Brontotheriidae, also called Titanotheriidae, is a family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

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

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

s, rhinoceros

Rhinoceros

Rhinoceros , also known as rhino, is a group of five extant species of odd-toed ungulates in the family Rhinocerotidae. Two of these species are native to Africa and three to southern Asia....

es, and tapir

Tapir

A Tapir is a large browsing mammal, similar in shape to a pig, with a short, prehensile snout. Tapirs inhabit jungle and forest regions of South America, Central America, and Southeast Asia. There are four species of Tapirs: the Brazilian Tapir, the Malayan Tapir, Baird's Tapir and the Mountain...

s. 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

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

.

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

Molar (tooth)

Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....

 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

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

. The earliest brontotheres, such as Eotitanops

Eotitanops

Eotitanops 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 tall at the shoulder. It probably resembled a larger, bulkier version of its...

, 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

Dolichorhinus

Sphenocoelus is an extinct genus of brontothere of the subfamily Dolichorhininae, family Brontotheriidae, endemic to North America during the Middle Eocene epoch , existing for approximately .-Taxonomy:...

, evolved

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 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 America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

n brontothere Megacerops

Megacerops

Megacerops is an extinct genus of the family Brontotheriidae endemic to North America during the Late Eocene epoch , existing for approximately .-Taxonomy:...

evolved large sexually dimorphic

Sexual dimorphism

Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

 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

Frontal bone

The frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....

 and nasal bone

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 form, by their junction, "the bridge" of the nose.Each has two surfaces and four borders....

, 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

Oligocene

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

.

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

Lambdotherium

Lambdotherium is a genus of North American brontothere....

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

Equidae

Equidae is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. All extant species are in the genus Equus...

.

Old Classification (summarized by McKenna Malcolm McKenna Malcolm Carnegie McKenna was an American paleontologist. He was the curator of vertebrate paleontology at the American Museum of Natural History and co-authored the book Classification of Mammals along with Susan K. Bell...  and Bell, 1997)

New classification (Mihlbachler et al., 2004a, 2004b; Mihlbachler, 2005)

Family Brontotheriidae Pakotitanops Pakotitanops Pakotitanops is an obscure genus of brontothere.The only known species is Pakotitanops latidentatus. It is represented only by a few tooth fragments from the middle Eocene Kuldana Formation, in the Ganda Kas area of Pakistan... incertae sedis Incertae sedis , is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any... , from Pakistan Pakistan Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan... Nanotitanops incertae sedis Incertae sedis , is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any... , from Asia Asia Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population... Subfamily Lambdotheriinae Lambdotherium Lambdotherium Lambdotherium is a genus of North American brontothere.... , from North America Xenicohippus, from North America Subfamily Palaeosyopinae Palaeosyops Palaeosyops Paleosyops is a genus of small brontothere.These animals are commonly found in Wyoming fossil beds primarily as fossilized teeth. From all of the species of this animal, it is concluded that P. major was the largest, reaching the size of a tapir... (including Eotitanops), from North America, 0.5 m tall Mulkrajanops Mulkrajanops Mulkrajanops is a type of brontothere endemic to India during the Eocene living from 55.8—48.6 mya, existing for approximately .-Taxonomy:... , from Pakistan Pakistan Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan... , 1.25 m tall Subfamily Dolichorhininae Metarhinus Metarhinus Metarhinus is a genus of brontothere endemic to North America during the Eocene living from 46.2—42 mya, existing for approximately .-Taxonomy:Metarhinus was named by Osborn . It is not extant. Its type is Metarhinus fluviatilis... , from North America, 1 m tall Sphenocoelus, from North America, 1.25 m tall Mesatirhinus Mesatirhinus Mesatirhinus is a genus of brontothere endemic to North America during the Eocene living from 50.3—42 mya, existing for approximately .-Taxonomy:... , from North America, 1 m tall Subfamily Brontotheriinae Duchesneodus Duchesneodus Duchesneodus is a large brontothere endemic to North America during the Eocene living from 46.2—38 mya, existing for approximately .-Taxonomy:... , from North America Brontotherium Brontotherium Brontotherium is an extinct genus of prehistoric odd-toed ungulate mammal of the family Brontotheriidae, an extinct group of rhinoceros-like browsers related to horses. The genus was found in North America during the Late Eocene.... , from North America Megacerops Megacerops Megacerops is an extinct genus of the family Brontotheriidae endemic to North America during the Late Eocene epoch , existing for approximately .-Taxonomy:... , from North America, 2.5 m tall Subfamily Embolotheriinae Titanodectes, from Asia Embolotherium Embolotherium Embolotherium is an extinct genus of brontothere that lived in Mongolia during the late Eocene period. It is most easily recognized by a large bony protuberance emanating from the anterior end of the skull... , from Mongolia Mongolia Mongolia is a landlocked country in East and Central Asia. It is bordered by Russia to the north and China to the south, east and west. Although Mongolia does not share a border with Kazakhstan, its western-most point is only from Kazakhstan's eastern tip. Ulan Bator, the capital and largest... , 2.5 m tall Protembolotherium, from Outer Mongolia, 2 m tall Subfamily Brontopinae Brachydiastematherium Brachydiastematherium Brachydiastematherium transylvanicum is the westernmost species of brontothere, with the first fossils of it being found in Transylvania, Romania. In comparison with other brontothere fossils, it is suggested that B... , from Eastern Europe Eastern Europe Eastern Europe is the eastern part of Europe. The term has widely disparate geopolitical, geographical, cultural and socioeconomic readings, which makes it highly context-dependent and even volatile, and there are "almost as many definitions of Eastern Europe as there are scholars of the region"... , 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 Rhinotitan Rhinotitan is an extinct genus of brontothere from the Eocene of China.-References:* Classification of Mammals by Malcolm C. McKenna and Susan K. Bell... , from Inner Mongolia, 2.5 m tall Metatitan, from Mongolia, 1.5 m tall Protitanotherium Protitanotherium Protitanotherium emarginatum is a genus of brontothere endemic to North America during the Eocene living from 46.2—33.9 mya, existing for approximately .-Taxonomy:... , from North America, 2 m tall Parabrontops, from Mongolia, 2 m tall Oreinotherium Oreinotherium Oreinotherium is a genus of brontothere.According to one source, the species of Oreinotherium are merged into the genus Megacerops.-References:... , from North America Brontops Brontops Brontops is an extinct genus of rhinoceros-like perissodactyl mammal.According to one source, Brontops is subsumed into genus Megacerops.-Appearance:... , from North America Protitanops Protitanops Protitanops was a genus of brontothere that lived during the Eocene, in the Western United States, especially in Death Valley, California, where the best specimens of the species P. curryi have been found. It bore a strong resemblance to Brontops brontotheres with its knob-shaped horns... , 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 Sthenodectes Sthenodectes is a genus of brontothere endemic to North America during the Paleogene living from 46.2—42 mya, existing for approximately .-Taxonomy:... , from North America, 1.25 m tall Telmatherium Telmatherium Telmatherium is a genus of a North American Brontothere. It stood tall. It lived during the Eocene epoch.... (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 Notiotitanops Notiotitanops is a brontothere endemic to North America during the Eocene living from 48.6—37.2 mya, existing for approximately .-Taxonomy:Notiotitanops was named by Gazin and Sullivan... , from North America, 2 m tall Menodus Menodus Menodus giganteus is a species of brontothere. The best known specimen is a mounted skeleton in the Field Museum of Natural History.According to one source , M. giganteus is merged into the genus Megacerops.... , from Europe and North America Ateleodon Ateleodon Ateleodon is a genus of brontothere from the late Eocene of North America-Taxonomy:According to Mihlbachler and others , Ateleodon is synonymized with Megacerops, which also includes the species of the genera Menodus, Brontotherium, Brontops, Menops, and Oreinotherium... , from North America

Family Brontotheriidae Pakotitanops Pakotitanops Pakotitanops is an obscure genus of brontothere.The only known species is Pakotitanops latidentatus. It is represented only by a few tooth fragments from the middle Eocene Kuldana Formation, in the Ganda Kas area of Pakistan... incertae sedis Incertae sedis , is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any... , from Pakistan Pakistan Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan... Mulkrajanops Mulkrajanops Mulkrajanops is a type of brontothere endemic to India during the Eocene living from 55.8—48.6 mya, existing for approximately .-Taxonomy:... incertae sedis Incertae sedis , is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any... , from Pakistan Pakistan Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan... , 1.25 m tall Eotitanops Eotitanops Eotitanops 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 tall at the shoulder. It probably resembled a larger, bulkier version of its... , from North America, 0.5 m tall Palaeosyops Palaeosyops Paleosyops is a genus of small brontothere.These animals are commonly found in Wyoming fossil beds primarily as fossilized teeth. From all of the species of this animal, it is concluded that P. major was the largest, reaching the size of a tapir... , from North America, 1 m tall Subfamily Brontotheriinae Bunobrontops, from Asia Asia Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population... Mesatirhinus Mesatirhinus Mesatirhinus is a genus of brontothere endemic to North America during the Eocene living from 50.3—42 mya, existing for approximately .-Taxonomy:... , from North America, 1 m tall Dolichorhinus Dolichorhinus Sphenocoelus is an extinct genus of brontothere of the subfamily Dolichorhininae, family Brontotheriidae, endemic to North America during the Middle Eocene epoch , existing for approximately .-Taxonomy:... , 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 Metarhinus Metarhinus is a genus of brontothere endemic to North America during the Eocene living from 46.2—42 mya, existing for approximately .-Taxonomy:Metarhinus was named by Osborn . It is not extant. Its type is Metarhinus fluviatilis... , from North America, 1 m tall Microtitan, from Inner Mongolia, 0.75 m tall Sthenodectes Sthenodectes Sthenodectes is a genus of brontothere endemic to North America during the Paleogene living from 46.2—42 mya, existing for approximately .-Taxonomy:... , from North America, 1.25 m tall Telmatherium Telmatherium Telmatherium is a genus of a North American Brontothere. It stood tall. It lived during the Eocene epoch.... , from North America, 1.25 m tall Metatelmatherium Metatelmatherium Metatelmatherium is a genus of brontothere endemic to North America during the Eocene living from 46.2—33.9 mya, existing for approximately .-Taxonomy:Metatelmatherium was named by Granger and Gregory... , from North America and Inner Mongolia, 1.25 m tall Epimanteoceras, from Inner Mongolia, 2 m tall Hyotitan incertae sedis Incertae sedis , is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any... , from Inner Mongolia, 2.2 m tall Nanotitanops incertae sedis Incertae sedis , is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any... , from Asia Asia Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population... Pygmaetitan incertae sedis Incertae sedis , is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any... , from China, 0.5 m tall Acrotitan incertae sedis Incertae sedis , is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any... , from Inner Mongolia, 0.3 m tall Arctotitan incertae sedis Incertae sedis , is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any... , from China Qufutitan incertae sedis Incertae sedis , is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any... , from China Tribe Brontotheriini Protitan, from Inner Mongolia, 2 m tall Protitanotherium Protitanotherium Protitanotherium emarginatum is a genus of brontothere endemic to North America during the Eocene living from 46.2—33.9 mya, existing for approximately .-Taxonomy:... , from North America, 2 m tall Rhinotitan Rhinotitan Rhinotitan is an extinct genus of brontothere from the Eocene of China.-References:* Classification of Mammals by Malcolm C. McKenna and Susan K. Bell... , from Inner Mongolia, 2.5 m tall Diplacodon (including Eotitanotherium), from North America, 2 m tall Pachytitan, from Inner Mongolia, 2 m tall Brachydiastematherium Brachydiastematherium Brachydiastematherium transylvanicum is the westernmost species of brontothere, with the first fossils of it being found in Transylvania, Romania. In comparison with other brontothere fossils, it is suggested that B... , from Eastern Europe Eastern Europe Eastern Europe is the eastern part of Europe. The term has widely disparate geopolitical, geographical, cultural and socioeconomic readings, which makes it highly context-dependent and even volatile, and there are "almost as many definitions of Eastern Europe as there are scholars of the region"... , 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 Embolotherium Embolotherium is an extinct genus of brontothere that lived in Mongolia during the late Eocene period. It is most easily recognized by a large bony protuberance emanating from the anterior end of the skull... (including Titanodectes), from Mongolia Mongolia Mongolia is a landlocked country in East and Central Asia. It is bordered by Russia to the north and China to the south, east and west. Although Mongolia does not share a border with Kazakhstan, its western-most point is only from Kazakhstan's eastern tip. Ulan Bator, the capital and largest... , 2.5 m tall Subtribe Brontotheriina Parabrontops, from Mongolia, 2 m tall Protitanops Protitanops Protitanops was a genus of brontothere that lived during the Eocene, in the Western United States, especially in Death Valley, California, where the best specimens of the species P. curryi have been found. It bore a strong resemblance to Brontops brontotheres with its knob-shaped horns... , from North America, 2 m tall Notiotitanops Notiotitanops Notiotitanops is a brontothere endemic to North America during the Eocene living from 48.6—37.2 mya, existing for approximately .-Taxonomy:Notiotitanops was named by Gazin and Sullivan... , from North America, 2 m tall Dianotitan, from China, 2 m tall Duchesneodus Duchesneodus Duchesneodus is a large brontothere endemic to North America during the Eocene living from 46.2—38 mya, existing for approximately .-Taxonomy:... , from North America Megacerops Megacerops Megacerops is an extinct genus of the family Brontotheriidae endemic to North America during the Late Eocene epoch , existing for approximately .-Taxonomy:... (including Menodus, Brontotherium, Brontops, Menops, Ateleodon, and Oreinotherium), from North America, 2.5 m tall

External links

• Brontotheroidea at Mikko's Phylogeny Archive