The ability to regulate body temperature internally, rather than relying on the Sun, may have first emerged among dinosaurs around 180 million years ago, coinciding with a period of extreme climate.
Dinosaurs were originally thought to be cold-blooded creatures. After all, their name means “terrible lizard,” and lizards need to be roasted on rocks to warm their blood. This idea has been called into question by the discovery of dinosaur species that lived near the poles, apparently surviving winter conditions that modern reptiles cannot cope with. For years, the warm-blooded versus cold-blooded dinosaur debate was among the most heated in paleontology.
It is now widely agreed that some, but not all, dinosaurs were warm-blooded, or endothermic, to use the more scientific term. After all, we now know that birds are part of the dinosaur branch, and lizards are not. Since dinosaur ancestors were almost certainly cold-blooded, this raises the question of when the first warm-blooded dinosaurs appeared. Looking at when they first colonized colder regions may provide an answer.
The first bones from both theropods and ornithischians – two of the three major branches of dinosaurs – appeared in cold regions in the early Jurassic, reports Dr Alfio Alessandro Chiarenza of University College London. No equivalent presence from sauropods was found, making it more likely that thermoregulation did not evolve in this branch, rather than that we made erroneous temperature reconstructions.
“Our analyzes show that different climatic preferences emerged among major groups of dinosaurs around the time of the Jenkyns Event 183 million years ago, when intense volcanic activity led to global warming and the extinction of plant groups,” Chiarenza said in a statement.
A period of high temperatures may seem like an unlikely catalyst for developing the capacity to survive cold conditions, but it appears to have been the case.
“Many new groups of dinosaurs appeared at that time,” Chiarenza continued. “The adoption of endothermy, perhaps a result of this ecological crisis, may have allowed theropods and ornithischians to thrive in colder environments, allowing them to be very active and maintain activity for longer periods, to develop and grow faster and produce more offspring.”
Although the most famous theropods are T. Rex and Velociraptorsco-author Dr Sara Barella of the Universidade de Vigo noted that the birds were also theropods, adding: “Our study suggests that the unique temperature regulation of birds may have originated in this early Jurassic period.”
Crucially, some dinosaurs laid eggs at high latitudes, proving that they didn’t just migrate there in the summer.
Image credit: Davide Bonadonna/Universidade de Vigo/UCL
At this time, sauropods grew to the enormous size that is their most famous characteristic, which may have been another consequence of the environmental changes that were occurring.
As further evidence for their claim that sauropods did not learn to thermoregulate while some members of each of the other branches did, the authors note that sauropods were common in relatively dry grasslands. This disproves the idea that they avoided the poles because they relied on the rich foods available in the rainforest. If lack of water did not prevent poleward expansion, the obvious explanation is cold. On the other hand, seeing the theropod predators heading towards the poles would not have given the sauropods much incentive to join them.
Sauropods eventually made it closer to the poles during the Cretaceous, as seen in the example of Patagotitan, a candidate for the largest beast to ever walk the Earth. However, the world was generally warmer during the Early Cretaceous than during the Jurassic period, and plant fossils found at sites similar to Titanosaurs suggest average annual temperatures of 10–15 °C (18–27 °F).
The authors suggest that although sauropods were able to survive at high latitudes during the Cretaceous, they did not do so well in the far north, perhaps because they were still less suited to do so than the ornithopods that preceded them. The enormous size sauropods had reached by this point may have helped their largest members survive near the South Pole during the Cretaceous, as all that thermal mass would have kept their body temperatures relatively stable at night.
Although the work suggests that warm-bloodedness in birds is very ancient, mammals probably predated it by 50 million years.
The research is published in Current Biology.
