Surprised that warm-bloodedness is still an issue. Aren't TRex bones cross-sections shown clearly to be of the warm- blooded marrow type? Makes sense for a predator needed bursts of speed.
The amount of evidence for them being endothermic is about as much as for them poikilothermic.
The bone evidence specifically is not particularly convincing on its own, it based on some microscopic analysis which indicates that (some) dinosaurs grew too fast to be poikilothermic but it’s far for being conclusive.
Other counter evidence is that some species were just too darn big to be endothermic and at that size would have high enough internal temperatures to cook themselves.
So far we also didn’t find a single species of dinosaurs even the ones that are closest to birds with respiratory turbinates which are structures within the skull that act like the HRV/ERV in your house’s ventilation system preventing loss of humidity and heat whilst breathing.
The lack of respiratory (or nasal) turbinates is one of strongest pieces of evidence against them being endothermic.
Dinosaurs being warm blooded as a given is ironically a modern misconception just like that all dinosaurs had feathers and that they are basically big chickens.
Dinosaurs lived for about 165 million years they came in all shapes and sizes, some had hides with or without hair, some had feathers or feather like structures some had scales and everything in between.
As for them needing to be warm blooded for speed we have modern day reptiles even fairly large ones like the Komodo Dragon which are capable of moving at very high speeds which are apex predators whilst being cold blooded.
There are also some studies that have shown many deep similarities between endothermy in birds and endothermy in mammals, which do not seem very likely to be the result of parallel evolution.
So there are chances that actually endothermy was developed in the common ancestor of all amniotes.
If that would be true, then various branches have reverted to poikilothermy, like modern crocodiles, turtles, lizards and snakes, in order to save energy and be able to live with less food, especially in water or at small sizes, when the energy consumption due to endothermy is great without a good thermal insulation.
In this variant some dinosaur branches could have also reverted to poikilothermy.
Regarding turbinates, even the crocodiles have a very simple form of them (and the lizards and snakes an even simpler form), so it is likely that most of the dinosaurs also had them, even if in a simpler form than in birds.
The lack of turbinates in most dinosaur fossils might be a preservation artifact, because these are very thin and fragile bones.
Googling shows that in cases of exceptionally good preservation of the skull, turbinates have been found in some dinosaurs, e.g. in Stegoceras.
I mostly agree with this, the point was that dinosaurs being warm blooded as a general rule isn’t well established or to be more exact no one is trying to establish it.
Dinosaurs span 165 million years there is enough room on an evolutionary time scale for them to take any shape and form.
That said the climate for much of their reign was very different, you probably didn’t had warm blooded aquatic life when the oceans were at 35c and finding dinosaurs in polar regions isn’t a proof for them being warm blooded as those region were tropical rain forests for about 90M years.
Without commenting on anything else in your reply,
> Other counter evidence is that some species were just too darn big to be endothermic and at that size would have high enough internal temperatures to cook themselves.
I don’t understand. Isn’t the existence of the (obviously endothermic) blue whale enough to obviate that argument, at least on its own?
The blue whale and all its relatives have a very good thermal insulation made of a thick fat layer. This allows it to keep a constant internal temperature even in the cold water.
In terrestrial conditions, a very big animal has the reverse problem, of cooling its body, which is more difficult than keeping it warm, which needs only insulation. Cooling requires body parts with very large area and energy lost with pumping blood through them, like the elephant ears. Such body parts may need to be so large at herbivore dinosaur sizes as to make difficult most activities.
An endothermic animal that becomes fully aquatic must either develop a very good thermal insulation or revert to being poikilothermic, to decrease the energy consumption.
It is possible that the ancestors of the crocodiles (which were terrestrial and apparently much more agile than the modern crocodiles) were endothermic and the modern crocodiles have evolved according to the second option, unlike the whales.
I’m really getting out of my area of expertise on this, but could the fact that whales live immersed in a heat sink be a factor? They’re the most massive animal that’s ever lived (as far as is known). Maybe an animal that size can only be endothermic if it lives in the water.
[Edit: the blue whale is the most massive animal known.]
Yes, I considered that. I think it's a very valid possibility and likely GP's argument is applicable to the titan dinosaur species. However in the context of this article, we're discussing much smaller ("regular mammal-sized") dinosaurs living in polar conditions, where the calculus changes enough that this discussion is rather besides the point.
The polar regions used to be warm rainforests up-to as recently as about 50m years ago.
The temperature of the oceans during the Cretaceous was 35c.
Finding dinosaurs in Antarctica or Lapland doesn’t mean anything really since those areas used to be quite warmer than most tropical regions we have today.
Blue whales didn’t had to live when the ocean temperature was on average 35c.
On land as someone else mentioned very large warm blooded animals need ways to cool themselves this is why elephants have huge ears and why they spray water on themselves when ever possible.
Marine mammals need to worry today about insulating themselves in order to not to loose heat, marine dinosaurs would need to worry about their insides cooking.