[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.