Equidae is the taxonomic family of horses and related animals, including the extant horses, asses, and zebras, and many other species known only from fossils. The family evolved around 50 million years ago from a small, multi-toed ungulate into larger, single-toed animals. All extant species are in the genus Equus, which originated in North America. Equidae belongs to the order Perissodactyla, which includes the extant tapirs and rhinoceros, and several extinct families.
Perissodactyla is an order of ungulates. The order includes about 17 living species divided into three families: Equidae, Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs). They typically have reduced the weight-bearing toes to three or one of the five original toes, though tapirs retain four toes on their front feet. The nonweight-bearing toes are either present, absent, vestigial, or positioned posteriorly. By contrast, artiodactyls bear most of their weight equally on four or two of the five toes: their third and fourth toes. Another difference between the two is that perissodactyls digest plant cellulose in their intestines, rather than in one or more stomach chambers as artiodactyls, with the exception of Suina, do.
Hyracotherium is an extinct genus of very small perissodactyl ungulates that was found in the London Clay formation. This small, fox-sized animal is 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.
Merychippus is an extinct proto-horse of the family Equidae that was endemic to North America during the Miocene, 15.97–5.33 million years ago. It had three toes on each foot and is the first horse known to have grazed.
Moropus is an extinct genus of large perissodactyl mammal in the chalicothere family. They were endemic to North America during the Miocene from ~20.4–13.6 Mya, existing for approximately 6.8 million years. Moropus belonged to the schizotheriine subfamily of chalicotheres, and has the best fossil record of any member of this group; numbers of individuals, including complete skeletons, have been found.
Anchitherium was a fossil horse with a three-toed hoof.
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.
Chalicotheriidae is an extinct family of herbivorous, odd-toed ungulate (perissodactyl) mammals that lived in North America, Eurasia, and Africa from the Middle Eocene until the Early Pleistocene, existing from 48.6 to 1.806 mya. They are often called chalicotheres, a term which is also applied to the broader grouping of Chalicotherioidea. They are noted for their unusual morphology compared to other ungulates, such as their elongated clawed forelimbs. They are thought to have been browsers.
Pakotitanops is a genus of poorly known brontothere represented only by a few tooth fragments. Its fossil dates from the middle Eocene Kuldana Formation, in the Ganda Kas area of Pakistan. Because this species is known only from a few tooth fragments it is difficult to compare it to other species to determine if it is indeed a distinct species and to what other species it is mostly closely related.
Megacerops is an extinct genus of the prehistoric odd-toed ungulate family Brontotheriidae, an extinct group of rhinoceros-like browsers related to horses. It was endemic to North America during the Late Eocene epoch, existing for approximately 4.1 million years.
Amynodontidae is a family of extinct perissodactyls related to true rhinoceroses. They are commonly portrayed as semiaquatic hippo-like rhinos but this description only fits members of the Metamynodontini; other groups of amynodonts like the cadurcodontines had more typical ungulate proportions and convergently evolved a tapir-like proboscis.
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. transylvanicum would have had an elongated head, not unlike Dolichorhinus, and be about 2 meters at the withers.
Telmatherium is a genus of a North American Brontothere. It stood 1.25 metres (4.1 ft) tall. It lived during the Eocene epoch.
Phenacodontidae is an extinct family of large herbivorous mammals traditionally placed in the “wastebasket taxon” Condylarthra, which may instead represent early-stage perissodactyls. They lived from the late early Paleocene to early middle Eocene and their fossil remains have been found in North America and Europe. The only unequivocal Asian phenacodontid is Lophocion asiaticus.
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 the earliest mammals to have evolved large body sizes. They lived around 56–34 million years ago, until the very close of the Eocene.
Balochititanops is an extinct genus of brontotheriid perissodactyl from late Early Eocene deposits of Pakistan. Balochititanops is known from the holotype GSP-UM 6532, right maxilla with teeth, from Kingri area, Balochistan. Many referred materials are known and include cranial and postcranial remains. All specimens were collected from numerous localities in Balochistan and northwestern Frontier Province, from the uppermost part of the Ghazij Formation. It was first named by Pieter Missiaen, Gregg F. Gunnell and Philip D. Gingerich in 2011 and the type species is Balochititanops haqi. It represents one of the oldest Asian brontotheres.
Diplacodon is a genus of prehistoric odd-toed ungulates in the family Brontotheriidae. It was the size of a rhinoceros, with the last two upper premolars molar-like.
Homogalax is an extinct genus of tapir-like odd-toed ungulate. It was described on the basis of several fossil finds from the northwest of the United States, whereby the majority of the remains come from the state of Wyoming. The finds date to the Lower Eocene between 56 and 48 million years ago. In general, Homogalax was very small, only reaching the weight of today's peccaries, with a maximum of 15 kg. Phylogenetic analysis suggests the genus to be a basal member of the clade that includes today's rhinoceros and tapirs. In contrast to these, Homogalax was adapted to fast locomotion.
Systemodon is a genus of early Eocene mammal of Wasatchian age. It was one of many mammals originally considered the earliest horses, long classified in the genus Hyracotherium. These were dog-sized animals that in life would have looked vaguely like a paca, mara, or chevrotain The type species, S. tapirinus, is represented by 24 individuals from a locale called the Castillo pocket in the Huerfano Formation of Colorado. This well-preserved deposit allows researchers to reconstruct aspects of the environment and lifestyle of the species.
Nanotitanops is an extinct genus of Brontothere from the middle Eocene of China. It contains a single species, N. shanghuangensis. It is known only from isolated teeth, the smallest of any known Brontothere.
