The fossils suggest that the of that time were small animals that would have looked more like mongooses than any dogs alive today and were well adapted to that habitat. Their forelimbs were not specialized for running, retaining the flexibility to grapple with whatever meal unwittingly walked by. But beginning just a few million years later, the global climate began cooling considerably. In North America the rocky mountains had reached a threshold of growth that made the continental interior much drier. The dense forests slowly gave way to open grasslands.
The research team examined the elbows and teeth of 32 species of dogs spanning the period from 40 million years ago to two million years ago. They noticed clear patterns in those bones at the museum. At the same time that climate change was opening up the vegetation, dogs were evolving from ambushers to pursuit-pounce predators like modern coyotes or foxes, and ultimately to those dogged, follow-a-caribou-for-a-whole-day pursuers like wolves in the high latitudes.
Janis said, "The elbow is a really good proxy for what carnivores are doing with their forelimbs, which tells their entire locomotion repertoire. While the herbivores of this time were evolving longer legs, the predator evolution evident in the study tracked in time directly with the climate-related changes to habitat rather than to the anatomy of their prey species. After all, it was not advantageous to operate as a pursuit-and-pounce predator until there was room to run. There is no point in doing a dash and a pounce in a forest. They'll smack into a tree."
The study appeared in Nature Communications.