Pliohippus

Pliohippus is an extinct genus of medium‑sized Equidae that lived during the Late Miocene, specifically from about 7.1 to 5.5 million years ago. Fossils have been found across North America, especially in the Great Plains and the western United States, indicating that the species occupied a broad geographic range during a period of significant climatic change. The genus was first described by Henry Fairfield Osborn in 1904, and its name derives from Greek words meaning “earlier horse.”

The animals of this genus are considered one of the earliest known true horses. Pliohippus exhibits a combination of primitive and derived traits: the teeth show a transition from multi‑crowned cheek teeth of earlier Miocene equids to the single‑rowed prismatic enamel typical of later perissodactyls. The limb bones are relatively robust, yet the metacarpals and metatarsals hint at the beginning of cursorial specialization. The pelvis is more slender than in its predecessors, and the skull shows a reduction in the palatine crests, an adaptation that has been linked to changes in diet and feeding behavior.

Pliohippus played a pivotal role in the evolutionary transition toward modern horses. Its dental and skeletal adaptations reflect adaptation to open grasslands and are considered an intermediate stage in the morphological trend toward increased limb length, reduced stomach capacity, and greater speed. The genus is often used by paleontologists to illustrate the early stages of equid evolutionary radiation and to study the ecological impacts of Miocene climate change on herbivorous mammals. Recent discoveries of well‑preserved skulls and limb bones have allowed researchers to refine phylogenetic relationships among early horses, placing Pliohippus near the base of modern Equus lineages while highlighting the stepwise acquisition of key locomotor and feeding specializations.