Brontotheriidae is a

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

of extinct mammals belonging to the order Perissodactyla, the order that includes

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million y ...

rhinoceros 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 ...

es, and

tapir Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk. Tapirs inhabit jungle and forest regions of South and Central America, with one species inh ...

s. Superficially, they looked rather like rhinos, although they were actually more closely related to horses;

Equidae Equidae (sometimes known as the horse family) is the taxonomic family of horses and related animals, including the extant horses, asses, and zebras, and many other species known only from fossils. All extant species are in the genus '' Equus'', ...

and Brontotheriidae make up the suborder

Hippomorpha Odd-toed ungulates, mammals which constitute the taxonomic order Perissodactyla (, ), are animals—ungulates—who have reduced the weight-bearing toes to three (rhinoceroses and tapirs, with tapirs still using four toes on the front legs) o ...

. They lived around 56–34 million years ago, until the very close of the

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''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 The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

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
/ref> The earliest brontotheres, such as ''

Eotitanops ''Eotitanops'' ('dawn titan-face') is an extinct genus of brontothere native to North America and Asia. ''Eotitanops'' is the earliest known genus of brontothere. While brontotheres generally known as very large animals, ''Eotitanops'' was only ...

'', were rather small, no more than a meter in height, and hornless. Brontotheres evolved massive bodies, although some small species such as ''

Nanotitanops ''Nanotitanops'' is an extinct genus of Brontothere from the middle Eocene of China. It contains a single species, ''N. shanghuangensis''. It is known only from isolated teeth, the smallest of any known Brontothere. Description This genus is c ...

'' did persist through the Eocene. Some genera, such as ''

Dolichorhinus ''Sphenocoelus'' is an extinct genus of brontothere endemic to North America during the Middle Eocene 46.2—40.4 mya), existing for approximately . Fossils have been found only in southern Wyoming and eastern Utah. In life, it would have resemb ...

'',

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

highly elongated skulls. Later brontotheres were massive, up to tall with horn-like skull appendages. The

n brontothere ''

Megacerops ''Megacerops'' ("large-horned face", from '' méga-'' "large" + '' kéras'' "horn" + '' ōps'' "face") is an extinct genus of the prehistoric odd-toed ungulate (hoofed mammal) family Brontotheriidae, an extinct group of rhinoceros-like browsers ...

'', for example, evolved large

sexually dimorphic Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

paired horns above their noses. The sexually dimorphic horns suggest that brontotheres were highly

gregarious Sociality is the degree to which individuals in an animal population tend to associate in social groups (gregariousness) and form cooperative societies. Sociality is a survival response to evolutionary pressures. For example, when a mother wasp ...

(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 Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...

, however, the horns of brontotheres are composed of bone (the

frontal bone The frontal bone is a bone in the human skull. The bone consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, pa ...

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 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'' 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,

References

External links

Brontotheroidea
at Mikko's Phylogeny Archive {{Taxonbar, from=Q132865 Eocene first appearances Eocene extinctions Prehistoric mammal families

Characteristics and evolution

Classification

References

Further reading

External links