Dinosaurs had to be cold enough to be warm-blooded

Enlarge / Later theropods had multiple adaptations to different temperatures.

Dinosaurs were once believed to be ectothermic, or cold-blooded, an idea that makes sense considering they were reptiles. Although scientists had previously discovered evidence of dinosaur species being warm-blooded, what could have caused this adaptation remained unknown. A team of researchers now think that dinosaurs that already had some cold tolerance evolved endothermy, or warm-bloodedness, to adapt as they migrated to regions with lower temperatures. They also think they have found a possible reason for the trip.

Using the Mesozoic fossil record, evolutionary trees, climate models and geography, plus taking into account drastic climate change causing global warming, the team found that theropods (predators and ancestors of birds such as Velociraptor and T. rex) and ornithischians ( such as triceratops and stegosaurus) must have found 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] an early realization of homeothermic (possibly endothermic) physiology [certain species]allowing them to colonize and survive even at 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 230 to 66 million years ago, proto-dinosaurs known as dinosauromorphs began diversifying in warm and dry climates. Early sauropods, ornithischians, and theropods all remained in these regions.

Sauropods (such as Brontosaurus and Diplodocus) would become the only dinosaur groups to bask in the heat; the fossil record shows that sauropods tended to remain in warmer areas, even when there was less food. This suggests the need for sunlight and heat associated with ectothermy. According to one hypothesis, they may have been able to survive in colder temperatures, but were not adapted enough to survive for long.

It’s also possible that living in cooler areas meant too much competition with other types of dinosaurs, as theropods and ornithiscians eventually moved into these cooler areas.

Almost apocalypse

Apart from the ecological possibilities that attracted dinosaurs to the cooler areas, it is possible that they were driven out of the warm areas. About 183 million years ago, a disruption in the carbon cycle occurred, along with extreme volcanism that spewed out enormous amounts of methane, sulfur dioxide and mercury. Life on Earth suffered from scorching heat, acid rain and forest fires. The researchers now think that these disturbances pushed theropod and ornithischian dinosaurs into cooler climates, as temperatures in warmer regions rose above the optimal temperatures for their survival.

The theropods and ornithischians that escaped the effects of the Jenkyns event may have had an important adaptation to cooler climates; It is now thought that many dinosaurs from these groups were feathered. Feathers can be used to retain and release heat, which allowed feathered dinosaurs to regulate their body temperatures in more diverse climates. Modern birds use their feathers in the same way.

Dinosaur species with feathers or special structures that improved heat management could have been homeothermic, meaning they could have maintained their body temperature with metabolic activity or even endothermic.

In addition to dinosaurs migrating to high latitudes and adapting to a drop in temperature, endothermy might have led to the emergence of new species and lineages of dinosaurs. It could have contributed to the rise of Avialae, the clade that includes birds (the only true dinosaurs in existence) and goes all the way back to their earliest ancestors.

“[Our findings] provide new insights into the origins of endothermy in birds, suggesting that this evolutionary trajectory within theropods… likely began in the latest Early Jurassic,” the researchers said in the same study.

That’s really something to think about the next time a sparrow flies by.

Current Biology, 2024. DOI: 10.1016/j.cub.2024.04.051

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