''Hyracotherium'' ( ; "

hyrax Hyraxes (), also called dassies, are small, stout, thickset, herbivorous mammals in the family Procaviidae within the order Hyracoidea. Hyraxes are well-furred, rotund animals with short tails. Modern hyraxes are typically between in length a ...

-like beast") is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of small (about 60 cm in length)

perissodactyl Perissodactyla (, ), or odd-toed ungulates, is an order of Ungulate, ungulates. The order includes about 17 living species divided into three Family (biology), families: Equidae (wild horse, horses, Asinus, asses, and zebras), Rhinocerotidae ( ...

ungulate Ungulates ( ) are members of the diverse clade Euungulata ("true ungulates"), which primarily consists of large mammals with Hoof, hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined ...

s that was found in the

London Clay The London Clay Formation is a Sediment#Shores and shallow seas, marine formation (geology), geological formation of Ypresian (early Eocene Epoch, c. 54-50 million years ago) age which outcrop, crops out in the southeast of England. The London C ...

formation. This small, fox-sized animal is (for some scientists) considered to be the earliest known member of

Equidae Equidae (commonly known as the horse family) is the Taxonomy (biology), taxonomic Family (biology), family of Wild horse, horses and related animals, including Asinus, asses, zebra, zebras, and many extinct species known only from fossils. The fa ...

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

palaeothere Palaeotheriidae is an extinct family of herbivorous perissodactyl mammals that inhabited Europe, with less abundant remains also known from Asia, from the mid-Eocene to the early Oligocene. They are classified in Equoidea, along with the living ...

, a perissodactyl

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

related 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 mi ...

s and

brontothere 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 with some developing bony nose horns, and were some of ...

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'' (now strictly defined as a member of the Palaeotheriidae rather than the Equidae). Its rem ...

'', 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 A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat ...

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 preserve ...

identified as being of this genus,

holotype specimen A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was Species description, formally described. It is either the single such physical example (or illus ...

BMNH M16336, was found in the cliffs of Studd Hill near

Herne Bay, Kent Herne Bay is a seaside town on the north coast of Kent in South East England. It is north of Canterbury and east of Whitstable. It neighbours the ancient villages of Herne and Reculver and is part of the City of Canterbury local government ...

, and described by the

paleontologist Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

Richard Owen Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomy, comparative anatomist and paleontology, palaeontologist. Owen is generally considered to have been an outstanding naturalist with a remarkabl ...

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 Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Pachydermata Pachydermata (meaning 'thick skin', from the Greek , and ) is an obsolete order of mammals described by Gottlieb Storr, Georges Cuvier, and others, at one time recognized by many systematists. The term ' is commonly used to describe elephants, ...

, 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

, 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 New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

, 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. 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 International_Code_of_Zoological_Nomenclature, 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 spe ...

, ''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'', '' 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 The evolution of the horse, a mammal of the family Equidae, occurred over a geologic time scale of 50 million years, transformin ...

'', ''

Protorohippus ''Protorohippus'' (Latin: "before" (pro), + Greek: "mountain" (oros), "horse" (hippos)) is an extinct genus of equid that lived in the Eocene of North America. Palaeobiology Based on oxygen and carbon isotope analysis of the teeth of ''P. mont ...

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

North American continent North America is a continent in the Northern and Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Caribbean Sea, and to the sou ...

References

{{Taxonbar, from=Q222191 Palaeotheriidae Eocene Perissodactyla Eocene mammals of Europe Ypresian genus first appearances Lutetian extinctions Fossil taxa described in 1841 Monotypic prehistoric placental genera