Hipparion is an extinctgenus of three-toed, medium-sized equine belonging to the extinct tribeHipparionini, who lived about 10-5 million years ago.[1][2] While the genus formerly included most hipparionines, the genus is now more narrowly defined as hipparionines from Eurasia spanning the Late Miocene.[2]Hipparion was a mixed-feeder who ate mostly grass, and lived in the savannah biome.[2][3]Hipparion evolved from Cormohipparion,[2] and went extinct due to environmental changes like cooling climates and decreasing atmosphericcarbon dioxide levels.[4]
The genus "Hipparion" was used for over a century as a form classification to describe over a hundred species of Holartic hipparionines from the Pliocene and Miocene eras that had three toes and isolated protocones. Since then, groups such as the genera Cormohipparionand Neohipparion were proposed to further sort these species, typically based on differences in skull morphology. These species are now known as "Hipparion" in sensu lato (s.l.), or a broad sense.[5]
Hipparioninsensu stricto (s.s.), or a strict sense, describes the genus of Old World hipparionines from remains found in Eurasia (France, Greece, Turkey, Iran, and China) from the Late Miocene era (~10-5 Ma, or million years ago). The assignment of remains from elsewhere to the genus, such as North America and Africa, is uncertain.[2]
Morphology
[edit]Life reconstruction of H. forceiH. gracile skullThree-toed Hipparion hind leg fossil
Hipparion generally resembled a smaller version of the modern horse, but was tridactyl, or three-toed. It had two vestigial outer toes on each limb in addition to its hoof.[2] In some species, these outer toes were functional.[6]
Hipparion was typically medium in size, at about 1.4 m (4.6 ft) tall at the shoulder.[7][8] The estimated body mass of Hipparion depends on the species, but ranges from about 135 to 200 kg (about 298 to 441 lbs).[2]
Hipparion had hypsodont dentition (high-crowned teeth) for its premolars and molars, with a crown height of about 60 mm (2.36 in). Hipparion had isolated protocones in the upper molars, meaning a cusp of the teeth called a protocone was not connected to a tooth crest called a protoloph.[2]Hipparion is also characterized by its facial fossa, or deep depression in the skull, located high on the head in front of the orbit.[9][10]
Life
[edit]Mauricio Antón's illustration of Hipparion cornelianum that resembles a modern-day zebra
Hipparion lived in the Old World Savannah Biome, or OWSB, consisting of woodlandstograsslands.[2]Hipparion ate a mixed-feed diet, mostly consisting of grass. This diet is indicated by fossil evidence of microscopic wear patterns of scratches and pits on the enamelofHipparion's teeth, observed using scanning electron microscopy (SEM).[3]
Hipparion achieved skeletal maturity and possibly sexual maturity at about 3 years old. Fossils of Hipparion individuals are up to 10 years old at death.[8]
Hipparion likely evolved from a species of Cormohipparion during the Late Miocene, about 11.4-11.0 Ma. This species, C. occidentale, came to Eurasia and Africa from North America.[2] The last common ancestor of Hipparion and the modern horse was Merychippus.[11]
In the Old World, Hipparion experienced population decline and extinction down a North to South gradient, as did many other Miocene vertebrates. This trend is believed to be due to environmental changes caused by global cooling and decreasing carbon dioxide levels in the atmosphere.[4]
^Williams, Wendy (2015). The Horse. Toronto, Canada: Harper Collins. p. 117. ISBN9781443417860.
^Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 257. ISBN1-84028-152-9.
^Kalmykov N. P. (2023). "New Data on Dental Morphology of Hipparion tchicoicum Ivanjev, 1966 from Western Transbaikalia, Russia". Dokl Biol Sci. 508 (1): 67–71. doi:10.1134/S001249662270017X. PMID37186049.
