Hyracotherium

''Hyracotherium'' ( ; " hyrax-like beast") is an extinct genus of very small (about 60 cm in length) perissodactyl ungulates that was found in the London Clay formation. This small, fox-sized animal was once considered to be the earliest known member of Equidae before the type species, ''H. leporinum'', was reclassified as a palaeothere, a perissodactyl family basal to both horses and brontotheres. The remaining species are now thought to belong to different genera, such as '' Eohippus'', 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 identified as being of this genus, holotype specimen BMNH M16336, was found in the cliffs of Studd Hill near Herne Bay, Kent, and described by the paleontologist Richard Owen in a paper read to the Geological Society of London 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, 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, which he placed in a new genus '' Eohippus''. 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, ''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'', '' Xenicohippus'', '' Pliolophus'', '' Protorohippus'' 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.

See also

*''Equus''
* Evolution of the horse
*'' Merychippus''
*'' Mesohippus''

References

{{Taxonbar, from=Q222191

Eocene odd-toed ungulates
Eocene mammals of Europe
Ypresian genus first appearances
Lutetian extinctions
Fossil taxa described in 1841