae (Mammalia, Perissodactyla)". Bulletin of the American Museum of Natural History. American Museum of Natural History (311). Retrieved 4 September 2013.
3. Mader, Bryn J. "A species-level revision of the North American brontotheres Eotitanops and Palaeosyops (Mammalia, Perissodactyla)" (PDF). Department of Biological Sciences and Geology, Queensborough Community College. Retrieved 4 September 2013.

References

• McKenna, M.C.; Bell, S.K. (1997). Classification of Mammals Above the Species Level. New York: Columbia University Press. ISBN 0231110138. OCLC 37345734.CS1 maint: ref=harv (link)
• Mihlbachler, M.C. (2004). Phylogenetic Systematics of the Brontotheriidae (Mammalia, Perissodactyla) (Ph.D. thesis). Columbia University.
• Mihlbachler, M.C.; Lucas, S.G.; Emry, R.J. (2004a). "The holotype specimen of Menodus giganteus, and the 'insoluble' problem of Chadronian brontothere taxonomy". In Lucas, S.G.; Zeigler, K.; Kondrashov, P.E. (eds.). Paleogene Mammals. Bulletin 26. New Mexico Museum of Natural History and Science. pp. 129–136. Retrieved 29 October 2013.CS1 maint: ref=harv (link)
• Mihlbachler, M.C.; Lucas, S.G.; Emry, R.J.; Bayshashov, B. (2004b). "A new brontothere (Brontotheriidae, Perissodactyla, Mammalia) from the Eocene of the Ily Basin of Kazakhstan and a phylogeny of Asian "horned" brontotheres" (PDF). American Museum Novitates. 3439: 1–43. doi:10.1206/0003-0082(2004)439<0001:ANBBPM>2.0.CO;2. Retrieved 29 October 2013.CS1 maint: ref=harv (link)

External links

• Brontotheroidea at Mikko's Phylogeny Archive