Andrewsarchus

Type skull compared to those of other carnivores, from Osborn (1924)

The only known skull was found at a locality in the lower levels of the middle Eocene Irdin Manha Formation of Inner Mongolia,[1][2] by the paleontological assistant Kan Chuen Pao during the spring of the second year (1923) of the Central Asiatic Expeditions (CAE) of the AMNH, led by the explorer and naturalist Roy Chapman Andrews. The skull is now on display at the American Museum of Natural History in New York.[3]

The genus name was dedicated to Andrews by Henry Fairfield Osborn, and it derives from the surname "Andrews" + Ancient Greek: ἀρχός archos ('leader', 'chief' or 'commander'). The species epithet mongoliensis refers to Inner Mongolia, the region where the type material was found.[1]

It was classified in the clade Mesonychia due to the similarity in structure between its teeth and skull with those of other mesonychid species known from complete skeletons; however, much of this was based only on Osborn's original publication, and more recent studies have found it to have no special mesonychid affinities, instead grouping it with various artiodactyl clades. Indeed, one study (Spaulding et al.) has not only found them to be closer to entelodonts, but as kin to Whippomorpha in the clade Cetacodontamorpha.[4]

Osborn (1924) declared Andrewsarchus as the largest terrestrial mammalian carnivore known on the basis of the length of the skull, which he used to estimate its size by comparing it to the mesonychid Mesonyx.[1] However, since the known morphology of Andrewsarchus is entelodont-like and consequently very different to mesonychids in habits and likely in body proportions, according to Szalay and Gould (1966) if a size estimate has to be made it would be more appropriate to follow the proportions of entelodonts.[5]

Life restoration

The type skull of Andrewsarchus mongoliensis (AMNH 20135) is 83.4 cm (32.8 in) in basal length,[1] with a long snout comprising 60% of that measurement. The orbits of the eyes are set low and widely separated from one another by the snout, the sagittal crest is small, and the articulation for the mandible is shallow.

Andrewsarchus mongoliensis has a complete placental tooth formula with 3 incisors, 1 canine, 4 premolars and 3 molars in each side of the jaws, as in entelodonts. The incisors are arranged in a semicircular configuration, the second and third premolars are elongated and single-cusped, the crowns of the molars are heavily wrinkled, and the first and second molars are much more heavily worn than the precedent and subsequent teeth. In fact, the molars are so similar to those of entelodonts it has been suggested that had they been found in isolation, they would have been classified as such.[5] There are also greatly enlarged second incisors, as big as the canines, which despite not being preserved can be estimated from the diameter of their tooth sockets. They were proportionally small compared to the whole dentition and the size of the skull according to Szalay and Gould, contrary to Osborn's description.[5]