The study looked at how climate change between 30 million and 9 million years ago affected the number of mammal species in the northwestern USA. They found that species richness in the region did not change much in response to global temperature changes. The peak in species richness around 15 million years ago in the Rocky Mountains may have been due to more diverse species within the mountains. This peak coincided with a warm period, but it's unlikely that temperature alone caused the increase in species. Instead, tectonic events in the region may have led to more unique species within the area.