Monday, March 24, 2014

Andrewsarchus mongoliensis

Andrewsarchus mongoliensis is one of the more famous mammals of the Eocene. It was long hailed as the largest mammalian predator to have ever lived until it became recognized that there were entelodonts and bears that grew just as large.

A cast of the only skull known of Andrewsarchus mongoliensis, on display
at the American Museum of Natural History in New York City. Wiki
Andrewsarchus was named for renowned explorer and fossil hunter Roy Chapman Andrews who led an expedition to the Gobi Desert in 1932, which was sponsored by the American Museum of Natural History. The actual fossil remains (see picture above) were discovered by Kan Chuan Pao, a member of Andrews' team. The original specimen is on display at the American Museum of Natural History and remains the only known evidence of this species to this day.

This species was once believed to be part of the family Mesonychidae, a group of predatory ungulates that lived from the early Paleocene to the late Eocene, and for a time was nicknamed "the giant Mesonychid of Mongolia". However, recent research has confirmed that this animal is actually a basal Artiodactyl, an early offshoot of the branch that gave rise to whales, hippos, and entelodonts. 

Habitat & Distribution
At the time Andrewsarchus lived, the former island continent of India had collided with southern Asia, closing off the eastern part of the Tethys Sea and pushing up the Himalayan Mountain Range. The resulting rain shadow resulted in intense monsoon seasons to the south, but robbed central Asia of much of its rainfall and caused it to become drier. Forests receded, causing drier open woodland and savanna habitats to open up. In response to these new open conditions, herbivores started to diversify and many became larger and longer-legged, and this in turn led to an increase in predator diversity as well. Andrewsarchus was among these new predators. In fact, Andrewsarchus was the largest land predator known to have walked the Earth since the extinction of the dinosaurs 25 million years earlier.

Physical Attributes
Andrewsarchus is known only from a single skull that was 83cm long and 56cm wide at the widest point. From this skull we know that its eyes were very small and set rather low close to the mouth. This resulted in limited binocular vision and the animal's sense of sight was probably somewhat less important to it. The long nasal tube implies an extended surface area for olfactory receptors and an acute sense of smell. The animal has the complete placental mammal dental formula of I3/3 + C1/1 + P4/4 + M3/3 x 2 = 44 teeth. The rostrum was narrow but strong, with large canines and incisors. This suggests that the jaws were capable of seizing and dispatching prey its own size or slightly larger. The molars and premolars are large, blunt, and somewhat triangular in shape; a good structure for gripping and perhaps cracking open bones, but ill-suited for grinding plant matter.

Based on the size of the skull, Andrewsarchus would have measured around 1.8m (6ft) high at the shoulder and 3.4m (11ft) in head-and-body length, with a weight of slightly over 1,000kg. This estimate puts it in the same league with other great mammalian predators such as the Short-faced Bear (Arctodus simus), the Terminator Pig (Daeodon shoshonensis), and the giant hyaenodont Megistotherium osteothlastes

Because the body of Andrewsarchus has never been found, any attempts to reconstruct it are purely speculative. The below restoration is based on similarly-sized land predators, specifically Daeodon shoshonensis and Arctodus simus (with a little hippo reference for the feet). As a large carnivore adapted to life in open country, this animal would have benefited from long, slender legs and a short torso allowing it to cover great distances with minimal energy.The large head would have been supported by a powerful neck and shoulders. As an artiodactyl, its toes would have ended in blunt hooves, which would have protected its foot bones when running over rough terrain. In the hot climate of the late Eocene, an animal this size is likely to have had a coat of short, sparse fur similar to that of large bovines in southern Asia and Africa of the genera Bubalus and Bos. As with other carnivores, thick skin and coarse fur around the neck region would have protected it from bites during aggressive encounters between males.

Ecology & Behavior
Based on the skull and dentition alone, the picture that emerges is of a carnivore that could attack and kill its own prey, but was also an active scavenger and frequent kleptoparasite. With its well-developed sense of smell it could detect the smell of blood or decaying flesh from miles away. Potential prey animals included brontotheres and rhinos. As the largest predator in its environment it could dominate any carcass it came across. Powerful jaws and sturdy teeth enabled it to smash open very large bones to utilize the highly nutritious marrow within. Other predators that lived in the same environment as Andrewsarchus included the creodont Sarkastodon mongoliensis, and the Mesonychids Harpagolestes immanis and Mongolestes robustus

References & Further Reading
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. doi:10.1371/journal.pone.0007062 <>

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