Bear-dogs varied in size and shape throughout their evolutionary history. The earliest members of the group were smaller and more agile, while later amphicyonids grew much bigger. In general, they had dog-like body proportions but a heavier, bear-like build. Also like bears but differently than dogs, amphicyonids were plantigrade. That means they walked with their foot bones flat against the ground and not on the tips of their toes, something that humans do too.
Though the bear-dogs are now extinct, their history is one of success. One of the oldest amphicyonids known is Daphoenus, a long-bodied, short-legged predator the size of a coyote that appeared in North America around 42 million years ago, not too long after the non-avian dinosaurs went extinct, in a time which mammals were rapidly diversifying. They were likely similar to modern-day bush dogs, resting in burrows and pursuing prey in short bursts of speed.
From then on, the group expanded into Asia and Europe. Cynodictis appeared 37 million years ago, and its remains were found from China to Britain. Still small and with a low-slung body, it lived like a fox, hunting small animals like rodents and taking advantage of the carcasses left by the bigger predators with which it lived.