[4] Like other mesonychids, they had large heads and long necks in proportion to their body size, and cranial material is most often preserved.

Although modern Carnivora have more complex brains, their ancestors did not; Mesonyx species would have been intelligent animals for their time.

[4] 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.

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