Stardate 20010910.1757 (On Screen): One of the big debates about Dinosaurs has been "active like birds and mammals" versus "lethargic like crocodiles". Crocs are capable of moving quite rapidly and expending a lot of energy, but only for short periods of time. That's because their circulatory systems are inefficient. In mammals and birds, there is a two-chamber pump which moves blood through the lungs, and a second two-chamber pump which takes oxygenated blood from the lungs and circulates it through the body. In crocs and lizards, there's a gap between those two chambers and the blood from both mixes. As a result, the blood sent to the body doesn't carry as much oxygen as it could and the animal isn't capable of extended effort -- it runs out of breath, in a sense. Well, which way did dinosaurs live? The debate continued.
Now it may have been settled. In an extremely fortuitous find, a dinosaur was dug up with a concretion inside its rib cage which turned out to be its heart, fossilized. Extensive use of CAT scans later, they've reproduced a 3D model of it and the word is now in: it is much more like mammals and birds than like crocodiles and lizards. This agrees with more indirect evidence from bone cross-sections which suggested the same thing, but that was always open to much more doubt. This doesn't yet settle the ectotherm-endotherm debate but it strongly weighs in favor of endothermy. Ectothermic critters are usually called "cold blooded" which is a poor description. Endothermic animals are popularly known as "warm blooded" which is equally inaccurate. Ectotherms derive much of their body heat from external sources and their blood temperature tends to very quite a lot over the course of a day; endotherms create most of theirs from metabolism of food and have much better control over blood temperature. Endotherms can also survive in a much broader range of habitats, but ectotherms eat much less (on the order of a tenth as much per unit body mass). A big anaconda can get by on one good-sized meal (a monkey or a big parrot) per month, a level of food intake on which a panther of comparable mass would starve. On the other hand, the panther can hunt at night and is capable of actually chasing down its prey.
Of course, it's just one species of dinosaur, but it's difficult to believe that one group could have a heart developed like this without most of the others being similar. Every mammal has a four-chamber heart; none of them are croc-like, and that's also true with the birds. This almost certainly settles the issue for the Ornithischians, and very likely also for the Saurischians. I have to say that I'm much more pleased with the image of active and alert dinosaurs than I am with slothfully dull ones spending most of their time laying in the sun. All the convincing bioengineering arguments about this I ever heard all argued in favor of endothermy; most of the arguments in favor of ectothermy seemed to come down to "But they're fucking LIZARDS for God's sake!" (discuss)
