Study reveals when the first warm-blooded dinosaurs roamed Earth

(CNN) — Were dinosaurs warm-blooded like birds and mammals or cold-blooded like reptiles? It’s one of paleontology’s oldest questions, and gleaning the answer matters because it illuminates how the prehistoric creatures may have lived and behaved.

Challenging the prevailing idea that they were all slow, lumbering lizards that basked in the sun to regulate their body temperature, research over the past three decades has revealed that some dinosaurs were likely birdlike, with feathers and perhaps the ability to generate their own body heat.

However, it’s hard to find evidence that unquestionably shows what dinosaur metabolisms were like. Clues from dinosaur eggshells and bones have suggested that some dinosaurs were warm-blooded and others were not.

A new study published in the journal Current Biology on Wednesday suggested that three main dinosaur groups adapted differently to changes in temperature, with the ability to regulate body temperature evolving in the early Jurassic Period about 180 million years ago.

Based on fossils from 1,000 dinosaur species and paleoclimate information, the new study looked at the spread of dinosaurs across different environments on Earth throughout the dinosaur era, which started some 235 million years ago and ended 66 million years ago when an asteroid slammed into Earth.

Two of the three main groups — meat-eating therapod dinosaurs, which included T. rex, and plant-eating ornithischians, whose notable members included Triceratops and Stegosaurus — spread to live in colder climates during the early Jurassic Period, the research suggested. These dinosaurs may have evolved endothermy, or the ability to internally generate body heat, according to the study.

Two adaptable dinosaur groups

Therapods and ornithischians lived in a broad range of thermal landscapes in their respective evolutionary histories and were “remarkably adaptable,” the researchers wrote. Recent fossil discoveries have shown that different species of dinosaurs even thrived in the Arctic, giving birth and living there year-round.

“Warm-blooded animals are generally more active, for example, cold-blooded animals usually don’t build nests,” said lead study author Dr. Alfio Alessandro Chiarenza, Royal Society Newton International Fellow at University College London’s department of Earth sciences.

By contrast, the towering, plant-eating sauropods kept to warmer, lower-latitude regions of the planet, and the availability of richer foliage in certain habitats wasn’t the only factor why, the study found. Sauropods, which included Brontosaurus and Diplodocus, also appeared to thrive in arid, savannahlike environments and practiced “prolonged climatic conservatism,” the researchers wrote.

“It reconciles well with what we imagine about their ecology,” Chiarenza said. “They were the biggest terrestrial animals that ever lived. They probably would have overheated if they were hot-blooded.”

What’s more, he added, the amount of plant matter they would have needed to consume if they were warm-blooded would have been unsustainable.

“(These animals) were living in herds and we know that each one of them was the equivalent of 10 African elephants. (If they were warm-blooded) they would just destroy plant life. It makes more sense, as living animals, for them to be more cold-blooded.”

However, Jasmina Wiemann, a postdoctoral research scientist at the Field Museum of Natural History in Chicago, said that the findings from this study contrasted with her own research, which looked at molecular traces of oxygen intake found in dinosaur fossils. Her 2022 study suggested that ornithischians were more likely cold-blooded and sauropods were warm-blooded.

She questioned to what extent the biogeographic range of a dinosaur was determined by its metabolic capacity as opposed to other factors such as behavior, growth strategy, dietary preferences and other ecological interactions.

“Some animals with incredibly fast growth rates (i.e., sauropods), and by requirement, fast metabolisms, are here found to be cold-blooded, while other animals with very slow growth rates (i.e., ceratopsians) are recovered as endotherms,” Wiemann said. “These discrepancies will need to be addressed.”

Evolutionary trigger

Chiarenza said that the model, developed by researchers at UCL and Universidade de Vigo in Spain, suggested that the earliest dinosaurs were more reptilian and cold-blooded. But a period of global warming resulting from volcanic activity 180 million years ago, known as the Jenkyns Event, may have been a trigger for the evolution of the ability to generate body heat internally.

“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,” he said in a news release.

As with all research based on models, the study made predictions grounded in existing information. New fossils or climatic information might alter that picture. “Of course, if a sauropod turned up in the Arctic that would change things,” Chiarenza said.

Paleontologist Anthony Fiorillo, executive director of the New Mexico Museum of Natural History & Science, said the study was “intriguing” and the “first real attempt to quantify broad patterns that some of us had thought about previously.” Fiorillo, who is also a senior fellow at Southern Methodist University in Dallas, wasn’t involved with the research.

“Their modeling helps create a robustness to our biogeographical understanding of dinosaurs, and their related physiology,” he said.

“This study provides a platform for us to further test what we think we might know.”

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