Ungulates are four-footed mammals with their toes modified into a hard, flexible covering called the hoof. They are currently placed under the clade Euungulata (true ungulates), which is further divided into two orders: the Perissodactyla (odd-toed ungulates) and the Artiodactyla (even-toed ungulates).
While the former includes equines, rhinoceroses, and tapirs, the latter comprises cattle, antelope, pigs, giraffes, camels, sheep, deer, and hippopotamuses. The cetaceans (whales, dolphins, porpoises, and narwhals) are also considered ungulates because they have evolved from early artiodactyl ancestors, even though they do not bear hooves on their toes.
Ungulates have special features, such as hooves, cranial appendages, and modified dentition, to adapt to their different habitats and lifestyles.
The tips of an ungulate’s toes are modified into hooves, which comprise a rubbery sole and a hard wall composed of thick keratin. They grow continuously and usually bear the entire body weight. The two forelimb bones, the radius and ulna, are fused in most modern ungulates, thereby restricting their rotation.
Perissodactyls have a mesaxonic foot, where the body weight is distributed on the third toe of all limbs. The number of toes in this group has reduced compared to their ancestors. In contrast, the body weight of terrestrial artiodactyls is distributed on the third and fourth toes of all limbs (paraxonic foot). Most artiodactyls have cloven hooves, split into two small toes called dew claws located further up on the limbs.
The earliest cetaceans possessed two characteristic ankle bones: the astragalus and cuboid. On the contrary, modern cetaceans, like whales and dolphins, have modified their forelimbs into pectoral fins, and the hind parts are internal and highly reduced to enable them to swim underwater. They also possess the tail fin or the fluke for propulsion and retain all the digits on their limbs.
The ungulates belonging to the superfamily Cervoidea have evolved a pair of characteristic cranial appendages or horns that often vary in shape and size but retain a simple basic structure. They are bony protrusions (twisted, spiral, or fluted in form) covered in a permanent keratinous sheath. However, in rhinos, the horn is present on the top of the nasal ridge and is completely made of keratin (lacks bony element).
Members of the family Bovidae bear unique horns (often more than two in number) that distinguish them from other cervoids. The horns of female bovids are smaller than those of males and are thought to have evolved for defense against predators through crypsis (avoidance), while in males, these appendages are used for sexual selection.
In deer (family Cervidae), most males possess antlers that develop from an attachment point on the skull called a pedicle. The only cervid females (does) that have antlers are caribou and reindeer, but these appendages are smaller than those in males. Some does of other species can also produce antlers due to increased testosterone levels. These appendages grow faster than any other mammal bone, starting from their cartilaginous tips that ossify into a bony structure. While growing, the antler is covered with highly vascular skin called velvet, providing nutrients to the bone. This skin is shed, and the bone cells (osteoclasts) die once the antler’s growth is complete.
In members of the Family Giraffidae (giraffes and male okapi), the skull is modified into antler-like ossicones, similar to the horns of antelopes and cattle. However, these structures remain covered with skin or fur.
Ungulates, such as pronghorn, possess a slender, laterally flattened blade of bone emerging from the frontal bones of the skull. This structure is covered by a keratinous sheath that sheds and regrows annually.
Most ungulates have reduced canines, except for narwhals, which possess remarkably long canines. They also have specialized molars, such as bunodont teeth with low, rounded cusps and high-crowned hypsodont teeth. It is often believed that these mammals switched from browsing to grazing diets, and thus, bunodonts are gradually replaced by hypsodont teeth.
Some ruminants, camels, and toothed whales lack upper incisors and instead have a dental pad specialized for browsing. Baleen whales have a keratinized bristle-like filter-feeding apparatus called the baleen for consuming tiny organisms in water.
Ungulates derive their name from the Late Latin term ‘ungulātus,’ meaning ‘having claws or hooves.’ They initially belonged to the clade Ungulata under Paenungulata (also a clade) but have been reclassified under the newer clade Euungulata in 2001. This new clade contains two extant orders, the Perissodactyla and Artiodactyla, along with an extinct order, Mesonychia. The three orders, Hyracoidea (hyraxes), Sirenia (sea cows), and Proboscidea (elephants), were initially grouped within the clade Ungulata but are now placed under another clade Afrotheria. This conclusion was based on molecular and morphogenetic studies (2009) that suggested hyraxes, sea cows, and elephants were more closely related to each other and to sengis, tenrecs, and golden moles than to the ungulates.
Whether Euungulata is an evolution-based cladistic group, taxon-based phenetic group, or folk taxon (similar, but not necessarily related) remains to be determined. However, some studies indicate that the Perissodactyla and the Artiodactyla form a monophyletic group, sharing a common ancestor, and are closely related to either the carnivorans and the pangolins in the clade Fereuungulata or to bats. In contrast, some other studies consider either perissodactyls or artiodactyls to be closely related to bats. Although internal relationships between the different ungulate groups remain uncertain, all ungulates are presently listed under 26 extant families.
The two groups of ungulates, Perissodactyls and Artiodactyls, first appeared during the late Paleocene Epoch (66 to 56 million years ago). Some scientists believe modern ungulates descended from a group called condylarths, the earliest known member being Protungulatum, an ungulate that co-existed with the last of non-avian dinosaurs. However, some scientists refuse to consider it to be true ungulate. The dinoceratans, the first large herbivorous mammals, are also currently considered within the true ungulate assemblage and thus might have been ancestors of present-day ungulates.
Perissodactyls (order Perissodactyla) are believed to have evolved from members of the family Phenacodontidae, which comprises small, sheep-like animals with reduced digits (first and fifth). Around 55 million years ago, by the beginning of the Eocene Epoch, they started to diversify and spread to different continents worldwide.
By the middle of the Eocene (45 million years ago), the rhinos migrated to the Americas, and only three out of fifteen families survived and could stabilize their populations. Around this time, the largest perissodactyl, Paraceratherium, attained twice the weight of an elephant (17 tons).
With time, the ancestral ungulates diverged into two large groups: Anthracobunidae and Desmostylia. The desmostylians were amphibious quadrupeds with massive limbs and a short tail and weighed more than 200 kilograms (440 lbs). Owing to their herbivorous diet, they developed highly distinctive molars, with each cusp modified into hollow columns. Similarly, the anthracobunids were amphibious too and probably inhabited marshy environments.
Over the years, the desmostylians and anthracobunids evolved further and are currently divided into the major groups meridiungulates, equids, rhinocerotoids, and tapirids.
Meridiungulates
All meridiungulates, including tapir-like pyrotheres and astrapotheres, litopterns, and notoungulates, are believed to have evolved from animals like Hyopsodus.
Equids
The earliest equids evolved in the early Eocene (54 million years ago). They were herbivorous fox-like animals with three-toed hind feet and four-toed front feet. However, these ancestors witnessed a reduction in the number of toes and gradually developed teeth adapted for grinding.
Rhinocerotoids
Rhinocerotoids diverged from ancestral perissodactyls around the early Eocene and evolved into three families, Hyracodontidae, Amynodontidae, and Rhinocerotidae, by the Late Eocene. Fossils of Hyrachyus eximus unearthed in North America resembled a tapir or small horse more than a rhinoceros.
Tapirids
Although the earliest tapirids were about half the size of modern forms and first appeared in the early Eocene, the first true tapirs appeared in the Oligocene Epoch (around 34 to 23 million years ago). By the Miocene Epoch (23 to 5 million years ago), the Asian and American tapirs diverged and migrated from North to South America during the Great American Interchange (3 million years ago).
Throughout the Oligocene, the perissodactyls dominated as large terrestrial browsers on land. However, as grass became the primary source of nutrition, the artiodactyls became better adapted to grazing and started rising in number.
This group is believed to have evolved from Arctocyonidae, a small family of condylarths comprising raccoon-like or bear-like omnivores from the Early Paleocene Epoch (about 65 to 60 million years ago). These omnivores had relatively short limbs, heavy tails, long canines, and claws and preyed on smaller animals. These ancestors soon evolved into two separate lineages: the mesonychians and the artiodactyls.
The earliest artiodactyls were short-legged and resembled present-day mouse deers or pigs. By the Late Eocene (46 million years ago), they had diversified into three suborders: Suina (including pigs), Tylopoda (including camels), and Ruminantia (including goats and cattle). However, they only occupied marginal niches around that time and became dominant only after developing complex digestive systems fit for less nutritious diet forms, like grass. While most artiodactyl groups were land dwellers, one particular lineage, the cetaceans, ventured into marine habitats.
Cetaceans
Scientists have long believed that cetaceans evolved from mesonychians since they had unusually triangular teeth. However, today, it is thought that they evolved from the same group of animals that gave rise to the hippopotamus. Around 54 million years ago, this hypothesized ancestral group had probably split into two branches, the first eventually evolving into aquatic cetaceans and the second giving rise to anthracotheres, a large family of four-legged hippopotamus-like animals. All lineages of anthracotheres (except the family Hippopotamidae) gradually became extinct during the Pliocene.
The first major mammalian predators, the mesonychians, originated in the Paleocene. While early mesonychians with five digits had plantigrade locomotion (entire feet rested flat on the ground), they later developed four digits that ended in tiny hooves and resorted to digitigrade locomotion (walking on the digits). Eventually, only one genus, Mongolestes, survived into the Oligocene.
Ungulates can inhabit a wide range of habitats, ranging from mountains and ocean depths (cetaceans) to grasslands and deserts.
Although most terrestrial ungulates are herbivores that graze on grass, others, such as pigs, peccaries, hippos, and duikers, are omnivores. On the contrary, cetaceans, like baleen whales, are carnivores that feed on fish, krill, baby sharks, and other mammals, such as seals.
As herbivores, many ruminants employ their gut bacteria to digest the hard-to-digest cellulose in plant matter.
According to the IUCN-World Conservation Union, most ungulates are threatened by habitat loss and trophy hunting, leading to a slow decline in their population sizes.
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