- General features
- Natural history
- Form and function
- Evolution and paleontology
The classification presented here follows that of U.S. paleontologist George Gaylord Simpson, which is generally accepted but modified in some fairly minor ways by other authors. The term Mesaxonia, introduced by the 19th-century paleontologist O.C. Marsh, is essentially synonymous with Perissodactyla. Simpson used it as a convenient designation for the superorder containing the single order Perissodactyla, as he used the parallel name Paraxonia for the superorder with the one order Artiodactyla.
Groups indicated by a dagger (†) are known only as fossils.
- Order Perissodactyla
- Herbivorous ungulates with either 3 digits or 1, at least in the hindfeet. Early forms digitigrade, this feature either retained or completely replaced by the unguligrade condition in later representatives. The body weight borne mainly or entirely on the third digit through which the long axis of the limb passes (mesaxonic condition). The talus (heel bone) with only 1, proximal, keeled surface, articulating with the tibia, no duplication of keel on the distal surface, as in artiodactyls. Nasals broad at the posterior end, alisphenoid canal present. Posterior premolars molariform; cheek teeth bunodont in early forms, typically lophodont or selenolophodont, rectangular; wide diastema separates them from incisors and canines, which may be reduced or absent. Testes inguinal, occasionally scrotal; mammary glands inguinal, with 2 teats; uterus bicornuate. Fifteen living species, but more than 200 fossil forms.
- Suborder Hippomorpha
- Superfamily Equoidea
- Dentition complete, upper molars with 6 tubercles, the 2 external ones united to form an ectoloph, median and internal tubercles generally fused into a single loph. Tendency to molarization of premolars and reduction of lateral digits.
- †Superfamily Brontotherioidea
- †Suborder Ancylopoda
- Suborder Ceratomorpha
- Superfamily Tapiroidea
- Brachydont forms, molars with simple ectoloph and strongly developed transverse protoloph and metaloph. Forefoot generally with 4 toes, hindfoot with 3.
- Superfamily Rhinocerotoidea
- Molars brachydont, upper with 2 transverse lophs fused with a well-developed ectoloph; lower with 2 asymmetrical crescents. Premolars more or less molariform. Rough processes on nasals (for horns) frequently present; forefoot with 3 or 4 toes.