Centre for GeoGenetics, Natural History Museum of Denmark, Øster Voldgade 5-7, 1350K Copenhagen, Denmark.
Curr Biol. 2015 Oct 19;25(20):R973-8. doi: 10.1016/j.cub.2015.09.005.
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.
除了家马和驴之外，马科动物（也称为马属动物）还包括六种现存的野驴和斑马（图 1）。马科动物在化石记录中有着极为丰富的代表，其进化历史可以追溯到 5500 万年前，期间经历了许多创新、灭绝和迁徙的阶段。如今，马科动物仅限于马属一种，在中新世（2300 万至 530 万年前），马科动物蓬勃发展，包括二十多个属。该物种起源于北美洲，最早的化石形式被称为雷兽，体型不超过小狗。这些动物是软叶食草动物，与现代马不同，现代马只用一个脚趾在坚硬的角质蹄上行走，它们的后肢有三趾，前肢有四趾。因此，马与犀牛和貘一起，构成了奇蹄目哺乳动物，这一目的哺乳动物的特征是奇数脚趾。与反刍动物不同，它们是后肠发酵者，在肠道中而不是在分化的多个胃室中消化植物纤维素。
