Mesonychid

Mesonychids were the first mammalian carnivores after the extinction of the dinosaurs.
These "wolves on hooves" are an extinct order of carnivorous mammals, closely related to artiodactyls.
Mesonychids first appeared in the early Palaeocene with the genus "Dissacus". They went in decline at the end of the Eocene, and became extinct in the early Oligocene.
Overview.
At the start of the Palaeocene, there were (so far as we know) no large carnivores on land. The surviving mammals and birds evolved into many of the ecological niches previously occupied by dinosaurs and pterosaurs. This process is called adaptive radiation. The mesonychids were the first group to become larger carnivores.
They soon had competition. By the later Palaeocene there were several other groups of land carnivores. The giant flightless bird "Gastornis" appeared in the Upper Palaeocene, and the Creodonts had also evolved. The Creodonts became an important order, and would certainly have been serious competitors of the mesonychids.
Most important of all, the modern order Carnivora became significant in the Eocene. They were a sister group to the mesonychids, and started as small forms in the Palaeocene. Once they increased in size they would have been direct competitors of the mesonychids.
The broken-up Gondwana gave rise to southern continents which the early placental mammals could not get to. Africa and India eventually bumped into Eurasia, and South America sometimes had land connection to North America. All this lead to 'faunal interchange'. Australasia developed its own carnivores, which became extinct only in modern times.
Size and habits.
Large size was common. The famous "Andrewsarchus" had a skull a metre long, which is much larger than that of the modern Kodiak bear.
Mesonychid teeth consisted of molars which were laterally compressed and often blunt and were probably used for shearing meat or crushing bones. Many species are suspected of being fish-eaters, and the largest species may have been scavengers.