Mesonychid

Mesonychids "Middle claws" 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.even-toed ungulates and cetaceans (dolphins and whales)

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.Benton M.J. 1997. Vertebrate palaeontology. 2nd ed, Chapman & Hall, London. The Palaeocene placental radiation, p327. This process is called adaptive radiation. The mesonychids were the first group to become larger carnivores.

They soon had competition. By the later Palaeocene,  several other groups of land carnivores had appeared. 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 had 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 joined into Eurasia, and South America sometimes had a land connection to North America. All of those factors led 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; they 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.

 References