Dinosaurs were once assumed to be ectothermic or cold-blooded, which makes sense given that they were reptiles. Although scientists have previously discovered evidence of dinosaur species that were warm-blooded, it remains unknown what could have triggered this adaptation. A team of researchers now thinks that dinosaurs that already had some tolerance for cold evolved endothermy, or warm-bloodedness, to adapt when 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, and taking into account the drastic climate change event that caused global warming, the team found that theropods (predators and ancestors of birds such as the velociraptor and T. rex) and ornithischians (such as such as Triceratops and Stegosaurus) had to make their way to colder areas during the Early Jurassic. Lower temperatures are thought to have selected for species that were partially adapted to endothermy.
“Early invasion of cool niches… [suggests] early achievement of homeothermal (possibly endothermic) physiology in [certain species]allowing them to colonize and persist 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 from 230 to 66 million years ago, proto-dinosaurs known as dinosauromorphs began to diversify in hot and dry climates. Early sauropods, ornithischians and theropods all tended to stay in these regions.
Sauropods (such as brontosaurus and diplodocus) became the only group 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 indicates the need for sunlight and warmth associated with ectothermy. According to one hypothesis, they may have been able to survive at lower temperatures, but were not adapted enough to last long.
It’s also possible that living in colder areas meant too much competition with other dinosaur species, as theropods and ornithischians eventually moved to those colder areas anyway.
Almost an apocalypse
In addition to ecological opportunities that may have attracted dinosaurs to colder areas, it is possible that they were driven away from warmer ones. About 183 million years ago, there was a disruption in the carbon cycle, along with extreme volcanism that spewed vast amounts of methane, sulfur dioxide and mercury. Life on Earth suffered from extreme heat, acid rain and forest fires. Known as the Early Jurassic Jenkyns Event, researchers now think that these disturbances pushed the theropod and ornithischian dinosaurs into colder climates because temperatures in warmer zones were above the optimal temperatures for their survival.
Theropods and ornithischians that escaped the effects of the Jenkyns event may have had a key adaptation to colder climates; many dinosaurs from these groups are now thought to have been feathered. Feathers can also be used to trap and release heat, which would allow feathered dinosaurs to regulate their body temperature in different climates. Modern birds use their feathers in the same way.
Dinosaur species with feathers or special structures that improved heat management may have been homeothermic, meaning they could maintain their body temperature through metabolic activity, or even endothermic.
In addition to dinosaurs that migrated to high latitudes and adapted to the drop in temperature, endothermy could have led to the emergence of new species and lineages of dinosaurs. This may have contributed to the rise of the Avialae, a class that includes birds – the only true 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 latest Early Jurassic,” the researchers said in the same study.
That’s something to think about the next time a sparrow flies by.
Current Biology, 2024. DOI: 10.1016/j.cub.2024.04.051
