Hesperocyoninae is the oldest of the three sub-families of the canid family. The first group evolved in North
America about 40 million years ago. Research done on fossil evidence from this group leans towards the hypothesis
that this animal looked like a cross between a weasel and a fox. The hesperocyonines lived only in North America
and became extinct approximately fifteen million years ago.
Borophaginae evolved about 34 million years ago and consisted of larger hyena-like animals with huge jaw muscles
and sturdy teeth. Similar to the hesperocyonines, the borophagines were confined to North America. Extinction for
these animals happened approximately 2.5 million years ago.
The Caninae sub-family is the only canid sub-family still in existence and contains all the living canids in the
world including the domesticated dog. The world-wide distribution of modern canids, unlike the hesperocyons and
borophagines who were restricted to North America, is the result of their migrations in the last five million years
during the late Cenozoic.
The Evolution Of Dogs by Bryan Perfetto. Geology 201L, Professor Wayne Henderson. California State University Fullerton, 2005.