This predatory mammal had heavy jaws and blunt robust teeth more suited for crushing bones, than slicing meat.

The biggest species, Palaeonictis peloria (meaning "terrible ancient weasel") is known from an incomplete jaw that must have measured over 20 centimetres (7.9 in) in length.

By the end of the early Eocene (50 million years ago), Palaeonictis disappeared from North America, and by the early Eocene (55 million years ago) the last species of P. gigantea had vanished from Europe.

[4] In fact, the entire family Oxyaenidae had become extinct worldwide (although its sister group Hyaenodonta continued to thrive for a while).

This is traditionally assumed to be due to increased competition from miacids and nimravids belonging to the more successful order Carnivora, which eventually replaced earlier carnivorous mammal clades in the later Neogene as the world's top predators, though no evidence of direct competition is known, and carnivorans may have simply filled vacated niches.