Archaeotherium

"Megachoerus" redirects here. Not to be confused with Megalochoerus.

Archaeotherium Temporal range: Late Eocene to Middle Oligocene,
Skeleton in American Museum of Natural History
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Family:	†Entelodontidae
Genus:	†Archaeotherium Leidy, 1850
Type species
†Archaeotherium mortoni Leidy, 1850
Other species
†A. crassum Marsh], 1873a †A. marshi Troxell, 1920 †A. scotti Sinclair, 1921 †A. wanlessi? Sinclair, 1921 †A. palustris Schlaikjer, 1935
Synonyms
Entelodon mortoni Leidy, 1853 Choerodon Troxel, 1920 (preoccupied) Megachoerus Troxel, 1920

Archaeotherium (Ancient Greek: αρχαιοθήριον, meaning "ancient beast") is an extinct genus of entelodont artiodactyl endemic to North America during the Eocene and Oligocene epochs (35—28 mya). Archaeotherium fossils are most common in the White River Formation of the Great Plains, but it has also been found in the John Day Basin of Oregon and the Trans-Pecos area of Texas.

Taxonomy

Partial skull and holotype skull fragment as figured by Joseph Leidy in 1853

Early history

Archaeotherium was named by Joseph Leidy in 1850, based on a partial skull (ANSP 10609) recovered from the White River Formation of the Great Plains. Its generic name derives from the Greek αρχαιο ("ancient") and θήριον ("beast"). The type species, A. mortoni, was named after Samuel George Morton, then the president of the Academy of Natural Sciences of Philadelphia. Three years later, Leidy synonymised Archaeotherium with Entelodon, though in 1857 would assign it to Elotherium under the name Elotherium imperator. A posthumous 1915 paper by Edward Drinker Cope listed it as Entelodon imperator. In a 1909 revision of Entelodontidae, Olaf August Peterson, resurrected Archaeotherium as a genus. The genera Choerodon and Megachoerus were split off from Archaeotherium by Edward Leffingwell Troxell in 1920, though were later synonymised. Specimens of Archaeotherium have since been recovered from the Turtle Cove member of the John Day Formation in Oregon, and the Trans-Pecos area of Texas.

Classification

Archaeotherium belongs to the family Entelodontidae. The exact taxonomic position of the entelodontids has been disputed. Often, they have been considered either close relatives of Suidae (pigs), as the sister taxon to a clade containing suids and tayassuids (peccaries), or as true suids themselves. Recent phylogenetic analyses suggest that the family lies close to Andrewsarchus, anthracotheres, hippopotamuses and whales, as part of Cetancodontamorpha.

Description

Skull cast in Staatliches Museum für Naturkunde Karlsruhe

Size

Archaeotherium was about 1.2 m (3 ft 11 in) tall at the shoulder and around 2 m (6 ft 7 in) long. Adults weighed 150–250 kg (330–550 lb). The largest specimens, though much less common, which had been described under the name Megachoerus had skulls up to 80 cm (31 in) long, stood about 1.5–1.6 m (4 ft 11 in – 5 ft 3 in) tall at the shoulder and weighed up to 500–600 kg (1,100–1,300 lb). Several Archaeotherium morphotypes have been identified: the containing the holotype (ANSP 10609) and plesiotype (USNM 146), one containing "Megachoerus" and "Pelonax", and one that is robust beyond what is observed in other entelodonts.

Skull

Archaeotherium is characterised by having a relatively elongated face. The jugal has a lightly developed posterior process that does not form a buttress on the front margin of the glenoid cavity. The Archaeotherium specimens initially assigned to "Megachoerus" and "Pelonax" bear massively enlarged jugal flanges and kob-like mandibular tubercles respectively. Like in other entelodonts, at the junction of the maxilla and jugal, there is an infraorbital foramen. The palatine process of the maxilla is convex.

Dentition

The first and second incisors of Archaeotherium mortoni are relatively large and well-spaced. The canines are long and pointed. The premolars are transversely compressed, with high, pointed crowns. The third premolar is narrower than the fourth, and is convex labially. It is double rooted, has a single cusp, and only has slight posterior cingulum. The fourth premolar is triple-routed and described as cuboidal in shape. The third molar lacks a hypocone. No sexual dimorphism is observed in the dentition.

Paleobiology

Archaeotherium lived in forested and riverbank environments before the evolution of grasslands. Like all entelodonts, the genus had typical artiodactyl legs but lacked specializations for fast running; though it supported its weight on cloven hooves, the foot bones remained unfused, and the toes could spread as camelid feet do. This structure, unique to entelodonts, may have helped the living animal move on soft ground. The head was unusually large, and the high spines on the vertebrae above the shoulders supported strong neck muscles and tendons to handle the weight of the head. The brain was tiny, but had relatively large olfactory lobes, suggesting that the animal had a keen sense of smell.

Reproduction

Archaeotherium, like all entelodonts, possessed huge jugal projections. These projections are sexually dimorphic, with the males having much larger jugals than the females. Due to dimorphism, the function of the expanded jugals was likely used more for display than diet. This same sort of dimorphism can be seen in giant forest hogs, so it can be reasonably assumed that entelodont jugals supported large preorbital glands used for chemical communication signaling the intention to mate. Males would have fought for dominance through non-lethal intraspecific biting as seen by bite marks. Their mandibular tubercles are also dimorphic, and may have offered protection during combat.

Feeding and diet

Restoration of Archaeotherium eating roots, by Robert Bruce Horsfall, 1913

The largest (and type) species, A. mortoni, has been analyzed as an omnivore with specializations for biting and chewing resistant objects, such as hard fruits, stems, and bones. Like all entelodonts, the teeth and jaws resemble no living animal, though there are some similarities to peccaries, pigs, bears, predatory carnivores, rhinos, and bone-crushing scavengers. There is a full dentition. The canines, premolars, and molars were all large and heavily enameled, and show heavy wear. The jaws were enormously strong and operated largely by chopping, though they could move laterally enough for the flat molars to grind. There are no blades or notches on any teeth for slicing meat, which are seen in all living and fossil taxa of predators that can chew. Archaeotherium did not have the ability to slice its meat like most modern predators, but they would have used their strong neck musculature, using their entire head and neck to rip off chunks instead. Fossil evidence suggests that in North America they may sometimes have hunted the early camel Poebrotherium, severing the body in half and crushing and swallowing the foot-long rear section Bite marks on the cervical vertebrae of the camels suggests they attacked by running alongside their prey, snapping at their necks. The remains of these camels have been found together, implying Archaeotherium brought its kills to caches for later consumption.

Unlike both grazers and hypercarnivores, Archaeotherium teeth frequently show uneven wear that indicates the animal favored chewing on one side of the jaw, usually the result of tooth damage from hard foods. But the teeth do not show the bone-eating "piecrust fractures" seen in the larger Daeodon (Dinohyus), which may have specialized more at eating large carcasses. Tooth wear patterns suggest the interlocking front teeth of A. mortoni were frequently used to strip leaves from plants, but do not show soil scratches from rooting in the ground.

Adult Archaeotherium had very large temporalis muscles, but they were normal-sized in juveniles and only developed as the animal matured. This suggests the expanded cheekbones and extreme jaw strength of the genus may have been involved in adult social behavior more than eating. It is also possible that younger animals had softer diets, or Archaeotherium had significant parental care. Like other entelodonts, the jaws had an unusually wide gape; one entelodont skull shows the animal survived a bite mark near the orbit by another of the same species. Adult entelodonts may have had aggressive jaw-gaping displays and biting fights like living hippos, which have the same adaptation; in male camels, similar wounds result when one animal gets a rival's head between its jaws and bites down with the canines. The gape may also have been used to grab and position large, hard food objects like bones or nuts between the jaws to be cracked by the rear teeth, as in pigs and peccaries.