Most of us of a certain age grew up with the prevailing idea that dinosaurs were lumbering, cold-blooded lizards. Their very name means ‘terrible lizard’, after all.
But that idea was tossed out decades ago. Multiple strands of evidence emerged to indicate that at least some types of dinosaur were warm blooded, and not least the realisation that modern birds are the living descendants of dinosaurs. Other strands of evidence include the presence of dinosaurs in what were cold, dark climates, such as Antarctica, and ratios of predator to prey species that correspond more to modern mammals than reptiles.
Dinosaurs are divided into three main groups: theropods (exemplified by the likes of tyrannosaurs and Velociraptors), ornithischians (which included the famous herbivores Stegosaurus and Triceratops) and, lastly, sauropods (the big plant-eaters like Brontosaurus and Diplodocus). It is now believed that only the sauropods were cold-blooded like modern lizards. Some of the theropods and ornithischians, like Velociraptors, had developed feathers.
But when did the theropods and ornithischians become warm-blooded (or endothermic, to use the technical term, which means that they were able to internally regulate their body temperatures)? A new study suggests that the crucial change came much earlier than previously suspected.
The ability to regulate body temperature, a trait all mammals and birds have today, may have evolved among some dinosaurs early in the Jurassic period about 180 million years ago, suggests a new study led by UCL and University of Vigo researchers.
This is slightly later than halfway through the dinosaurs’ dominance of Earth. The new study examined distribution of dinosaurs across different climates during the Mesozoic (the ‘dinosaur’) Era, from 230 to 66 million years ago. The study drew on 1,000 fossils, climate reconstructions and dinosaurs’ evolutionary trees.
The researchers found that the theropods and ornithiscians spread to cooler climates during the early Jurassic period. The sauropods, by contrast, stuck to the warmer regions. This indicates that the first two groups had developed endothermy (warm-bloodedness), which allowed them to thrive in colder climates.
First author Dr Alfio Alessandro Chiarenza, of UCL Earth Sciences, said: “Our analyses show that different climate preferences emerged among the main dinosaur groups around the time of the Jenkyns event 183 million years ago, when intense volcanic activity led to global warming and extinction of plant groups.
“At this time, many new dinosaur groups emerged. The adoption of endothermy, perhaps a result of this environmental crisis, may have enabled theropods and ornithischians to thrive in colder environments, allowing them to be highly active and sustain activity over longer periods, to develop and grow faster and produce more offspring.”
Co-author Dr Sara Varela, of the Universidade de Vigo, Spain, said: “Theropods also include birds and our study suggests that birds’ unique temperature regulation may have had its origin in this Early Jurassic epoch.”
Another clue that the sauropods, lurking in warmer climates, remained cold-blooded is their characteristic gigantic size. Their smaller surface area to volume ratio meant that they lost heat slower than smaller creatures, which would allow them to stay active for longer.
In the paper, the researchers also investigated if sauropods might have stayed at lower latitudes to eat richer foliage unavailable in colder polar regions. Instead, they found sauropods seemed to thrive in arid, savannah-like environments, supporting the idea that their restriction to warmer climates was more related to higher temperature and then to a more cold-blooded physiology. During that time, polar regions were warmer, with abundant vegetation.
Besides purely academic interest, the study suggests that life on Earth is well capable of adapting to even the most extreme climate changes.
If dinosaurs could do it, what’s stopping humans adapting rather than, Canute-like, trying to stop the tides of climate?
