Hyracotherium

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."

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 was questioned in 2002.

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.

Palaeobiology

Palaeoecology

Hyracotherium is believed to have been a browsing herbivore that ate primarily soft leaves as well as some fruits and nuts and plant shoots. Dental microwear indicates that it was a folivore that facultatively ate non-fibrous food such as fruits and seeds when they were available.