''Mesonyx'' ("middle claw") is a genus of extinct

mesonychid Mesonychia ("middle claws") is an extinct taxon of small- to large-sized carnivorous ungulates related to artiodactyls. Mesonychids first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely ...

mesonychia Mesonychia ("middle claws") is an extinct taxon of small- to large-sized carnivorous ungulates related to artiodactyls. Mesonychids first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely ...

n mammal: fossils of the various species are found in Early to Late Eocene-age strata in the

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and Early Eocene-aged strata in China, 51.8—51.7 Ma ( AEO). 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. ''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 The hoof (plural: hooves) is the tip of a toe of an ungulate mammal, which is covered and strengthened with a thick and horny keratin covering. Artiodactyls are even-toed ungulates, species whose feet have an even number of digits, yet the rumi ...

. The long skull had a relatively large sagittal crest 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. Though modern Carnivora have more complex brains, their ancestors did not; ''Mesonyx'' species would have been intelligent animals for their time. Andrewsarchusscull2

''Mesonyx'' specimens have been unearthed in

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and China. Like other mesonychids, they have 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). Another specimen of ''Mesonyx uintensis'' is known from the Upper Eocene of northern Utah. An additional two species – ''Mesonyx uqbulakensis'' and ''M. nuhetingensis'' – have been described from the early Eocene Arshanto Formation in China. Size estimates of ''Mesonyx'' were used to generate the often-quoted idea that '' Andrewsarchus'' 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.

Phylogeny

In the generally accepted

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

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