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Order Perissodactyla
Odd-toed ungulates
Order PerissodactylaPerissodactyla, as we know it today, is a small order of hoofed mammals, containing 17 Recent species in three families: Equidae (horses), Tapiridae (tapirs), and Rhinocerotidae (rhinoceroses). Despite being such a small order, the odd-toed ungulates have a wide distribution, being found across the Ethiopian, Palearctic, Oriental, and Neotropical zoogeographic regions. Two species have been domesticated - the horse (Equus caballus) and the donkey (Equus asinus); feral populations of these two species have been established across the globe, (re)expanding this order's influence to the Nearctic and Australasian regions.

All modern perissodactyls are large to very large animals, ranging in size from the smallest equids (200 kg) to the largest rhinoceroses (3,500 kg). Despite the broad range in their physical appearances, perissodactyls are united by their mesaxonic limb structure, with most of the body weight being borne by the large central digit. Although a small order now, there are fourteen known families (living and extinct), which can be split into three suborders:

  1. Hippomorpha - horse-like perissodactyls.
  2. Ceratomorpha - rhinoceroses and relatives, including tapirs.
  3. Ancylopoda - the now-extinct chalicotheres.

Pe·ris·so·dac·ty·la (pai'ris-oh dak ti'lah)
From Greek perissos, strange, of numbers odd; daktulos, a finger or toe


Perissodactyls evolved on the Mesozoic continent of Laurasia, diversifying rapidly in what is now North America. The ungulate fauna of the Eocene was dominated by Perissodactyls, with thirteen different families evolving and spreading across the globe. Their ranks included some of the largest land mammals to have ever existed, including the ceratomorph Indricotherium which stood over 5 meters tall at the shoulder and weighed an estimated 15-20 tons! The diversity of perissodactyls has continually declined since the Oligocene, simultaneous with the rise in the artiodactyls, or even-toed ungulates. Only four perissodactyl families (the three extant families, plus the Chalicotheriidae) survived to the Pleistocene, and only 16 species in 6 genera have survived to the present day (a 17th species, the quagga Equus quagga became extinct in 1883 due to hunting and habitat loss). Modern perissodactyls are a last remnant of a once exceptionally successful order, and today, with the exception of domestic horses and donkeys, all species are found in relatively low numbers.

Diagnostic Characteristics

In all species, digit III is the most prominent on all feet, and, as the plane of symmetry of the foot passes through this digit, perissodactyls are said to have a mesaxonic foot. The first digit (equivalent the thumb or big toe of humans) is lost in all species. The Equidae have a single functional toe on each foot (the third digit), while the Rhinocerotidae have three toes per foot. The Tapiridae - the closest family to the ancestral perissodactyl condition - possess four toes on the forefeet (digit V is used on wet or marshy ground) and three on the hind. Perissodactyls are truly unguligrade, with the heel, sole, and digits of the foot never touching the ground. The ulna and fibula (bones in the forearm and lower leg) are reduced, simplifying the wrist/ankle joint considerably. The clavicle (collar bone) is absent, allowing for efficient running - the main driving force behind unguligrade evolution.

The skull is elongated, with the expansion occuring as a result of the facial bones being stretched (rather than the braincase). The nasal bones in the skull are expanded posteriorly, and project freely for at least part of their length. There is a well-developed paroccipital process. 'Horns' are present on the midline of the nasal and/or frontal bones in all living members of the Rhinocerotidae, but these are dermal in origin and have no bony core as in the artiodactyls. The dental formula is quite variable among modern species, I 0-3/0-3, C 0-1/0-1, P 3-4/3-4, M 3/3 x 2 = 24-44. In grazing species such as the horses the molars and premolars are hypsodont (high-crowned to allow for wear), while browsing forms such as the tapirs have brachydont (low-crowned) teeth. The pattern of dental ridges in modern perissodactyls is lophodont.

The Perissodactyla Family Tree
Branch lengths are not proportional to time
(From Norman and Ashley, 2000)

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Literature Cited

Klingel, H and E. Thenius. 1990. Odd-toed Ungulates. In Grzimek's Encyclopedia of Mammals. Volume 4. Edited by S. P. Parker. McGraw-Hill, New York. Pp. 547-556.

Norman, J. E., and M. V. Ashley. 2000. Phylogenetics of Perissodactyla and tests of the molecular clock. Journal of Molecular Evolution; 50(1): 11-21.

Nowak, R. M. [Editor]. 1991. Walker's Mammals of the World. Fifth Edition. Johns Hopkins University Press, Baltimore.

Vaughan, T. A., J. M. Ryan, and N. J. Czaplewski. 2000. Mammalogy. Fourth Edition. Saunders College Publishing, Philadelphia.

Webb, J. E., J. A. Wallwork, and J. H. Elgood. 1979. Guide to Living Mammals. Second Edition. Bell and Blain Ltd., Glasgow.