Horse Evolution — Eocene Epoch
Earliest members: Hyracotherium (Eohippus) and Orohippus.
1. Hyracotherium (Eohippus) — Dawn Horse
Earliest known member of the horse lineage (~50 MYA) discovered in North America.
Small fox-sized browser (~40 cm tall at shoulders) with 4 functional toes on forelimbs and 3 on hindlimbs.
Had a short head and neck. The low-crowned molar teeth were adapted for browsing soft vegetation.
Figure: Hyracotherium (Eohippus) — 4-Toed Dawn Horse (~40 cm)
Hyracotherium — Foot Structure Comparison
Detailed comparison showing the 4-toed forelimb structure and how it evolved over millions of years.
Figure: Eohippus Limb & Foot Structure — Detailed View
2. Orohippus — Early Eocene (~50 MYA)
Slightly larger than Hyracotherium with changes in tooth structure. Lost the first premolar and developed sharper crests on molars.
Still a forest browser. Subtle shift toward harder foods. Represents early diversification of the horse lineage.
Figure: Orohippus — 4-Toed Eocene Browser