Mesonychidae ("Middle Claws") is a supposedly extinct family of medium to large-sized omnivorous-carnivorous mammals closely related to artiodactyls (even-toed ungulates) which were endemic to North America and Eurasia during the Early Paleocene to Late Eocene living from 65—33.9 mya, existing for approximately 31.1 million years.
The mesonychids were an unusual group of condylarths with a specialized dentition featuring tri-cuspid upper molars and high-crowned lower molars with shearing surfaces. They were once viewed as primitive carnivores, like the Paleocene family Arctocyonidae, and their diet probably included meat and fish. In contrast to this other family of early mammals, the mesonychids had only four digits furnished with hooves supported by narrow fissured end phalanges. 
They first appeared in the Early Paleocene, undergoing numerous speciation events during the Paleocene, and Eocene. Mesonychidae faired very poorly at the close of the Eocene epoch, with only one genus, Mongolestes, survived into the Early Oligocene epoch.
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. Throughout the Paleocene and Eocene, several genera, including Dissacus, Pachyaena and Mesonyx would radiate out from their ancestral home in Asia and into Europe and North America, where they would give rise to new mesonychid genera. These animals would have migrated to North America via the Bering land bridge.
The term "mesonychid" is often used to refer to any of the various members of the order Mesonychia, though most experts prefer to use it to refer to the members of the family Mesonychidae, with many experts using the term "mesonychian" to refer to the order as a whole.
Mesonychidae was named by Cope (1880). It is not extant. Its type is Mesonyx. It was assigned to Creodonta by Cope (1880); to Creodonta by Cope (1889); to Carnivora by Peterson (1919); to Mesonychia by Carroll (1988) and Zhou et al. (1995); and to Cete by Archibald (1998).
* Genus Ankalagon
A theoretical surviving population of mesonychids seems the most likely creature to account for a series of sightings and over 60 deaths in France, in the 1760s. Most witnesses described the Beast of Gevaudan as fitting to a description such as this: The Beast is a quadruped about the size of a horse. It reminds witnesses of a bear, hyena, wolf and panther all at once. It has a long wolf-like or pig-like snout, lined with large teeth. The ears are small and round, lying close to the head. The neck is long and strong. The tail somewhat resembles the long tail of a panther, but it is so thick and strong that the Beast uses it as a weapon, knocking men and animals down with it. Anyone struck by the tail reports that it hits with considerable force. The feet of the Beast are the hardest to describe. Some say that it has cloven hooves, or that each digit is tipped with a hoof. Others say that the claws are so heavy, thick and formidable that they merely resemble hooves.
1. ^ PaleoBiology Database: Mesonychidae, basic info
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