[ComputerGuru]

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?

[adrian_b]

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.

[dogma1138]

This impacted both land and oceanic dinosaurs at least in certain periods.

During the Cretaceous period the ocean temperature was 35c and the north and south poles were tropical rain forests devoid of any ice.

[andrewl]

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.]

[ComputerGuru]

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.

[dogma1138]

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.

[ComputerGuru]

The article claims that the areas where fossils were found had winters with temperatures just above freezing, and that some evidence (fossilized young, growth patterns in bones, etc) indicates that the species lived there year-round rather than only migrating there during the warmer months.

[dogma1138]

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.

[LeFantome]

Brilliant, concise, and humble. An inspiration…