Did warmbloodedness occur once, in an ancestor common to birds and mammals? If so, how does this affect the current opinion that dinosaurs were endotherms? This would make the common ancestor pre-dinosaur, presumably after the split with the lines that would remain ectotherms (fish; amphibians-reptiles).
This would mean, I think, that mammals did NOT come from amphibia, which is the popular understanding, no? They would have descended from dinosaurs, right? like modern birds?
Unless of course endothermia evolved more than once, like wings and eyes.
(Posted in GD rather than GQ because these things usually end up in GD territory.)
There are animals other than birds and mammals that are endothermic. Some fish, even some insects maintain higher body temperatures. And there are some mammals that barely qualify as “endothermic”, like sloths. But what we are learning is that the whole warm-blooded/cold-blooded thing is more complex. Some organisms maintain higher temperatures just in some tissues (usually muscles), and let the rest of their bodies match the background. Or they have other strategies for maintaining higher body temperatures…very large creatures like dinosaurs could have warmed up during the day and stayed warmed up all the time, even if they don’t expend very much metabolic energy maintaining their body temperature. A large creature has much less surface area to lose heat from, and so can stay warmer than a small creature.
And of course, different dinosaurs could have used different techniques to maintain their temperature. What works for a Brachiosaurus won’t necessarily work for a Compsognathus. Some dinosaurs might have had feathers, even if the larger ones didn’t.
Anyway, to answer your question specifically, it is thought that endothermy/homeothermy evolved multiple times. It is certainly likely that birds inherited their homeothermy from their theropod dinosaur ancestors, even if the other dinosaur branches weren’t quite as homeothermic. But mammals evolved much earlier than dinosaurs. I don’t know what research has been done on the metabolism of the early mammal-like reptiles, but they certainly didn’t have an endothermic common ancestor with dinosaurs.
Speaking of early mammals, it is one of my favorite bits of trivia that the mass extinction of the Permian ~250 million years ago wiped out the ruling mammals (OK, mammal-like reptiles), opening the way for the smaller but smarter and faster dinosaurs to take their place.
The common ancestor of mammals and birds would have been an amniote, not an amphibian. This ancestral amniote line then split, with one branch going off to become reptiles (including dinosaurs and birds), and the other going on to become mammals. Given this interpretation of the various lineages involved, homeothermy would necessarily have evolved multiple times.
Actually they evolved pretty close to one another. The earliest mammal fossil found thus far is from about 210-220 million years ago (from the Late Triassic). The earliest dinosaur is from about 235 million years ago - also during the Late Triassic.
Well, I was conflating mammals and mammal-like reptiles. But the clade that includes mammal-like reptiles (with mammals as a sub-clade of that) existed before there were dinosaurs, and mammal-like reptiles did rule the earth in Permian times.
Did you ever see that Ray Troll cartoon, with tiny little trout waiting for dinosaurs to go extinct? That’s kind of true too, since back in the Mesozoic there weren’t as many fish! The Mesozoic extinctions that killed off the dinosaurs made room in the oceans for the fish by killing off the vast diversity of marine reptiles and ammonites. Yes, the Cenozoic could be called the Age of Fishes rather than the Age of Mammals.
The clade that includes “mammal-like reptiles” would be Synapsida - the earliest members of which, it should be noted, were neither mammals nor reptiles (the phrase “mammal-like reptile” is one of those hold-overs from the idea that fish begot amphibians which begot reptiles which begot mammals). Its sister group, Sauropsida (the earliest members of which were likewise still neither reptiles nor mammals) eventually did beget Reptilia proper.
And for those who don’t already know, this guy was one of those early Synapsids which were so prominent during the Permian. That site calls it a reptile, but, again, based on modern terminology, it was not.