The other lineage of amniotes were our ancestors: the therapsids. The therapsids differed from reptiles at the time in one important way: they developed warm-bloodedness. Therapsids were the first vertebrates to evolve the ability to use energy to generate their own internal heat.* This was a gamble. They would require far more food to survive, but in return they had the ability to hunt at any time, including the cold nights when their reptile cousins lay immobile—an easy feast offered on a Permian platter.
