family of extinct South American marsupial mammals occurring from the Late Paleocene Epoch into the Pliocene (from about 63.5 to 5 million years ago). It is named for the genus Borhyaena; hyena-like specimens of this genus, found in early Miocene rocks of Argentina (26,000,000 years old), had large skulls and heavy crushing teeth. Not all borhyaenids, however, were hyenoid. Thylacosmilus was a Pliocene-aged marsupial counterpart of the sabre-toothed tiger. Many other forms were wolflike and foxlike.