Mesonychia

Mesonychia ("middle claws") is an extinct order of small to large-sized omnivorous to carnivorous hoofed mammals related to ungulates. Originally, it was hypothesized that mesonychians were a group of ungulates, however recent analysis now found them to be outside of the group entirely. Instead were a basal order of hoofed mammals within Pan-Euungulata. The order consisted of two families, Hapalodectidae and the more diverse and widespread Mesonychidae. It is believed that mesonychians evolved during the Late Cretaceous, at least 66.7 Ma. Mesonychids, as well as mesonychians as a whole, first appeared in fossil record in the Early Paleocene of East Asia with Dissacus rotundus, Yantanglestes, Hukoutherium, and Dissaccussium. Dissacus would later disperse into North America during the Early Paleocene, before dispersing into Northwestern Europe by the end of it. Dissacus was a jackal-sized predator, but taxon 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. Mesonychids were largest predatory mammals in North America and Europe during the Late Paleocene to Middle Eocene. In Asia, the record of their history suggests they grew gradually larger and more predatory over time, then shifted to scavenging and bone-crushing lifestyle. During the Middle Eocene, mesonychians saw a decline in diversity across their distribution. Originally, it was suggested mesonychids, such as Mongolestes, survived into the Early Oligocene. This has proven to be questionable as the Ulan Gochu Formation of Mongolia has been revised to the late Middle to earliest Late Eocene. The latest occurrence of the order was within the Ergilin Dzo Formation.