Equidae

Anchitheres were three-toed, primarily browsing horses that appear in the North American fossil record starting from the late Eocene and became extinct during the middle Miocene (Duchesnean to Clarendonian NALMA). During this time, they experienced a significant adaptive radiation during the late Oligocene and early Miocene, even spreading to the Old World (MN 3; 20-18mya). They are more derived in several ways compared to earlier horses, most notably in that the second-fourth premolars had become fully molariform and the tooth crown area is increased to enable more efficient grinding. The dentition itself is brachydont-lophodont, adapted to browsing foliage, though some taxa had developed slightly increased crown heights and were able to process more abrasive plants in a limited capacity. In all but the earliest anchitheres, the manus completely loses the fifth digit and becomes fully tridactyl. The limbs, which are adapted to movement on soft substrates, are digitigrade with all 3 toes touching the ground and the weight being partially supported by cushioned pads. The skull retains a post-orbital process rather than a complete post-orbital bar, resulting in the orbit being confluent with the temporal fossae. Their extinction in the middle Miocene may be attributed to the diversification and dispersal of mixed-feeding hipparionine horses and browsing ruminants during this time.