> It is not certain that the slow metabolism of reptiles is primitive and that birds and mammals have developed homeothermy independently. Reptiles are just as

> It is not certain that the slow metabolism of reptiles is primitive and that birds and mammals have developed homeothermy independently. Reptiles are just as evolved as mammals. So their metabolism is not primitive, it's just not high-intensive. > Because there are certain similarities between the homeothermy of birds and that of mammals that seem less likely to be coincidences, there exists an alternative hypothesis, that homeothermy and fast metabolism has already evolved in the ancestor of all amniotes. Nope. Warm-blooded metabolism is not that hard to evolve, it's just that the intermediate stages are not viable in the presence of very adapted mammals with which you'll have to compete. For example, tegu lizards are optionally endothermic. They can raise their body temperature by more than 10C, they use that during the mating season. > Later, in the ancestors of crocodyles, of turtles and in that of lizards and snakes Crocodiles and snakes are just as [un]related as birds and mammals. If you want to separate birds from crocodiles, then you need to include mammals into reptiles.