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Eocene epoch

Carnivorans evolved after the extinction of the non-avian dinosaurs 66 million years ago. Around 50 million years ago, or earlier, in the Paleocene, the carnivorans split into two main divisions: caniforms (dog-like) and feliforms (cat-like). By 40 Mya, the first identifiable member of the dog family had arisen. Named Prohesperocyon wilsoni, its fossilized remains have been found in what is now the southwestern part of Texas. The chief features which identify it as a canid include the loss of the upper third molar (part of a trend toward a more shearing bite), and the structure of the middle ear which has an enlarged bulla (the hollow bony structure protecting the delicate parts of the ear). Prohesperocyon probably had slightly longer limbs than its predecessors, and also had parallel and closely touching toes which differ markedly from the splayed arrangements of the digits in bears.

The canid family soon subdivided into three subfamilies, each of which diverged during the Eocene: Hesperocyoninae (about 39.74–15 Mya), Borophaginae (about 34–32 Mya), and Caninae (about 34–30 Mya). The Caninae are the only surviving subfamily and all present-day canids, including wolves, foxes, coyotes, jackals, and domestic dogs. Members of each subfamily showed an increase in body mass with time and some exhibited specialized hypercarnivorous diets that made them prone to extinction.