Archaeotherium
Archaeotherium is an extinct genus of entelodont artiodactyl endemic to North America during the Eocene and Oligocene epochs. Archaeotherium fossils are most common in the White River Formation of the Great Plains, but they have also been found in the John Day Basin of Oregon and the Trans-Pecos area of Texas. Archaeotherium
Archaeotherium was distinguished from most entelodonts by having an unusually long snout and large jugal flanges, extensions of the zygomatic arches that are characteristic of the group. The latter may be the result of sexual dimorphism or species differentiation. Healed bite marks on several Archaeotherium specimens suggest that it engaged in facial biting behaviours, similar to dromedary camels, and their faces consequently may have had thickened skin for protection. Archaeotherium had powerful neck muscles, which were supported by a bony hump comprising the first six thoracic vertebrae. Its digits were unfused, capable of spreading, and may have supported a fleshy pad, enabling it to move more effectively on soft terrain. Like other entelodonts, it had large incisors and canines, triangular premolars, and small, bunodont molars. Archaeotherium species varied in size, with A. mortoni having a skull length of, and A. zygomaticus having a skull length of.
Archaeotherium is thought to have been omnivorous, and exhibited strong adaptations for carnivory. Fossils of the early camelid Poebrotherium bearing entelodont bite marks indicate that Archaeotherium cached food, consuming the front half and saving the rear half to eat later. It was incapable of slicing meat due to its bunodont molars, and compensated for this by using its head and neck muscles together to rip off chunks of flesh. Archaeotherium mostly inhabited woodlands and open plains, though occasionally associated with riparian environments.
Taxonomy
Early history
In 1850, Alexander Culbertson collected several fossils from the area around Fort Laramie, Wyoming. His father, Joseph, presented them to the Academy of Natural Sciences of Philadelphia. Two of the fossils were named by anatomist and palaeontologist Joseph Leidy. One of them was a small skull fragment, preserving the third and fourth left premolars. It was determined to be a new species, and was designated the holotype of a new taxon, Archaeotherium mortoni. Its generic name derives from the Greek αρχαιο and θήριον. The type species, A. mortoni, was named after Samuel George Morton, then the president of the Academy of Natural Sciences of Philadelphia. Several more complete specimens were described in an 1853 paper, also by Leidy: a fragment of a mature specimen's skull, preserving in their entirety the two front molars and parts of the last molar and last premolar, all on both sides; and the rear of a young individual's skull, broken in two and missing the upper left half of the skull. Though perplexed by the anatomy it displayed, Leidy suggested that it was related to Entelodon magnus from Eurasia, if it did not represent the same taxon outright.Image:Archaeotherium skull fragments.jpg|thumb|left|upright|Partial skull and holotype skull fragment as figured by Joseph Leidy in 1853In 1873, Othniel Charles Marsh named a new species of Elotherium, E. crassum, based on remains found in Colorado. Shortly thereafter, Edward Drinker Cope named another new species, Elotherium ramosum. Subsequently, he reassigned both E. crassum and E. ramosum to a genus of their own, Pelonax. In 1951, James Reid MacDonald lumped Pelonax into Archaeotherium, though opted to retain it as a subgenus. In 2007, Scott Foss fully synonymised Pelonax with Archaeotherium.
In 1920, geologist Edward Leffingwell Troxell described a new entelodont genus, Megachoerus, as part of a series of papers discussing Marsh's entelodontid collection. In that paper, he designated M. zygomaticus as the type species. A second species, M. latidens, was also assigned. In 2007, Scott Foss synonymised Megachoerus with Archaeotherium. In the same series of papers that named Megachoerus, Troxell named the new taxon "Choerodon" calkinsi, for remains formerly assigned to Daeodon and Elotherium; however, Choerodon is preoccupied by a genus of wrasse. C. calkinsi is now regarded as a species of Archaeotherium, and may represent part of its own subgenus.
In 1922, William John Sinclair erected the new taxon Scaptohyus altidens, based on a partial skull, mandibles, and several bone fragments recovered from the Corral Draw locality of South Dakota in 1893 by R. E. Zuver. The species name, altidens, is in reference to the height of the third lower premolar. By 2007, Scaptohyus was regarded as a junior synonym of Archaeotherium.
In 1935, Erich Maren Schlaikjer named Dinohyus ''minimus, based on the symphyseal region of a juvenile's lower jaw. After comparisons with A. trippensis, D. minimus was reassigned to Archaeotherium in 1998. It may be the same taxon as A. trippensis''.
Proposed synonymy with ''Entelodon''
As far back as 1853, Joseph Leidy, the describer of Archaeotherium, suggested that it may have been part of the same genus as Entelodon magnus. In 1857, he reassigned Archaeotherium mortoni to the genus Elotherium without comment under the binomial name Elotherium imperator. A posthumous paper by Edward Drinker Cope, published in 1915, listed the same taxon as Entelodon imperator. In a 1909 revision of Entelodontidae, Olaf August Peterson resurrected Archaeotherium as a genus. He suggested that Archaeotherium and Entelodon could be distinguished by geography, as the former was North American and the latter was Eurasian. In 1940, William Berryman Scott and Glenn Lowell Jepsen noted strong similarities between the two genera, though they stopped short of synonymising them due to the incompleteness of the latter. In 1979, the relationship between the two genera was re-examined by French palaeontologist Michel Brunet. He contended that the differences between Archaeotherium and Entelodon were insufficient, and that the two genera should be synonymised; in this case, Entelodon, being named earlier, would take priority. This total synonymy has not been followed by subsequent authors, though Scott Foss noted that it remained a topic for investigation.Inner systematics
Many species have been assigned to Archaeotherium over the years. In 2007, Foss divided Archaeotherium into three subgenera: Archaeotherium proper, subgenus A, and subgenus B. Subgenus A consists predominantly of very large species formerly referred to as Megachoerus and Pelonax, whereas Subgenus B consists of A. calkinsi, a species known from a single specimen from the John Day Formation of Oregon that has features of both Archaeotherium and Daeodon. Foss suggested the possibility of elevating them to genus level, though did not do so in that work.A list of species according to work of Foss is as follows:
| Taxon | Proposed subgenus | Author of taxon | Taxon publication year |
| A. altidens | Subgenus A | Troxell | 1920 |
| A. calkinsi | Subgenus B | Sinclair | 1905 |
| A. caninus | Subgenus A | Troxell | 1920 |
| A. crassum | Archaeotherium | Marsh | 1873 |
| A. latidens | Subgenus A | Troxell | 1920 |
| A. lemleyi | Subgenus A | Macdonald | 1951 |
| A. marshi | Archaeotherium | Troxell | 1920 |
| A. mortoni | Archaeotherium | Leidy | 1850 |
| A. palustris | Archaeotherium | Schlaikjer | 1935 |
| A. praecursor | Subgenus A | Scott & Jepsen | 1940 |
| A. ramosum | Subgenus A | Cope | 1874 |
| A. scotti | Archaeotherium | Sinclair | 1921 |
| A. trippensis | Subgenus A | Skinner et al.. | 1968 |
| A. wanlessi? | Archaeotherium? | Sinclair | 1922 |
| A. zygomaticus | Subgenus A | Troxell | 1920 |
An additional species, "A." coarctatum, was formerly assigned to Archaeotherium. In 2007, it was assigned to a new genus, Cypretherium.
Classification
Archaeotherium belongs to Entelodontidae, a family whose exact taxonomic position has long been disputed. Similarities to members of Suina were recognised as far back as 1853. Henri Marie Ducrotay de Blainville is said to have suspected that the genus belonged to a carnivoran family he dubbed Subursi, based on characters now understood to be convergent. Karl Alfred von Zittel believed that Archaeotherium was a true suid, placing it within the subfamily Achaenodontinae. William Berryman Scott, in 1940, concurred that it was a relative of Achaenodon, though placed it within "Palaeodonta". This interpretation would be followed by authors who did not regard them as suiforms, and instead preferred to associate them with cebochoerids, choeropotamids, and helohyids. In 1955, Charles Lewis Gazin suggested that entelodonts were offshoots of Helohyinae, if not direct descendants of Helohyus proper. Since then, entelodontids have mostly been regarded as close relatives of Suidae within Suina/Suiformes.More recent phylogenetic analyses, such as that of Yu et al.., diverge from the classical suiform model. Instead, they suggest that the family lies close to Andrewsarchus, anthracotheres, hippopotamuses and whales, within Cetancodontamorpha.
Below is a reproduction of the Yu et al.. cladogram of Cetancodontamorpha:In 1996, Spencer G. Lucas and Robert J. Emry suggested that Archaeotherium represented the terminal stage of a North American entelodont clade, which became extinct and was subsequently replaced by an Asian clade which entered North America near the end of the Oligocene. In 2007, Scott Foss instead proposed that Archaeotherium represents a late stage of a continuous North American lineage, beginning with Brachyhyops and terminating in Daeodon. Conversely, Yu et al.. recovered Archaeotherium as belonging to a polytomy with Brachyhyops and a clade consisting of Entelodon and Paraentelodon.