Archaeotherium

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 ("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. 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 (including the zygomatic arch). 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. 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 synonymised Megachoerus with Archaeotherium. for remains formerly assigned to Daeodon 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. By 2007, Scaptohyus was regarded as a junior synonym of Archaeotherium. It may be the same taxon as A. trippensis. 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. a family whose exact taxonomic position has long been disputed. Similarities to members of Suina were recognised as far back as 1853. Since then, entelodontids have mostly been regarded as close relatives of Suidae (pigs) within Suina/Suiformes. Below is a reproduction of the Yu et al.. cladogram of Cetancodontamorpha: }} }} }} }} }} }} }} }}|label1=Cetacodontamorpha}}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 (itself ending with Daeodon) 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.. (2023) recovered Archaeotherium as belonging to a polytomy with Brachyhyops and a clade consisting of Entelodon and Paraentelodon. ==Description==

Size The many species assigned to Archaeotherium differ dramatically in size. The type species, A. mortoni, is relatively small and slender, Skull The skull length of Archaeotherium mortoni, 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. These 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 orbits (eye sockets) were forward-facing and fully enclosed by a postorbital bar. Dentition Entelodonts such as Archaeotherium had the same general tooth morphology: large incisors and canines, triangular premolars, and small, bunodont molars. They possessed the typical placental dental formula of . this, in conjunction with hypothetical foot pads, may have helped Archaeotherium move on soft terrain. Unlike the humerus, the femur was long and relatively slender, whereas the tibia was shorter and more robust; the fibula is very much reduced, though is not co-ossified. == Palaeobiology ==

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. whilst others interpret it as evidence of intraspecies variation. 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. 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. Bite marks on the cervical vertebrae of the camels suggest that Archaeotherium ran alongside 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. Brain and senses Endocasts of Archaeotherium's skull suggest that the brain was very small. The cerebellum was small, short and narrow, and the cerebral hemispheres were proportionally very small. However, the olfactory bulbs were relatively large, indicating that Archaeotherium had a keen sense of smell. == 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. Specimens have been recovered from the White River Formation, John Day Formation, Brule Formation and Chadron Formation, as well as from the Trans-Pecos area of Texas. The latest known occurrence of the genus comes from the Turtle Butte Formation, which bears fossils of A. trippensis; Palaeoenvironment The White River Formation, from which many Archaeotherium fossils are known, The depositional environments of the Brule Formation, also part of the White River Group, are believed to have been a gallery forest and a wooded grassland, populated at least in part by hackberry trees (Celtis). The John Day Formation underwent a similar transformation to the White River Formation: the mid-Eocene Clarno Nuts Beds flora was tropical, while early Oligocene Slanting Leaf Beds held mostly temperate, deciduous taxa. Examples were maples (Acer), plane trees (Platanus), and oaks, (Quercus), along with an extinct member of the avocado family (Cinnamomophyllum). Smaller representatives of the late John Day Formation's fauna were soapberries (Dipteronia), roses (Rosa), and the evergreen shrub genus Mahonia. Archaeotherium is known to have inhabited all of these formations, preferring woodlands and open plains, though may have frequented riparian habitats. Archaeotherium is known from the White River Group (including the Brule Formation and Chadron Formation), and the John Day Formation. The Chadron Formation preserves taxa such as the brontothere Megacerops and the hyaenodont Hyaenodon. The amynodont Metamynodon is known from certain parts of the Brule Formation. The Turtle Butte Formation, from which the latest species (A. trippensis) is known, preserves the canids Enhydrocyon and Leptocyon, the nimravid Hoplophoneus, the equid Archaeohippus, the merycoidodonts Megoreodon and Paramerychyus, and the camelid "Protomeryx" (Miotylopus) leonardi. ==References==