Temporal range: Middle Eocene, 48–41 Ma
|A cast of the only known skull of Andrewsarchus, at the American Museum of Natural History, New York City.|
Andrewsarchus (//) is an extinct genus of mammal that lived during the middle Eocene epoch in what is now Inner Mongolia, China. Only one species is usually recognized, A. mongoliensis, known from a single skull of great size discovered in 1923 during the expeditions of central Asia by the AMNH. Generally classified as a mesonychid since its original description, most recent studies recover it as an artiodactyl, in one study specifically, as a member of the clade Cetacodontamorpha, closely related to entelodonts, hippos and whales.
The genus name was dedicated to Roy Chapman Andrews by Osborn and it derives from the surname "Andrews" + Greek: ἀρχός (archos), "leader", "chief" or "commander". The species epithet mongoliensis refers to the region where the type material was found, Inner Mongolia.
The only known skull was found at a locality in the lower levels of the middle Eocene Irdin Manha Formation of Inner Mongolia, by the paleontological assistant Kan Chuen Pao during the spring of the second year (1923) of the Central Asiatic Expeditions (CAE) of the AMNH, leadered by the famous explorer and naturalist Roy Chapman Andrews. The skull is now on display at the American Museum of Natural History in New York. 
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 skeleton, 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 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.
The type skull of Andrewsarchus mongoliensis (AMNH 20135) is 83cm in basal length 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 saggital 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, like 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 alike to those of entelodonts that it has been suggested that had they been found in isolation they would have been assigned to entelodonts. Between the dental features unique to A. mongoliensis we find greately 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 (1966) contra Osborn (1924).
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 comparing it to the mesonychid Mesonyx, however, since the known morphology of Andrewsarchus is entelodont-like and concecuently 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 appropiate to follow the prorpotions of entelodonts.
- Osborn, H. F. (1924). "Andrewsarchus, giant mesonychid of Mongolia" (PDF). American Museum Novitates (146).
- Wang Y Q, Meng J, Beard C K; et al. (2010). "Early Paleogene stratigraphic sequences, mammalian evolution and its response to environmental changes in Erlian Basin, Inner Mongolia, China". Science China Earth Sciences 53 (12): 1918–1926.
- AMNH (7 March 2013). "Andrewsarchus, "Superb Skull of a Gigantic Beast"". Retrieved 18 February 2014.
- Spaulding, M; O'Leary MA, Gatesy J (2009). "Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution". PLoS ONE 4 (9): e7062.
- Szalay, F. S.; Gould, S. J. (1966). "Asiatic Mesonychidae (Condylartha, Mammalia)". Bulletin of the American Museum of Natural History 132: 127–174.
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