Absurdly, birds are dinosaurs. Palaeontologists insist they're close relatives, but to nearly everyone else these animals are polar opposites. Birds are graceful, musical, fluffy and so athletic that many travel between continents without resting. In contrast, dinosaurs hardly seem graceful. They're at once fierce, scaly monsters and fodder for goofy cartoons because of features like Brachiosaurus’s oversized neck or Tyrannosaurus’s clumsy arms. But despite their apparent differences, somehow, birds are theropods – the dinosaur group that includes T. rex. This bird–dinosaur connection is increasingly clear in the fossil record, which includes fossils of feathered and winged dinosaurs, suggesting many weren't as reptilian as movies depict. And now, in the biggest departure yet from Jurassic Park, new evidence shows plenty of dinosaurs weren't even cold blooded (ectothermic). Like mammals, birds are warm blooded (endothermic). Birds have the highest metabolisms of all living animals and much of this energy is spent on keeping their bodies warm. But when did their warm-blooded metabolisms evolve? Was it recent, or did ancestral dinosaurs heat themselves, too?
To investigate, a team of scientists from across the USA and Spain, led by Jasmina Wiemann and Derek Briggs (Yale University), found a way to check an animal's metabolism by scanning their bones, even after they are fossilized, as some biological molecules remain. But it's unrealistic to extract those molecules, because you can't grind up precious artefacts to test what's inside. Instead, the researchers used harmless light to scan the fossils without destroying or even touching them. When illuminated with infrared light, each biological chemical responds with its own unique light scattering signature. Using this signature, the researchers could tell which biomolecules were preserved in the fossilized eggshells, teeth or bones, and some turned out to be metabolic waste products. With these fossil scans, the researchers revealed details about how much energy the dinosaurs burned, even though they had been extinct for millions of years.
The scientists applied this technique to dinosaur fossils to detect any remaining biological molecules – as well as the bones of modern-day reptiles, mammals and birds, whose metabolic rates are already known – to estimate the dinosaurs’ metabolic rates. Surprisingly, the metabolisms of the dinosaurs were more varied than expected. Some had slow metabolisms, including plant-eaters such as the spikey Stegosaurus or the three-horned Triceratops, more in line with the metabolisms of modern cold-blooded reptiles. However, others had faster, more bird-like metabolisms, such as the flying pterosaurs or the turkey-sized predator Velociraptor. In fact, many dinosaurs, including T. rex, were warm blooded.
The researchers then made a family tree to track when and how warm bloodedness evolved, and it seems that the dinosaurs and closely related reptiles that lived at the time evolved warm-blooded metabolisms more than once. In addition, investing in heating paid off: every time they evolved warm bloodedness, they quickly evolved a new way of moving. For example, plesiosaurs – marine behemoths that grew almost as long as a ten-pin bowling lane – probably began to swim as they became more warm blooded. And metabolism appears to have increased in pterosaurs right before they started to fly. In addition, dinosaurs such as Velociraptor and T. rex increased their metabolic rates before they learned to walk on two legs. Remarkably, birds evolved from these bipedal dinosaurs when their arms were free to evolve into wings, probably benefiting from their ancestors’ high metabolisms to fuel flight.
Despite this work, I still can't imagine dinosaurs with bird-like grace, but this is why this study is so fascinating: it bridges the bird–dinosaur divide. Our simplistic notions of dinosaurs are dissolving, causing us to rethink how we typecast dinosaurs as bland reptilians. In reality, dinosaurs were more diverse and interesting than anything in Jurassic Park.