''Hyracotherium'' ( ; " hyrax-like beast") is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of very small (about 60 cm in length)

perissodactyl 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 ...

ungulate Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, ...

s that was found in the

London Clay The London Clay Formation is a marine geological formation of Ypresian (early Eocene Epoch, c. 56–49 million years ago) age which crops out in the southeast of England. The London Clay is well known for its fossil content. The fossils from t ...

formation. This small, fox-sized animal was once considered to be the earliest known member of

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'', ...

before the type species, ''H. leporinum'', was reclassified as a

palaeothere Palaeotheriidae is an extinct family of herbivorous perissodactyl mammals related to equids. They ranged across Europe and Asia from the Eocene through to the early Oligocene 55–33 Ma, existing for approximately . Living in dense fore ...

, a perissodactyl

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

basal to both

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 yea ...

s and

brontothere Brontotheriidae is a family (biology), family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinoceroses, and tapirs. Superficially, they looked rather like Rhinoceros, rhinos, although they were actua ...

s. The remaining species are now thought to belong to different genera, such as ''

Eohippus ''Eohippus'' is an extinct genus of small equid ungulates. The only species is ''E. angustidens'', which was long considered a species of ''Hyracotherium''. Its remains have been identified in North America and date to the Early Eocene (Ypresian ...

'', which had previously been synonymised with ''Hyracotherium''.

Description

''Hyracotherium'' averaged 78 cm (2.5 feet) in length and weighed about 9 kg (20 pounds). It had a short face with eye sockets in the middle and a short diastema (the space between the front teeth and the cheek teeth). The skull was long, having 44 low-crowned teeth. Although it had low-crowned teeth, the beginnings of the characteristic horse-like ridges on the molars can be seen. ''Hyracotherium'' is believed to have been a browsing herbivore that ate primarily soft leaves as well as some fruits and nuts and plant shoots.

Discovery

The first

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

identified as being of this genus,

holotype specimen A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

BMNH M16336, was found in the cliffs of Studd Hill near Herne Bay, Kent, and described by the

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

Richard Owen in a paper read to the

Geological Society of London The Geological Society of London, known commonly as the Geological Society, is a learned society based in the United Kingdom. It is the oldest national geological society in the world and the largest in Europe with more than 12,000 Fellows. Fe ...

on 18 December 1839 as a "small mutilated cranium about the size of that of a hare". He identified it as belonging to an extinct order of

Pachydermata Pachydermata (meaning 'thick skin', from the Greek grc, παχύς, pachys, thick, label=none, and grc, δέρμα, derma, skin, label=none) is an obsolete order of mammals described by Gottlieb Storr, Georges Cuvier, and others, at one time ...

, with teeth resembling those of the ''Chaeropotamus'' and the general form of the skull "partaking of a character intermediate between that of the hog and the hyrax, though the large size of the eye must have given to the physiognomy of the living animal a resemblance to that of the Rodentia." Referring to this resemblance to the hyrax, Owen proposed the genus name ''Hyracotherium'' for this new genus. In his formal description published by the Geological Society in 1841, Owen wrote "Without intending to imply that the present small extinct Pachyderm was more closely allied to the Hyrax than as being a member of the same order, and similar in size, I propose to call the new genus which it unquestionably indicates, ''Hyracotherium'', with the specific name ''leporinum.''"Owen, Richard (1841). �
Description of the Fossil Remains of a Mammal (''Hyracotherium leporinum'') and of a Bird (''Lithornis vulturinus'') from the London Clay.
�� ''Transactions of the Geological Society of London'', Series 2, 6: 203–208A horse is a horse, unless of course… « Playing Chess with Pigeons
25 May 2008 In 1876 Othniel C. Marsh published the description of a full skeleton found in

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ke ...

, which he placed in a new genus ''

''. Its similarities with the fossils described by Owen were pointed out in a 1932 paper by Sir

Clive Forster Cooper Sir Clive Forster Cooper, FRS (3 April 1880 – 23 August 1947) was an English palaeontologist and Director of the Cambridge University Museum of Zoology and Natural History Museum in London. He was the first to describe ''Paraceratherium'', als ...

. The only species, ''E. angustidens'', was moved to the genus ''Hyracotherium'', which had priority as the name for the genus, with ''Eohippus'' becoming a junior synonym of that genus. Many other North American equids were subsequently classified as species of ''Hyracotherium'' as well, but this synonymy has recently been questioned.

Taxonomy and evolution

The

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specime ...

, ''H. leporinum'', is now regarded as a paleothere, rather than a horse proper. Most other species of ''Hyracotherium'' are still regarded as equids, but they have been placed in several other genera, such as ''

Arenahippus ''Sifrhippus'' is an extinct genus of equidae, equid containing the species ''S. sandrae''. ''Sifrhippus'' is the oldest known equid, living during the early Eocene. Its fossils were discovered in the Bighorn Basin of Wyoming. Description ''Sif ...

'', '' Minippus'', ''

Sifrhippus ''Sifrhippus'' is an extinct genus of equid containing the species ''S. sandrae''. ''Sifrhippus'' is the oldest known equid, living during the early Eocene. Its fossils were discovered in the Bighorn Basin of Wyoming. Description ''Sifrhippus' ...

'', '' Xenicohippus'', ''

Pliolophus ''Pliolophus'' is an extinct equid that lived in the Early Eocene of Britain. See also * Evolution of the horse References

Eocene horses Prehistoric placental genera Eocene odd-toed ungulates Eocene genus extinctions Eocene mammals of ...

'', '' Protorohippus'' and the resurrected ''Eohippus''. At one time, ''Xenicohippus'' was regarded as an early brontothere. The main stream of

horse evolution The evolution of the horse, a mammal of the family Equidae, occurred over a geologic time scale of 50 million years, transforming the small, dog-sized, forest-dwelling ''Eohippus'' into the modern horse. Paleozoologists have been able to piece ...

occurred on the

North American continent 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 ...

References

{{Taxonbar, from=Q222191 Eocene odd-toed ungulates Eocene mammals of Europe Ypresian genus first appearances Lutetian extinctions Fossil taxa described in 1841