Absorbing heat
We know that dinosaurs, such as Stegosaurus undoubtedly developed very effective methods of absorbing heat quickly from the sun and thereby maintained higher than ambient body temperatures. Mammals, however, evolved from an earlier group of reptiles (the Synapsids, often referred to as mammal-like reptiles). One of the earliest groups of synapsids, the pelycosaurs, also had similar adaptations to the Stegosaurusdinosaurs. Dimetrodon, grew long spines from its backbone which supported a sail of skin which must have served as a solar panel in a similar way to the Stegosaur's plates. Although the pelycosaurs persisted for a considerable time their sail like crests disappeared in later forms. In seems extremely unlikely that even if there was a warming of the climate, the forces of evolution would allow an animal to lose such a valuable method of heat control unless it was able to replace it with an adaptation that is more efficient. It has been hypothesized that the pelycosaurs and their successors, the therapsids, were to some degree endothermic.
Dimetrodon, grew long spines from its backbone which supported a sail of skin which must have served as a solar panel in a similar way to the Stegosaur's plates.
