Equids.

Alongside domestic horses and donkeys, the horse family, also known as equids, comprises six extant wild species of asses and zebras (Figure 1). Equids are extremely well represented in the fossil record, comprising a 55 million-year evolutionary history, punctuated by many episodes of innovation, extinction and migration. Limited to the single genus Equus today, in the Miocene (23.0–5.3 million years ago) the equid family flourished, comprising more than twenty genera. The group originated in Northern America, where the earliest fossil forms have been found, the so-called Hyracotheres, no larger than small dogs. These animals were soft-leaf browsers and in contrast to modern equids, which roam on a single toe with a solid keratin hoof, their hindlimbs were three-toed and their forelimbs four-toed. Equids thus form, together with rhinos and tapirs, the perissodactyls, an order of mammals characterized by an odd number of toes. Unlike ruminants, they are hindgut fermenters, which digest plant cellulose in their intestines and not in differentiated multiple stomach chambers.