Brontotheriidae 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
A rhinoceros (; ; ), commonly abbreviated to rhino, is a member of any of the five extant species (or numerous extinct species) of odd-toed ungulates in the family Rhinocerotidae. (It can also refer to a member of any of the extinct species o ...
, 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.
Characteristics and evolution
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
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
.
[Titanotherium](_blank)
/ref> 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 tall with horn-like skull appendages. The North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
n 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
The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose.
Eac ...
) 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
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
.
Classification
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
''Lambdotherium'' ("wave beast") is a genus of North American brontothere
Brontotheriidae is a family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinoceroses, and tapirs. Superficially, they loo ...
'' 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.
References
Further reading
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External links
Brontotheroidea
at Mikko's Phylogeny Archive
{{Taxonbar, from=Q132865
Eocene first appearances
Eocene extinctions
Prehistoric mammal families