Temporal range: Early Eocene
Sifrhippus is an extinct, monotypic genus of equid containing the species S. sandrae and S. grangeri. Sifrhippus is the oldest equid known from North America, and its fossils come from the earliest Eocene of the Bighorn Basin, Wyoming. A 2012 study found Arenahippus to be synonymous with Sifrhippus.
Sifrhippus was a very small equid the size of a house cat, varying from 12 to 8.5 pounds (5.4 to 3.9 kilograms), the size variance, according to one theory, depending on the warmth of the climate. Sifrhippus sandrae is referred to in earlier literature as Hyracotherium sandrae, but Froehlich, arguing that the traditional genus Hyracotherium was not monophyletic, reassigned many of its species to other genera, and re-using the old name "Eohippus", for one. Froehlich give H. sandrae the new generic name Sifrhippus, derived from the Arabic صِفْر (ṣifr), "zero", and Greek ἵππος (híppos), "horse".
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- Froehlich, D.J. (2002). "Quo vadis eohippus? The systematics and taxonomy of the early Eocene equids (Perissodactyla)". Zoological Journal of the Linnean Society 134 (2): 141–256. doi:10.1046/j.1096-3642.2002.00005.x.
- Secord, R.; Bloch, J. I.; Chester, S. G. B.; Boyer, D. M.; Wood, A. R.; Wing, S. L.; Kraus, M. J.; McInerney, F. A.; Krigbaum, J. (2012). "Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum". Science 335 (6071): 959–962. doi:10.1126/science.1213859. PMID 22363006.
- Gorman, James (23 February 2012). "A Tiny Horse That Got Even Tinier as the Planet Heated Up". The New York Times.