Perissodactyla

Perissodactyla (from the Ancient Greek perissos, "odd-numbered," and daktylos, "finger" or "toe," coined by Sir Richard Owen in 1848) is the order of odd-toed ungulates (hoofed placental mammals) within the superorder Laurasiatheria (clade Euungulata, sister to Cetartiodactyla or Ferae). Today, Perissodactyla is represented by 17 extant wild species distributed across three families—Equidae, Tapiridae, and Rhinocerotidae.

1. Equidae (~7–8 species): Horses (Equus caballus and Przewalski's horse Equus ferus przewalskii), African and Asiatic wild asses/donkeys (Equus africanus, E. hemionus, E. kiang), and three species of zebras (plains zebra Equus quagga, Grevy's zebra E. grevyi, and mountain zebra E. zebra);
2. Tapiridae (4 species): Neotropical Baird's, lowland, and mountain tapirs (Tapirus bairdii, T. terrestris, T. pinchaque) and the Southeast Asian Malayan tapir (Tapirus indicus); and
3. Rhinocerotidae (5 species): African white rhinoceros (Ceratotherium simum) and black rhinoceros (Diceros bicornis), plus the Asian greater one-horned (Indian) rhinoceros (Rhinoceros unicornis), Javan rhinoceros (R. sondaicus), and Sumatran rhinoceros (Dicerorhinus sumatrensis).

Although living perissodactyls are outnumbered more than 13-to-1 by even-toed ungulates (Artiodactyla), the Paleogene fossil record reveals that during the Eocene and Oligocene Epochs (~56 to 23 million years ago), Perissodactyla was the dominant, most diverse clade of large terrestrial herbivores on Earth—spanning 14 families, including horned Brontotheriidae† (Megacerops), clawed gorilla-like Chalicotheriidae† (Moropus), cursorial Hyracodontidae†, and the giant Oligocene hornless rhino Paraceratherium (Indricotherium)†, which stood 4.8 meters tall at the shoulder, reached 7.5 meters in length, and weighed 15 to 20 metric tons—making it the largest land mammal in Earth's history!

1. Mesaxonic Foot Symmetry (Weight Centered on the Third Digit, Digit III): Whereas Artiodactyla carry their weight equally between two central toes (digits III and IV, paraxonic), all perissodactyls have a mesaxonic foot in which the plane of mechanical symmetry passes straight down the middle of an enlarged central third digit (digit III):
   • Tapirs (Tapiridae) retain the primitive Eocene toe formula: 4 functional toes on the front foot (digits II, III, IV, and a small lateral digit V used on soft mud) and 3 toes on the hindfoot (digits II, III, and IV);
   • Rhinoceroses (Rhinocerotidae) have 3 functional toes (digits II, III, and IV) on both front and hind feet, backed by an elastic shock-absorbing subcutaneous fat pad;
   • Equids (Equidae — Horses, Zebras, and Asses) carried mesaxonic specialization to the ultimate cursorial extreme (monodactyly): only the central third toe (digit III) touches the ground, encased in a cylindrical keratinous hoof wall, while digits I and V were completely lost and digits II and IV were reduced to vestigial bony "splint bones" fused along the upper sides of the third metacarpal/metatarsal (cannon bone) (Brianna K. McHorse, Stephanie E. Pierce, and John R. Hutchinson, 2017–2019)!
2. Saddle-Shaped Astragalus and Third Trochanter of the Femur: Unlike the double-pulley astragalus of artiodactyls, the perissodactyl astragalus has a single, deeply grooved proximal pulley for the tibia and a flat or slightly concave distal facet articulating strictly with the navicular bone, while the femur bears a prominent lateral crest—the third trochanter—for insertion of the gluteus superficialis running muscle.
3. Hindgut (Cecal–Colonic) Fermentation vs. Ruminant Foregut Fermentation: Why did large-bodied perissodactyls (horses, zebras, and white rhinos) survive alongside four-chambered ruminants on fibrous Miocene–Pleistocene C₄ savannas despite having a simple, single-chambered stomach? As analyzed in Christine M. Janis's (1976) classic eco-physiological synthesis, perissodactyls are hindgut fermenters: microbial fermentation of plant cellulose takes place behind the stomach and small intestine inside a gigantic, sacculated cecum and ascending colon (capacity >100 liters in a horse!).
   • When Ruminants Win (Moderate-Quality Forage in Limited Quantity): A ruminant cannot pass fibrous food out of its rumen until particles are chewed below ~1–2 mm, extracting higher efficiency per gram of good forage;
   • When Hindgut Perissodactyls Win (Abundant, Very-Low-Quality, High-Silica Fibrous Grass): If a cow eats coarse, lignified, silica-rich dry grass stem, its rumen becomes physically clogged with un-fermentable fiber and the cow literally starves with a full stomach! In contrast, a zebra or horse has no particle-size bottleneck: it simply accelerates gut transit time (30 h vs. 70 h), pumping massive volumes of poor-quality grass rapidly through its stomach (absorbing soluble sugars and proteins first in the small intestine before microbes can consume them!), fermenting the rapidly digestible cellulose fraction in its cecum, and voiding the indigestible lignin stems in its feces!

The 55-Million-Year Equid Fossil Evolutionary Radiation

Originally assembled by Othniel Charles Marsh (1874, 1879) and Thomas Henry Huxley and refined by George Gaylord Simpson (1951) and Bruce J. MacFadden, the North American fossil record of Equidae demonstrates that horse evolution was not a straight single-file ladder ("orthogenesis"), but a bushy adaptive radiation with dozens of browsing and grazing species coexisting simultaneously:

1. Eohippus (Hyracotherium) (Early Eocene, ~55–50 Ma): Fox-terrier-sized (~5–9 kg) forest browser with an arched back, 4 toes on the forefoot and 3 on the hindfoot, and low-crowned (brachyodont) bunolophodont teeth for eating soft leaves and fruits.
2. Mesohippus and Miohippus (Late Eocene–Oligocene, ~37–25 Ma): Sheep-sized (~25–45 kg); 3 toes on both fore- and hindfeet (with the central digit III noticeably larger); premolars molarized into a continuous grinding row of six cheek teeth.
3. Parahippus and Merychippus (Miocene, ~20–10 Ma): As global cooling expanded silica-rich grasslands across North America, the Anchitheriinae clade (Anchitherium, Hypohippus) remained 3-toed forest browsers, while the Equinae clade (Merychippus) evolved high-crowned (hypsodont), cementum-reinforced grazing teeth, elongated fused radius–ulna and tibia–fibula limb segments, and spring-loaded central digit III hooves (with side toes II and IV suspended clear of the ground during normal trotting).
4. Pliohippus, Dinohippus, and Equus (Late Miocene–Pleistocene, ~10–0.01 Ma): Complete reduction of lateral toes into internal splint bones (monodactyly). Dispersing across the Bering Land Bridge into Eurasia and Africa (~2.6 Ma) and across the Isthmus of Panama into South America (*Hippidion*†), wild Equus strangely went completely extinct across its native North and South American homeland at the end of the Pleistocene (~10,000–8,000 years ago), before being reintroduced to the Americas by Spanish conquistadors in the 1500s!

Why Do Zebras Have Black-and-White Stripes? (The Biting Tabanid Fly Hypothesis Confirmed)

For 150 years since Charles Darwin and Alfred Russel Wallace, biologists proposed four competing hypotheses for why the three species of zebras (Equus quagga, E. grevyi, E. zebra) evolved high-contrast black-and-white stripes over darkly pigmented skin: (1) camouflage in tall grass, (2) "motion dazzle" confusing lions during a chase, (3) convective cooling vortices across black vs. white stripes, or (4) deterring blood-sucking vector flies. Through exhaustive field mapping and controlled behavioral experiments dressing domestic horses in striped, black, white, and checkered coats (Tim Caro, Martin How, Ádám Egri, Gábor Horváth, and colleagues, 2012–2019):

• Lions capture zebras at the exact same rate as unstriped antelopes, and striped barrels do not stay cooler than grey barrels in the African sun;
• Instead, stripe density across equid species correlates strictly with the geographic intensity of blood-sucking horseflies (Tabanidae) and tsetse flies (Glossina) (which transmit lethal trypanosomiasis [nagana] and African horse sickness, to which zebras have high mortality!). High-speed videography by Caro et al. (2019) revealed that tabanid flies approach horses and zebras at the same rate from a distance, but within the final 50 cm before landing, high-spatial-frequency stripes disrupt the fly's optic-flow landing deceleration reflex—causing flies to crash into the zebra or veer off without landing, reducing tabanid biting rates by >75%!

What is the Difference Between a Rhinoceros Horn, a Bovid Horn, and a Deer Antler?

Unlike bovid horns (which have an internal bony core) and deer antlers (which are solid dermal bone), a rhinoceros horn contains zero bone whatsoever! A rhino horn is an epidermal outgrowth glued directly onto a roughened nasal/frontal bone boss and composed entirely of thousands of tightly packed, vertical tubular filaments of α-keratin (similar to horse hooves and human fingernails) cemented together by interfilamentous matrix keratin. Furthermore, as shown by Lawrence M. Witmer, Tobin L. Hieronymus, and colleagues (2006) using CT scans, the center of a rhino horn is mineralized with calcium phosphate (hydroxyapatite) and pigmented with UV-protective melanin—so as the outer perimeter of the horn is worn down by sun exposure and rubbing against bushes, the harder calcified/melanized core remains standing as a self-sharpening conical point!

1. Janis, C. (1976). The evolutionary strategy of the Equidae and the origins of rumen and cecal digestion. Evolution, 30(4), 757–774. doi:10.1111/j.1558-5646.1976.tb00957.x
2. Biewener, A. A. (1998). Muscle-tendon stresses and elastic energy storage during locomotion in the horse. Comparative Biochemistry and Physiology Part B, 120(1), 73–87. doi:10.1016/S0305-0491(98)00024-800024-8)
3. Hieronymus, T. L., Witmer, L. M., & Ridgely, R. C. (2006). Structure of white rhinoceros (Ceratotherium simum) horn investigated by X-ray computed tomography and histology with implications for growth and external form. Journal of Morphology, 267(10), 1172–1176. doi:10.1002/jmor.10465
4. Steiner, C. C., & Ryder, O. A. (2011). Molecular phylogeny and evolution of the Perissodactyla. Zoological Journal of the Linnean Society, 163(4), 1289–1303. doi:10.1111/j.1096-3642.2011.00752.x
5. McHorse, B. K., Biewener, A. A., & Pierce, S. E. (2017). Mechanics of evolutionary digit reduction in fossil horses (Equidae). Proceedings of the Royal Society B, 284(1861), 20171174. doi:10.1098/rspb.2017.1174
6. Caro, T., Argueta, Y., Briolat, E. S., Bruggink, J., Kasprowsky, M., Lake, J., ... & How, M. (2019). Benefits of zebra stripes: Behaviour of tabanid flies around zebras and horses. PLOS ONE, 14(2), e0210831. doi:10.1371/journal.pone.0210831