| This is not completely certain. There are similarities between the warm-bloodedness of mammals and birds that might not be coincidences. An alternative possibility is that some ancestor of all extant amniotes already had some kind of warm-bloodedness. Later, in the ancestors of crocodiles, turtles and lizards (including snakes), the capacity for generating heat has been abandoned, in order to save energy and allow them to survive with much less food than birds and mammals. There is some evidence in favor of this hypothesis, besides the similarities in temperature regulation between birds and mammals. For instance, in contrast with the amphibians, the lizards, snakes, crocodiles and turtles are dependent on high internal temperatures for their bodies to function correctly. Because they cannot generate internally the required heat, they must take it from the environment, so most of them can live only in warmer climates and they may need every day to do things like basking in solar light, before any sustained activity. It is also known that already the ancestors of pterosaurs and dinosaurs had their bodies covered by some kind of hair, which might have had the purpose of thermal insulation. Later, that hair has evolved into the feathers of birds and of those dinosaurs more closely related to them, while in the biggest dinosaurs the hair or the feathers were lost, like also in elephants and other such big animals where cooling becomes the problem, not heating. At least for some dinosaur or pterosaur fossils bone growth patterns are consistent with high body temperature. In the line of synapsid amniotes leading to mammals, high body temperature also appeared earlier than any ancestor of the extant mammals, but it is not known when exactly this happened. In conclusion, perhaps warm-bloodedness (homeothermy) has appeared independently in the ancestors of birds and of mammals, but perhaps not, it could have also appeared before the split of amniotes into these 2 branches. In general, this is the most difficult in guessing the past evolution, when you have a feature that exists only in some of the descendants of a common ancestor, is this because of independent gains of that feature, or because all the groups that do not have the feature have lost it. Most mistakes made in the past about the evolution of living beings have been caused by underestimating the probability of multiple losses, because it was wrongly believed that evolution goes from simple to complex. Now we know that losses and simplifications are extremely frequent, typically more frequent than the development of complex features, which happens independently more seldom than assumed in the past. |