Mesonychids are medium-to-large-sized carnivorous mammals closely related to even-toed ungulates (pigs, camels, goats, cattle) and cetaceans (whales and dolphins) that lived in the Paleogene, evolving soon after the extinction of the dinosaurs 65 million years ago and going extinct around 30 million years ago. Mesonychids had a superficial resemblance to wolves, though they had hooves instead of paws, and much larger heads. They were also larger in general, with some species exceeding the size of bears. In North America, mesonychids were the largest carnivores for millions of years.
Mesonychids were typically larger than there other two groups of carnivores that they shared the planet with at the time: the miacids (which evolved into modern carnivorans) and creodonts, another carnivorous group which mesonychids were once classified as. Mesonychids originated in Asia (which was an island continent) and quickly spread across much of the northern hemisphere, including Europe (which was an archipelago at the time), and North America (which was separated from South America by the ocean).
Though mesonychids have skulls similar to canids, the two are quite different. Mesonychids had longer, flatter skulls, and an exaggerated sagittal crest, a skull crest which would have been used as an anchor for jaw muscles. The mesonychids' large sagittal crests indicates that they would have had tremendous biting power, with some species more powerful than any living carnivorans. Mesonychids' canine teeth were slightly longer and thinner than canids', better at piercing flesh but slightly worse at holding onto the kill.
Mesonychids had triple-tipped mandibles which suggests that some of them were adapted for hunting fish, like their relatives the toothed whales. Male mesonychids had thick carnassals which would have been useful for crushing bone, pointing to a scavenger lifestyle. Evolutionarily, mesonychids are thought to be descended from condylarths, the first hoofed animals. For many years, it was thought that whales, which are mammals, descended from mesonychids, but more recent fossil finds make it seems more likely that they descended from the ancestors of hippos.