Andrewsarchus

Andrewsarchus is an extinct genus of ungulate that lived during the Middle Eocene in China. It contains two species, A. mongoliensis and A. crassum. It was formerly placed in the families Mesonychidae or Arctocyonidae, but is now the sole member of a distinct family, Andrewsarchidae. Only known from a largely complete skull as well as isolated teeth, it is notable for being estimated as the largest terrestrial, carnivorous mammal, but that status has been disputed.

Taxonomy
Andrewsarchus mongoliensis was named by Henry Fairfield Osborn in 1924 for a partial skull from the Irdin Manha Formation of Inner Mongolia. The genus was named in honor of Roy Chapman Andrews, the leader of the expedition on which it was discovered, with the Ancient Greek archos (ἀρχός, "ruler") added to his surname. A second species, A. crassum, was named by Ding Suyin and colleagues in 1977 for two premolars from the Dongjun Formation of Guangxi, but its specimens are too fragmentary to establish broader affinities of the genus.

Paratriisodon henanensis was named by Minchen Chow in 1959 for a partial skull and mandible, a fragmentary maxilla, and several isolated teeth from the Lushi Formation of Henan. A second species, P. gigas, was named by Chow and colleagues in 1973 for a molar also from the Lushi Formation. Three molars and an incisor from the Irdin Manha Formation were later referred to P. gigas. Both species are considered junior synonyms of A. mongoliensis.

Andrewsarchus is the only member of the family Andrewsarchidae, which was named by Frederick Szalay and Stephen Jay Gould in 1966. It was originally named as a subfamily of Mesonychidae, but was later raised to the rank of family.

Phylogeny
Andrewsarchus was originally classified as a member of the family Mesonychidae and Paratriisodon was originally classified as a member of the family Arctocyonidae. Andrewsarchus was then reassigned to its own family Andrewsarchidae, but retained within the order Mesonychia. After that, Paratriisodon was synonymized with Andrewsarchus and the latter was moved to Arctocyonidae. More recently, it has been recovered as a member of the clade Cetancodontamorpha, being most closely related to entelodonts, hippos, and whales.

Below is a simplified cladogram based on the results of Spaulding et al. (2009) and Yu et al. (2023).

Description
When first describing Andrewsarchus, Osborn believed it to be the largest terrestrial, carnivorous mammal. Based on the 83.4 cm length of the A. mongoliensis holotype skull, and using the proportions of Mesonyx, he estimated a total body length of 3.82 m and body height of 1.89 m. However, considering cranial and dental similarities with entelodonts, Szalay and Gould proposed that it had proportions more like them than mesonychids and that Osborn's estimates were inaccurate. They also suggested that it was likely an omnivore. These ideas are also supported by the close phylogenetic relationship between Andrewsarchus and entelodonts.