Dinosaurs were once thought to have been ectothermic, or cold-blooded, an idea that makes sense given that they were reptiles. While scientists have previously found evidence of dinosaur species that were warm-blooded, what may have caused this adaptation remains unknown. A team of researchers now believe that dinosaurs, which already had some cold tolerance, evolved endothermy, or warm-bloodedness, to adapt when they migrated to regions with colder temperatures. They also believe they have discovered a possible cause of the transition.
Using data from Mesozoic fossils, evolutionary trees, climate patterns and geography, as well as taking into account drastic climate change caused by global warming, the team found that theropods (carnivores and ancestors of birds, such as Velociraptor and T. rex) and ornithischians (such as such as Triceratops and Stegosaurus) must have made their way to colder regions during the Early Jurassic. Lower temperatures are thought to have selected for species that were partially adapted to endothermy.
“The early invasion of cool niches… [suggests] early attainment of homeothermic (probably endothermic) physiology c [certain species]allowing them to colonize and persist even in extreme latitudes since the early Jurassic,” the researchers said in a study recently published in Current Biology.
Hot real estate
During the Mesozoic Era, which lasted from 230 to 66 million years ago, protodinosaurs, known as dinosauromorphs, began to diversify in hot, dry climates. Early sauropods, ornithischians, and theropods tended to remain in these regions.
Sauropods (such as Brontosaurus and Diplodocus) would become the only groups of dinosaurs that enjoyed the heat—the fossil record shows that sauropods tended to stay in warmer areas, even if there was less food. This suggests the need for sunlight and heat associated with ectothermy. They may have been able to survive colder temperatures, but they weren’t adapted enough to last long, according to one hypothesis.
It’s also possible that living in cooler areas meant too much competition with other dinosaur species, as theropods and ornithischians eventually moved into those cooler areas.
Almost an apocalypse
In addition to environmental opportunities that may have attracted dinosaurs to cooler areas, it is possible that they were driven away from warmer ones. About 183 million years ago, there was a disturbance in the carbon cycle, along with extreme volcanism that spewed massive amounts of methane, sulfur dioxide and mercury. Life on Earth suffered from searing heat, acid rain and forest fires. Known as the Early Jurassic Jenkins Event, researchers now believe that these disturbances pushed theropods and ornithischian dinosaurs into cooler climates as temperatures in warmer areas exceeded the optimal temperatures for their survival.
Theropods and ornithischians that escaped the effects of the Jenkyns event may have had a key adaptation to the cooler climate; many dinosaurs from these groups are now believed to have been feathered. Feathers can be used to both trap and release heat, which would have allowed feathered dinosaurs to regulate their body temperature in a more varied climate. Modern birds use their feathers in the same way.
Dinosaur species with feathers or special structures that improve heat management could be homeothermic, meaning they could maintain their body temperature with metabolic activity, or even endothermic.
Beyond dinosaurs that migrated to high latitudes and adapted to falling temperatures, endothermy may have given rise to new species and lineages of dinosaurs. This may have contributed to the rise of the Avialae, the clade that includes birds—the only actual dinosaurs still in existence—and traces back to their earliest ancestors.
“[Our findings] provide new insights into the origins of avian endothermy, suggesting that this evolutionary trajectory within theropods … probably began in the Early Jurassic at the latest,” the researchers said in the same study.
It’s really something to think about the next time a sparrow flies over.
Current Biology, 2024. DOI: 10.1016/j.cub.2024.04.051
