Mesonyx
Mesonyx is an extinct genus of mesonychid, one of the families that is part of the extinct order Mesonychia. Fossils of the various species are found in Early to Late Eocene-age strata in the United States and Early Eocene-aged strata in China, 51.8—51.7 Ma.
Taxonomy
In the generally accepted cladogram by Spaulding et al., Mesonyx is classified, together with other mesonychians, outside of ungulates.Description
Mesonyx species have been estimated as 1.25-1.5m long in life, not including the tail. Weight estimates vary, from 20 to 55 kg. 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. and a facial length of 206 mm. 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. Although modern Carnivora 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 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.