''Sifrhippus'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of equid containing the species ''S. sandrae''. ''Sifrhippus'' is the oldest known equid, living during the early

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

. Its fossils were discovered in the

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Wyoming Wyoming () is a state in the Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the southwest, and Colorado to the s ...

Description

''Sifrhippus'' looked similar to modern horses, but were more slender and much smaller. Individuals likely weighed between ; the size variance, according to one theory, depended on the warmth of the climate.

Taxonomy

''Sifrhippus sandrae'' is referred to in earlier literature as ''Hyracotherium sandrae'', but Froehlich, arguing that the traditional genus ''

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 know ...

'' was not monophyletic, reassigned many of its species to other genera. Froehlich gave ''H. sandrae'' the new generic name ''Sifrhippus'', derived from the

Arabic Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walter ...

'' صِفْر'' (''ṣifr''), "zero", and

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

'' ἵππος'' (''híppos''), "horse". Since Froehlich's reorganization of ''Hyracotherium'' based on the morphological differences between fossil specimens, one further study has suggested that ''Sifrhippus sandrae'' and ''Minippus jicarillai'' are synonymous, and also that ''Minippus index'', although not seen by the authors of this study, is a synonym for the same taxon, which should then be named ''Sifrhippus index''. It has been argued that the differences in the fossils are more likely caused by individual variation in animals.

References

{{Taxonbar, from=Q291458 Eocene horses Eocene odd-toed ungulates Prehistoric placental genera Ypresian life Wasatchian Paleogene mammals of North America Fossil taxa described in 2002