Andrewsarchus (), meaning "Andrews' ruler", is an extinct genus of artiodactyl that lived during the Middle Eocene in what is now China. The genus was first described by Henry Fairfield Osborn in 1924 with the type species A. mongoliensis based on a largely complete cranium. A second species, A. crassum, was described in 1977 based on teeth. A mandible, formerly described as Paratriisodon, probably belongs to the Andrewsarchus as well. The genus has been historically placed in the families Mesonychidae or Arctocyonidae, or was considered to be a close relative of whales. It is now regarded as the sole member of its own family, Andrewsarchidae, and may have been related to entelodonts. Fossils of Andrewsarchus have been recovered from the Middle Eocene Irdin Manha, Lushi and Dongjun Formations of Inner Mongolia, each dated to the Irdinmanhan Asian land mammal age (Lutetian–Bartonian stages, 48–38 million years ago).
Andrewsarchus has historically been reputed as the largest terrestrial, carnivorous mammal given its skull length of , though its overall body size was probably overestimated due to inaccurate comparisons with mesonychids, and it did not have the meat-slicing carnassial teeth of a hypercarnivore. Its incisors are arranged in a semicircle, similar to entelodonts, with the second rivalling the canine in size. The premolars are again similar to entelodonts in having a single cusp. The crowns of the molars are wrinkled, suggesting it was omnivorous or a scavenger. Unlike many modern scavengers, a reduced sagittal crest and flat mandibular fossa suggest that Andrewsarchus likely had a fairly weak bite force.
Taxonomy
Early history
The holotype of Andrewsarchus mongoliensis is a mostly complete cranium (specimen number AMNH-VP 20135). It was recovered from the lower Irdin Manha Formation of Inner Mongolia during a 1923 palaeontological expedition conducted by the American Museum of Natural History of New York. Its discoverer was Kan Chuen-pao, also known as "Buckshot"', a local man who trained at the AMNH and became assistant to the expedition's lead paleontologist, Walter Granger. Granger initially identified the fossil as the skull of an Entelodon. a pair of teeth (the second and third lower premolars) recovered from the Dongjun Formation of Guangxi.
left|thumb|Illustrated holotype skull of A. mongoliensis
In the 1957, Zhou Mingzhen and colleagues recovered a mandible, a fragmentary maxilla and several isolated teeth from the Lushi Formation of Henan, China, which correlates to the Irdin Manha Formation. The maxilla belonged to a skull that was crushed beyond recognition; it is likely from the same individual as the mandible. Comparisons between the two genera were drawn as far back as 1969, when Frederick Szalay suggested that they either evolved from the same arctocyonid ancestors or that they were an example of convergent evolution. Paratriisodon was first properly synonymised with Andrewsarchus by Leigh Van Valen in 1978, who did so without explanation. Regardless, their synonymy was upheld by Maureen O'Leary in 1998, based on similarities between the molars and premolars of the two genera and their comparable body sizes. The subfamily was elevated to family level by Philip D. Gingerich in 1998, who tentatively assigned Paratriisodon to it. In 1988, Donald Prothero and colleagues recovered Andrewsarchus as the sister taxon to whales. It has since been recovered as a more basal member of Cetancodontamorpha, most closely related to entelodonts, hippos and whales. In 2023, Yu and colleagues conducted a phylogenetic analysis of ungulates, with a particular focus on entelodontid artiodactyls. Andrewsarchus was recovered as part of a clade consisting of itself, Achaenodon, Erlianhyus, Protentelodon, Wutuhyus and Entelodontidae. It was found to be most closely related to Achaenodon and Erlianhyus, with which it formed a polytomy. A cladogram based on their phylogeny is reproduced below: The mandible itself is long and shallow, characterised by a straight and relatively shallow horizontal ramus. Lars Werdelin further suggested that it was a scavenger, or that it might have preyed on brontotheres.
Palaeoecology
right|thumb|upright=1.2|[[Palaeogeography of Europe and Asia during the Middle Eocene with possible artiodactyl and perissodactyl dispersal routes]]
For much of the Eocene, a hothouse climate with humid, tropical environments with consistently high precipitations prevailed. Modern mammalian orders including the Perissodactyla, Artiodactyla and Primates (or the suborder Euprimates) appeared already by the Early Eocene, diversifying rapidly and developing dentitions specialized for folivory. The omnivorous forms mostly either switched to folivorous diets or went extinct by the Middle Eocene (Lutetian–Bartonian, 48–38 million years ago) along with the archaic "condylarths". By the Late Eocene (Priabonian, 38–34 million years ago), most of the ungulate form dentitions shifted from bunodont cusps to cutting ridges (i.e. lophs) for folivorous diets.
The Irdin Manha Formation, from which the holotype of Andrewsarchus was recovered, consists of Irdinmanhan strata dated to the Middle Eocene. Andrewsarchus mongoliensis comes from the IM-1 locality, dated to the lower Irdinmanhan, from which the hyaenodontine Propterodon, the mesonychid Harpagolestes, at least three unnamed mesonychids, the perissodactyls Deperetella and Lophialetes, the omomyid Tarkops, the glirian Gomphos, the rodent Tamquammys, and various indeterminate glirians are also known. The Dongjun Formation, from which A. crassum originates, is similarly Middle Eocene. It preserves the nimravid Eusmilus, the anthracotheriid Probrachyodus, the pantodont Eudinoceras, the brontotheres Metatelmatherium and cf. Protitan, the deperetellids Deperetella and Teleolophus, the hyracodontid Forstercooperia, the rhinocerotids Ilianodon and Prohyracodon, and the amynodonts "Amynodon", Gigantamynodon and Paramynodon.