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Evolution artiodactyla

Artiodactyla > General of Artiodactyla

Understanding the evolutionary beginnings of the early artiodactyls, like that of the early ungulates, is hampered by an incomplete fossil record. Also, the artiodactyls appeared abruptly, along with early perissodactyls, without any clear intermediate forms between the early ungulates and the early artiodactyls. Some aspects of the evolutionary story are difficult to follow because the characteristics used to assign taxonomic position do not fossilize.

The hooves of pigs

For example, modern artiodactyls are divided into three suborders-non-ruminants, tylopods, and ruminants-based on the morphology of their digestive tracts, soft internal structures that are not preserved in fossils. The oldest known fossils, clearly referable to artiodactyls, are in early Eocene deposits from Holarctica (Asia, Europe, and North America). These earliest artiodactyls were relatively abundant and widespread, and include Diacodexis and related genera in the Dichobunidae or Diacodexeidae. All were small mammals about the size of a rabbit or hare, weighing probably less than 11 lb (5 kg). They are considered to be early artiodactyls because they had a double-pulley astragalus (part of the ankle joint), which is a defining characteristic of this order, as well as other limb adaptations such as increased length for cursorial locomotion.

However, the teeth of Diacodexis were still bunodont (low-crowned with rounded cusps), suggesting omnivorous food habits, and the skull shows no other traits diagnostic of artiodactyls. The sudden and widespread appearance of the early artiodactyls at the beginning of the Eocene about 55 million years ago (mya) suggests that they might have evolved elsewhere other than Holarctica. Perhaps they arose in Africa, India, or Central or South America and entered the northern continents only when physical or climatic barriers disappeared. What is even less certain are the ancestors of these early artiodactyls from amongst the known fossil condylarths. So far, the closest condylarths to Diacodexis are the raccoon-like arctocyonids of the middle Paleocene.

They were also small, being probably no more than 11 lb (5 kg) with long tails and teeth, suggesting an omnivorous diet. There is a tentatively identified arctocyonid similar to Chriacus, a primitive oxyclaenid condylarth from the Paleocene, as being closest to the oldest artiodactyls so far discovered. The family Dichobunidae, to which Diacodexis probably belonged, is the most primitive group of artiodactyls discovered so far. They are placed in the suborder Paleodonta, along with the closely related Leptochoeridae and the Entelodontidae. The entelodontids were much more advanced than either of the other two families and resembled giant pigs. In one genus, Archaeotherium, their elongated skulls had characteristic processes protruding from the jugal bone, as well as bony knobs on the lower jaw, reminiscent of the modern African warthog (Phacochoerus africanus). The incisors were blunt and heavy, while the canines were robust and capable of inflicting serious injury. The small molars were almost piglike and, along with the premolars, were well spaced along the jaw. Their limbs had between two and three digits, with separate metapodials, although the ulna and radius were fused. Paleodonts have been found mainly in Europe, but also North America. They appeared in the early Eocene and became extinct by the end of the Miocene. The suborder Ancodonta, another group of primitive, presumably non-ruminant artiodactyls, includes a rather loose grouping of three families: Anoplotheriidae, Anthracotheriidae, and Caenotheriidae. The anoplotheres were medium-sized ungulates that became extinct in the Oligocene. Anthracotheres, which probably evolved in the Eocene and flourished in the Oligocene, began as dog-sized mammals that evolved to hipposize and are thought to have eaten soft plants and lived a semiaquatic life in swamps. Their remains have been found in Africa, Asia, Europe, and North America, and representatives of this family lasted until the Pleistocene. In the early Miocene (about 18 mya), they are believed to have led to the modern hippopotamuses. The caenotheres were small, four-toed ungulates ranging in size from rabbits to small antelope that became extinct in the Miocene. Although no representatives of these three families exist today, genetic evidence supports the idea that hippos did not descend from the Old World representatives of Tyassuidae as previously suspected. They are more likely to have originated from within the suborder Ancodonta and, based on morphological characteristics, thought to have evolved from the anthracotheres.

Two families, the Agriochoeridae and Merycoidodontidae, are grouped together in the suborder Oreodontae. Their remains have been found only in deposits from Central and North America. The Merycoidodontidae were a very diverse group of small- and medium-sized, stocky-built herbivores, the largest of which was said to be about the size of a wild boar. They were highly successful having appeared in the late Eocene, flourished in Oligocene and Miocene, before becoming extinct in the Pliocene. Early forms had four prominent toes on the fore and hind feet, with a small almost vestigial fifth toe on the fore feet; in later forms, this fifth toe was lost. Their teeth showed interesting and characteristic modifications. There was no diastema (gap), but the lower canines had become incisor-like, while the first premolars had replaced the lower canine in form and function. In most forms, the orbit was closed and the skull often relatively large compared to the rest of the body so that they resembled modern peccaries in many ways. At least two forms showed skull characteristics suggestive of a proboscis. Most members of this family were plains-dwellers, but some were thought to have had an aquatic lifestyle similar to modern hippos. This is based on their skulls in which the eyes and nostrils are located high on the skull, similar to aquatic species. The Agriochoeridae, while less successful, are an equally interesting group of early oreodontid artiodactyls. They shared the same modifications of the lower canines and first premolars, but had a diastema after the upper canine and lower canine-like (caniniform) first premolars. Unlike the merycoidodontids, these animals had an open orbit and a defined saggital crest. Their lumbar region suggested an animal that could leap like a cat, and they also had a long heavy tail. They had five toes, although the first digit was much reduced, and rather than hooves, the toes terminated in claws. Despite this resemblance to carnivores, the teeth suggested that they were herbivorous. They also appeared in the late Eocene, but became extinct at the end of the Oligocene.

Toward the end of the Eocene, the world’s climate began to change and by the beginning of the Oligocene epoch 38 mya, the Northern and Southern Hemispheres experienced definite seasons. Seasonality of climate resulted in significant and predictable variation in the growth and abundance of plants. Under these new conditions, both plants and the herbivores feeding on them evolved rapidly. Artiodactyls especially began to diversify and many large species evolved, with all but the pigs and peccaries becoming obligate herbivores. Molars of herbivorous artiodactyls evolved selenodont (crescent-shaped) enamel patterns adapted to grind plant food into small particles, to be followed much later in the Miocene by hypsodont (high-crowned) cheek teeth when the grasslands became established as an important terrestrial ecosystem. When grasses first flourished about 20 mya in the Miocene, open savannas became a widespread ecosystem and the first specialized grazing ungulates began to appear. Browsing was not abandoned, but many species had mixed grazer-browser habits. Large pecoran ruminants, having either horns or antlers on their skulls, appeared in the early Miocene. These included cervids, bovids, giraffids, and the okapi-like palaeomerycids. By the end of the Miocene, all modern artiodactyl families were present. In the mid Eocene, helohyids such as Helohyus appeared. These were primitive artiodactyls, somewhat larger than Diacodexis and with more robust limbs. These probably gave rise to the primitive pig-like Propalaeochoerus, which appeared in the late Eocene. By the beginning of the Oligocene, about 40 mya, the pig-like mammals had split into two families, the true Old World pigs (Suidae) and the New World peccaries (Tayassuidae). Both these modern groups have bunodont, relatively low-crowned molars, although in some ancestral forms, the molars are referred to as bunoselenodont because they show similar features to the crescent-shaped molars typical of ruminants. The earliest known peccary is Perchoerus, while the first recognized true pig is Paleochoerus. The ancestor of the modern Sus is probably Hyotherium from the early Miocene, which exhibited an elongated skull and tusks oriented laterally. The earliest primitive tylopod was probably the small, four-toed Poebrodon of the late Eocene, followed in the mid Oligocene by Poebrotherium, a taller, longer-necked species with fused metapodials and each foot reduced to two toes. Subsequently, evolution in the camels saw the pads replacing hooves and a digitigrade rather than unguligrade posture. Miocene fossil footprints indicate that they had also developed a pacing rather than trotting gait. Miocene camels included Protomeryx in the early Miocene and Procamelus later in this epoch. Lama appeared to have split off at this time, but the other camels continued with the Pliocene (Pliauchenia) and then modern camels (Camelus) appeared in the late Pliocene. Camels probably originated in North America before dispersing to South America and Eurasia in the late Pliocene.

The late-Eocene Hypertragulidae showed signs of selenodont enamel and higher-crowned cheek teeth and so were probably primitive ruminants. Their upper incisors were reduced in size and, while the upper canine was little changed, the lower canine was reduced and incisiform (incisor-like). Although they had only four toes on the hind feet, they still retained five on the front, but the lateral toes were reduced in size. Their limbs were elongated, which together with their foot structure suggested they were able to run fast. The tragulids proper appeared in the Oligocene, showing a further reduction in the number of toes to four, with reduced lateral digits on both front and hind feet. The most advanced ruminants appeared sometime in the late Eocene-early Oligocene. They had further modifications of the foot bones, and more complex cheek teeth. One group of these early ruminants gave rise to the Giraffidae, Bovidae, Moschidae, Antilocapridae, and Cervidae. Eumeryx was a deer-like animal that appeared in the Oligocene and probably gave rise to the earliest giraffes and deer. At the beginning of the Miocene, two early giraffids appeared, Climacoceras and Canthumeryx, followed shortly after by Paleomeryx and Palaeotragus. The latter was short-necked, but possessed bony projections from the skull similar to the horn structures of modern giraffes. In the late Miocene, Samotherium, another short-necked giraffid appeared, after which the first members of the genus Okapia were recognized. It was not until the Pliocene that the long-necked giraffids (Giraffa) were seen. Bovids first appeared in the fossil record in the late Oligocene, however, they were represented only by teeth typical of this family. The first, more complete representative of the early bovids was Eotragus from the middle Miocene. It was perhaps similar in size and habits to modern duikers (Cephalophinae), with small horns and slender limbs. Following Eotragus, the Tragocerinae evolved later in the Miocene. This was a primitive bovid group with a variety of horn shapes. The closest modern forms to these are probably the chousinga (Tetracerus quadricornis) and the nilgai (Boselaphus tragocamelus), both from India. The earliest sheep (Oioceros) and gazelles (Gazella) made their appearance in the mid to late Miocene about 14 mya. Pachyporlax, another genus from the late Miocene and a relative of the nilgai, is thought to be related to the earliest wild cattle such as Leptobos and Parabos, which appeared in the early Pliocene.

Since the order Cetartiodactyle is not yet accepted, the Artiodactyla is considered to be divided into three suborders with 10 families. The suborder Suina contains three families: the Suidae (pigs), Tayassuidae (peccaries and javelinas), and Hippopotamidae (hippopotamuses). The Tylopoda contains only the family Camelidae (camels and llamas), while the suborder Ruminantia is comprised of the Tragulidae (mouse deer and chevrotains), Giraffidae (giraffe and okapi), Cervidae (deer), Antilocapridae (pronghorn), and Bovidae (antelopes, cattle, sheep, and goats). The modern Artiodactyla include a total of 79–81 genera and 217–223 species. A meaningful number of subspecies is difficult because many are disputed, but there are probably more than 800 recognized.

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