Which Dinosaurs Were Warm Blooded or Cold Blooded?

[BillTre]

In another well directed chemical analysis of fossils, a research group has indications that some but not all dinosaurs were warm blooded (endothermic), although some (Stegosauri and Triceratops) had apparently reverted to being cold blooded (ectothermic). In addition, it seems that the flying non-avian reptiles were also warm blooded.
They did this by testing sections of fossilized femurs in a variety of known and extinct animals (all were amniotes, animals with eggs protected by a shell that could be laid on land (amphibians are not amniotes)) and determined the relative amounts of metabolic lipoxidation signals using in situ Raman and Fourier-transform infrared spectroscopy. The amounts of these chemicals are expected to reflect the warm or cold blooded state of the animals. This was confirmed by examining fossils of extent (still around) animals.
This paper is open access and available here.

Here is their phylogenetic diagram summarizing their findings:

How to read the phylogenetic diagram:

• Time goes left to right (scale at the bottom).
• Color of the lines in the phylogeny are colored (temp. scale upper left)
• Lines that go all the way to the right are still alive. Those ending more to the left are extinct.
• Changes in the temperature trait are indicated by small circles on the lines of the phylogenetic diagram (yellow: transitioning cold to warm blooded, blue: transitioning warm to cold blooded).
• Birds are at the top and branch off from the dinosaurs near the raptors (Deinonychous).
• Hadrosaur = the group of duck billed dinosaurs (more cold blooded than their close relatives).
• The flying non-avian reptiles (Pterodons (they are in Jurassic world for example) and Phanphorhinchoid) branch off from pre-dinosaurs just before they become dinosaurs. they are warm blooded.
• Plesioaurs (long necked large aquatic reptiles with paddle-like feet) were somewhat warm blooded. Not sure where mososaurs (large whale-like extreme carnovores, also in Jurassic world) would fit in. Maybe they were not tested.

Its thought that being warm blooded would provide a more sustained higher rate of metabolism, which would have some advantages.
On the other hand, its not entirely clear why some of them would switch back to being cold blooded, but being warm blooded continuously requires a lot of energy (unless hibernating like some mammals can do).

 

[Godot_]

If you're a carnivore / omnivore, being warm blooded is a big advantage in a world full of cold-blooded stuff: You can simply ... graze on them when they're down at night or in the winter. Ask e.g. the hedgehog...

Re. reverting: Obviously, when you're living in a constantly warm environment, like equatorial rainforest, or subtropical forests (not deserts - those get really cold at night), the "warm-bloodedness" advantage disappears, so keeping the metabolic rate as low as possible becomes advantageous again.