ADREWSARCHUS: THE TERROR OF THE EOCENE

Andrewsarchus mongoliensis
, was a mammal that lived during the Eocene epoch, roughly between 45 and 36 million years ago. It had a long snout with large, sharp teeth and flat cheek teeth that may have been used to crush bones. Because Andrewsarchus is only known from a single skull, whether it was an active predator or a large scavenger is open to debate, as is its exact time range.
Andrewsarchus is named for the famous explorer and fossil hunter Roy Chapman Andrews. It was discovered in June 1923 by Kan Chuen Pao, a member of Andrews' expedition, at a site in the Gobi Desert in Mongolia known as Irdin Manha [variants: Erdeni-Mandal and Erdenemandal ('jeweled mandala')] on the third Asiatic expedition that was led by Andrews and sponsored by the American Museum of Natural History. The skull is now on display at the American Museum of Natural History in New York; the lower jaw was not found. 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 Cetancodonta in their Cetacodontamorpha.

The appearance and behavioral patterns of Andrewsarchus are virtually unknown and have been topics of debate among paleontologists ever since it was first discovered. All that is known about Andrewsarchus comes chiefly from the single meter-long skull found in Late Eocene sediments in what is now Mongolia. New theories indicate that the teeth of Andrewsarchus may have been blunt and uncharacteristic of predators. Its diet could have been more omnivorous than carnivorous, consisting of carrion, bones, rooted plants, or mollusks rather than freshly killed meat. As a scavenger, Andrewsarchus may have gained access to freshly killed carcasses by using its formidable size to scare away other smaller predators and scavengers. Until more fossil evidence that may provide insight into these areas of uncertainty is uncovered any reconstructions remain highly speculative.

Andrewsarchus possessed some of the strongest jaws ever evolved in a land mammal, able to bite through large bones if needed. To judge from its immense jaws, and the coastal location of the fossils, Andrewsarchus may have fed on beached primitive whales, shellfish and hard-shelled turtles, and contemporary large mammals at various periods during its existence. Toward the end of the Eocene very large mammals (such as the brontotheres) had evolved in the region of Central Asia.

Despite the enormous jaws and very sturdy teeth, Andrewsarchus did not have teeth adapted for the carnassial shear. Judging by its size, the animal most likely fed on large animals such as the extinct brontotheres, which were among the largest herbivorous mammals at the time, possibly both hunting them, and scavenging already dead carcasses.\
 If plant material was also eaten, Andrewsarchus would have had a lifestyle similar to entelodonts.
Due to the food requirements of Andrewsarchus, sources of large animals are thought to have been present in Central Asia during the Eocene, most likely on a year-round basis. When the Indian subcontinent collided with Asia during the Late Eocene/Early Oligocene, this event caused the uplift of the Himalaya mountains while closing off the eastern Tethys Ocean, thus changing weather patterns, and caused Central Asia to dry out, ultimately resulting in a dramatic faunal turnover. It is suggested that Andrewsarchus became extinct due to this orogeny.