Some 55.8 million years ago, Earth underwent a really dramatic heat wave known as the Paleocene-Eocene Thermal Maximum, or PETM. What happened, exactly? We don't know how it happened, but we do know that within 20,000 years, the temperature climbed by five to eight degrees Celsius (or nine to 14 degrees Fahrenheit) and persisted for 200,000 years afterwards. Many animals responded to this change by downsizing, not only to better maneuver around the spreading jungles, but also to lose more heat more quickly.
You might think that this would be good news for the carnivores. While small miacids and viverravids were around long enough to witness the PETM, they did not become the modern order Carnivora until after the fact, 42 million years ago. Until then, they'd have to stay in the shadows of two other groups of carnivores--the creodonts, a group popularized by such megapredators as Hyaenodon and Simbakubwa; and the meat-eating ungulates known as mesonychians. The former died out 11 million years ago due to overspecialization, limited mobility and smaller and simpler brains, which made them unable to deal with the changing climate from one side and the onslaught of smarter, more flexible carnivorans on the other. The latter died out early in the Oligocene probably due to competition with the creodonts.
In this alternate Earth, the PETM lasted longer than in our timeline--three to four times longer. Many lifeforms couldn't take the extra heat. Coral reefs died out, taking another 25% of all marine species with them. Ginkgophyta, Cycadophyta and Gnetophyta became extinct with no living descendants, whereas only 38% of the conifer species and one-third of all angiosperm orders were gone. And the extra heat forced many animals--like the creodonts and mesonychians--to get smaller and smaller. Would this sort of downsizing make the creodonts and mesonychians smaller and more flexible, perhaps evolve into carnivores analogous to cats, dogs, bears, pandas, skunks, weasels, raccoons, linsangs, civets, hyenas and mongeese, thus surviving well into the present day?