''Mesonyx'' ("middle claw") is a genus of extinct mesonychid mesonychian mammal. Fossils of the various species are found in Early to Late Eocene-age strata in the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

and Early Eocene-aged strata in

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

, 51.8—51.7 Ma ( AEO).

Description

''Mesonyx'' species have been estimated as 1.25-1.5m (4.5–5 ft.) long in life, not including the tail. Weight estimates vary, from 20 to 55 kg (about 45-120 lbs). Like other mesonychids, the toes ended in small hooves. Like other mesonychids, they had large heads and long necks in proportion to their body size, and cranial material is most often preserved. ''Mesonyx uintensis'' from the Upper Eocene of Wyoming is described as having a total cranium length of 429 mm. (17 inches) and a facial length of 206 mm (8 inches). The long skull had a relatively large

sagittal crest A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are excepti ...

above the braincase to anchor large jaw muscles and give it a powerful bite. Brain casts show that ''M. obtusidens'' had an unusually well-developed neocortex for an Eocene mammal. Although modern

Carnivora Carnivora ( ) is an order of placental mammals specialized primarily in eating flesh, whose members are formally referred to as carnivorans. The order Carnivora is the sixth largest order of mammals, comprising at least 279 species. Carnivor ...

have more complex brains, their ancestors did not; ''Mesonyx'' species would have been intelligent animals for their time. Size estimates of ''Mesonyx'' were used to generate the proposal that ''

Andrewsarchus ''Andrewsarchus'' (), meaning "Roy Chapman Andrews, Andrews' ruler", is an extinct genus of artiodactyl that lived during the Eocene, Middle Eocene in what is now China. The genus was species description, first described by Henry Fairfield Osb ...

'' was the largest predatory land mammal that ever lived. Since ''Andrewsarchus'' is known only from a single isolated skull, the estimate of its size was based on scaling up material from ''Mesonyx.'' However, it is now known that the two genera are not closely related, their skulls indicate different lifestyles, and they likely had very different proportions in life.

Paleoecology

While this was the earliest genus of mesonychid to be named (by Edward Drinker Cope in 1872) and the group is named after it, ''Mesonyx'' was one of the most derived genera of mesonychids, evolving features for active running. These animals had a reduced sense of smell and likely relied on sight and hearing to find food. They were probably active hunters.

Distribution and habitat

''Mesonyx'' specimens have been unearthed in

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Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

and

. An additional two species – ''Mesonyx uqbulakensis'' and ''M. nuhetingensis'' – have been described from the early Eocene Arshanto Formation in China.

Phylogeny

In the generally accepted

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

by Spaulding ''et al.'' (2009), ''Mesonyx'' is classified, together with other mesonychians, outside of ungulates.

References

{{Taxonbar, from=Q289716 Mesonychids Eocene mammals of Asia Eocene mammals of North America Eocene United States Eocene genus first appearances Eocene genus extinctions Fossil taxa described in 1872 Prehistoric placental genera Taxa named by Edward Drinker Cope