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Mesonychia ("middle ") is an extinct taxon of small- to large-sized carnivorous related to . Mesonychians 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. In Asia, the record of their history suggests they grew gradually larger and more predatory over time, then shifted to scavenging and bone-crushing lifestyles before the group became extinct.
Mesonychians probably originated in China, where the most primitive mesonychian, Yantanglestes, is known from the early Paleocene. They were also most diverse in Asia, where they occur in all major Paleocene . Since other predators, such as Creodonta and Carnivora, were either rare or absent in these animal communities, mesonychians most likely dominated the large predator Ecological niche in the Paleocene of eastern Asia.
One genus, Dissacus, had successfully spread to Europe and North America by the early Paleocene. Dissacus was a jackal-sized predator that has been found all over the Northern Hemisphere, but species of a closely related or identical genus, Ankalagon, from the early to middle Paleocene of New Mexico, were far larger, growing to the size of a bear. A later genus, Pachyaena, entered North America by the earliest Eocene, where it evolved into species that were at least as large. Mesonychians in North America were by far the largest predatory mammals during the early Paleocene to middle Eocene.
Mesonychians varied in size; some species were as small as a fox, others as large as a horse. Some members of the group are known only from skulls and jaws, or have fragmentary postcranial remains. But where skeletons are known, they indicate that mesonychians had large heads with strong jaw muscles, relatively long necks, and robust bodies with robust limbs that could run effectively but not rotate the hand or reach out to the side. An unrelated early group of mammalian predators, the creodonts, also had unusually large heads and limbs that traded flexibility for efficiency in running; large head size may be connected to inability to use the feet and claws to help catch and process food, as many modern carnivorans do. Some mesonychians are reconstructed as predatory (comparable to canids), others as or carnivore-scavengers with bone-crushing adaptations to their teeth (comparable to the large hyenas), and some as Omnivore (comparable to pigs, humans, or black bears). They may not have included (comparable to felids); their teeth were not as effective at cutting meat as later groups of large mammalian predators. In some localities, multiple species or genera coexisted in different ecological niches. There is evidence to suggest that some genera were sexually dimorphic. Some genera may need revision to clarify the actual number of species or remove ambiguity about genera (such as Dissacus and Ankalagon).
These "wolves on hooves" were probably one of the more important predator groups in the late Paleocene and Eocene ecosystems of Europe (which was an archipelago at the time), Asia (which was an island continent), and North America. Mesonychian dentition consisted of molars modified to generate vertical shear, thin blade-like lower molars, and carnassial notches, but no true carnassials. The molars were laterally compressed and often blunt, and were probably used for shearing meat or crushing bones. The largest species are considered to have been . Many species are suspected of being fish-eaters, though some of these reconstructions may be influenced by earlier theories that the group was ancestral to .
The order is sometimes referred to by its older name Acreodi.
A recent study found mesonychians to be basal eu most closely related to the "Arctocyonidae" Mimotricentes, Deuterogonodon and Chriacus. "Triisodontidae" may be paraphyletic.Sarah L. Shelley, Thomas E. Williamson, Stephen L. Brusatte, Resolving the higher-level phylogenetic relationships of “Triisodontidae” (‘Condylarthra’) within Placentalia, October 2015, Society of Vertebrate Paleontology (abstract)
Most paleontologists now doubt that whales are descended from mesonychians, and instead suggest mesonychians are descended from basal , and that are descended from advanced ungulates (Artiodactyla), either deriving from, or sharing a common ancestor with, (the semiaquatic ancestors of hippos). However, the close grouping of whales with hippopotami in cladistic analyses only surfaces following the deletion of Andrewsarchus, which has often been included within the mesonychians. One possible conclusion is that Andrewsarchus has been incorrectly classified. The current uncertainty may, in part, reflect the fragmentary nature of the remains of some crucial fossil taxa, such as Andrewsarchus.
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