Archaeotherium (, 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 they have also been found in the John Day Basin of Oregon and the Trans-Pecos area of Texas. Archaeotherium 's fossils come from North America, between the Priabonian and Rupelian stages of the Eocene and Oligocene (35–28 million years ago). Up to fifteen species of Archaeotherium have been identified, which are divided into three subgenera. One contains the type species, A. mortoni, among others; another contains very large taxa formerly named Megachoerus and Pelonax; and the last contains A. calkinsi. Archaeotherium was distinguished from most entelodonts by having an unusually long snout and large jugal flanges, extensions of the zygomatic arches. 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. On average, Archaeotherium weighed around 150–250 kg, and stood around 1.2 m at the shoulder. The very biggest specimens may have been around 500-600 kg and stood around 1.5-1.6 m at the shoulder.

Taxonomy

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 they were later synonymised. Similarly, Pelonax, named by Edward Drinker Cope in 1874, was synonymised with Archaeotherium, though was subsequently resurrected in the form of a subgenus containing A. lemleyi.

Classification

Archaeotherium belongs to the family Entelodontidae, a family whose exact taxonomic position has long been disputed. In 1955, Charles Lewis Gazin suggested that entelodonts were relatives, if not direct descendants, of Helohyus. 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, such as that of Yu et al. (2023), 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 terms of entelodont interrelationships, in 2007, Scott Foss suggested that Archaeotherium represents a late stage of a continuous North American lineage, beginning with Brachyhyops and terminating in Daeodon. Conversely, Yu et al. (2023) recovered Archaeotherium as belonging to a polytomy with Brachyhyops and a clade consisting of Entelodon and Paraentelodon.

Historical synonymy with Entelodon

In 1909, Olof August Peterson suggested that Archaeotherium and Entelodon be distinguished solely by geographical factors, 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 proposed 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: An additional species, "A." coarctatum, was formerly assigned to Archaeotherium. In 2007, it was assigned to a new genus, Cypretherium.

Description

Size

Archaeotherium stood about 1.2 m tall at the shoulder and around 2 m long. Adults generally weighed 150–250 kg. The largest specimens, though much less common, which had been described under the name Megachoerus had skulls up to 80 cm long, stood about 1.5-1.6 m tall at the shoulder and weighed up to 500-600 kg. A. calkinsi is noted as being robust beyond what is observed in other entelodonts.

Skull

The skull length of Archaeotherium mortoni, measured from the tip of the premaxilla to the back of the occipital condyles, is about 27% of the total head–body length, based on the partly restored skeleton of SDSM 3346: this is 8% smaller, proportionally, than in Daeodon. The genus Archaeotherium is characterised by having a particularly elongated face and prominent jugal flanges, which were extensions of the zygomatic arches analogous to those of hippopotamuses. 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 a combination of a deep jaw and knob-like mandibular tubercles, respectively. Unusually among entelodonts (with the exception of Brachyhyops), Archaeotherium 's pterygoids bore a midline synarthrosis, meaning they were essentially incapable of movement. This was likely an adaptation to resisting stresses exerted on the back of the skull by the jaw muscles. The articular surface of Archaeotherium 's dentary condyle is comparable in orientation to that of modern carnivorans. As such, it may have been able to open its jaws to a maximum gape of 109°.

Dentition

The first and second incisors of Archaeotherium mortoni are relatively large, procumbent, and well-spaced. The canines are long and pointed. Together, the canines and incisors formed an effective device for grasping and puncturing. There are slight gaps between the premolars (intermolar diastemata), unlike in Daeodon. 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.

Palaeobiology

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 like those of camelids, and the toes could spread. This, in conjunction with hypothetical foot pads, may have helped them 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.

Intraspecific interactions

Young Archaeotherium had fairly small temporalis muscles, which increased in size as the animal matured. This suggests that, as in other entelodontids, the jugal flanges and strong jaws of the genus were involved in adult social interactions over obtaining and processing food. Further, the jugal flanges of Archaeotherium differ in shape and size among specimens. Some interpret this as evidence of sexual dimorphism, whilst others interpret it as evidence of intraspecies variation. Assuming the former, it is similar to that seen in giant forest hogs. Thus, it can be reasonably assumed that Archaeotherium 's jugals supported larged preorbital glands used for chemical communication, signalling readiness for mating. Healed bite-marks on the frontals, lacrimals and maxillae, as well as an A. scotti specimen with a damaged left cheek flange, suggest that at least some Archaeotherium populations engaged in agonistic facial biting. In such confrontations, one animal may have attempted to fit the head of the other in its mouth and bite down with the canines and incisors, similar to modern dromedary camels. The anterior tubercles of entelodonts such as Archaeotherium may have supported toughened skin, which would act as a buffer during such interactions.

Feeding and diet

Like other entelodonts, Archaeotherium is generally seen as a mixed feeder, with an inclination towards carnivory. The type species, A. mortoni, bore specialisations for biting and chewing resistant objects, such as hard fruits, stems, and bones. The jaws were enormously strong and operated largely by chopping, though exhibited enough lateral movement for the molars to grind objects. Archaeotherium 's teeth show signs of uneven wear, indicating that it often favoured chewing on one side of the jaw. Unlike Daeodon, the teeth do not exhibit so-called "piecrust fractures". This suggests that, whereas Daeodon was more specialised for the consumption of large carcasses, Archaeotherium was not. Tooth wear patterns suggest that its front teeth were often used to strip leaves from plants, though there is a lack of soil scratches that would indicate rooting in the ground. Scott Foss, in 2001, interpreted this as the result of feeding on plants such as lianas. Archaeotherium 's dentition was incapable of slicing meat, like most extant mammalian predators: rather, it compensated using its strong neck musculature, using its head and neck together to tear off chunks of flesh. Fossil evidence suggests that in North America they may sometimes have hunted the early camelid Poebrotherium. Bite marks on the cervical vertebrae of the camels suggest that Archaeotherium ran parallel to its prey while hunting, delivering crushing a bite to the neck and the back of the skull. The prey animal's body was then severed in half, and the rear section was consumed. The front half was stockpiled in a food cache to be consumed later.

Palaeoecology

Archaeotherium lived from the Chadronian to the Arikareean North American Land Mammal Ages (NNLMAs). The earliest occurrences of the genus are from the Chadronian of the White River Group. The last occurrence is from the late early Arikareean of either the White River or the John Day Formation in Oregon, roughly 28 million years ago. Specimens of Archaeotherium have also been recovered from the Trans-Pecos area of Texas.