Mesonychid

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

Mesonychids probably originated in Asia, where the most primitive mesonychid, Yangtanglestes, is known from the early Paleocene. They were also most diverse in Asia, where they occur in all major Paleocene faunas. Since other carnivores, such as the creodonts and condylarths, were either rare or absent in these animal communities, mesonychids most likely dominated the large predator niche in the Paleocene of Asia.

One genus, Dissacus, had successfully spread to Europe and North America by the early Paleocene. Dissacus was a jackal-sized carnivore that has been found all over the Northern Hemisphere, but its daughter genus, Ankalagon, from the early to middle Paleocene of New Mexico, was far larger, growing to the size of a bear.<ref name=O'Learya>O'Leary, Maureen A.; Lucas, Spencer G.; Williamson, Thomas E. (2000). "A new specimen of Ankalagon (Mammalia, Mesonychia) and evidence of sexual dimorphism in mesonychians". Journal of Vertebrate Paleontology 20 (2): 387–93. doi:10.1671/0272-4634(2000)020[0387:ANSOAM]2.0.CO;2. PMID 4524103. </ref> Species of the later genus, Pachyaena, entered North America by the earliest Eocene, where they evolved into huge species surpassing even Ankalagon in size. Mesonychids in North America were by far the largest predatory mammals during the early Paleocene to middle Eocene.