Dinosaurs started hot, then some got cold

Paleontologists have long argued with the question of dinosaur metabolism – whether it was hot, like modern birds and mammals, or similar to the slower metabolism of modern reptiles. In a surprise, the answer appears to be both.

“While we assumed that most dinosaurs were warm-blooded, there was no way to measure baseline metabolic capabilities,” said Jasmina Wiemann, a paleontologist at the California Institute of Technology. In the absence of dinosaurs available, she said, paleontologists grappling with questions about prehistoric metabolism — whether it was a warm-blooded or cold-blooded monster, for example — had to rely on indirect evidence, such as isotopic evidence. or growth rates of bone segments.

Now, Dr. Weiman and her colleagues have devised a new method for directly measuring the metabolic rate of extinct animals. their conclusions, Published Wednesday in Nature, asserted that many dinosaurs as well as their winged relatives, pterosaurs, were warm-blooded than their ancestors. But in a twist, the research also indicates that some herbivorous dinosaurs spent tens of millions of years developing a cold-blooded metabolism like that of modern and ancient reptiles.

The team analyzed more than 50 extinct and modern vertebrates from the collections of the Yale Peabody Museum of Natural History, including mammals, lizards, birds, and 11 different non-bird dinosaurs. Using laser spectroscopy, they identified a specific molecular marker of metabolic stress in both fossils and modern bone – an indicator that correlates directly with the amount of oxygen the animal breathes in. This, in turn, is a direct indication of the metabolism.

The team found that both mammals and plesiosaurs — long-necked marine reptiles — independently evolved elevated metabolisms. Dinosaurs and pterosaurs, which together form a group called Ornithodira, appear to have descended from warm-blooded ancestors – a condition that persisted in long-necked sauropods, and predatory theropods such as Tliranosaurus rexAnd their surviving feathered descendants, like chickens.

The presence of highly metabolized sauropods is unexpected, says Stephen Brusatte, a paleontologist at the University of Edinburgh who was not involved in the study. Researchers have suggested in the past that if any dinosaurs had a lower metabolism, they would have been gigantic, lumbering herbivores.

“Just imagine hundreds or thousands of pounds of plants that they would have to eat every day to fuel such a rapid metabolism,” Dr. Brusatte said.

Still, the team’s findings about another group of dinosaurs — the large, diverse family of herbivores called ornithischians — are still more surprising. While the ancestors of ornithischians shared the metabolism of other warm-blooded dinosaurs, Dr. Weiman said, their greater descendants like Stegosaurus and Triceratops actually reduced their metabolisms over time, ending up at metabolic rates closer to those of modern reptiles. And like modern reptiles, they may have needed to maintain their core temperature through behavior — basking in the sun or migrating seasonally to warmer climates.

Jingmay O’Connor, associate curator of fossil reptiles at Chicago’s Field Museum, who was also not involved in the study, said. “This work will fundamentally change how we interpret the lifestyles and behaviors of these animals.”

More research – and several fossil specimens – will be necessary to measure the temperature of all limbs on the ornithischian family tree. But they wouldn’t be the first members of the much broader family of dinosaurs, members of the archosaurs, to potentially usher in the change. Dr. Weiman said that the growth rates of some extinct groups of crocodiles indicate that they may have been warm-blooded, While their modern relatives developed slower metabolisms.

Now that they’ve demonstrated the technology’s potential, Dr. Weiman said more detailed studies could help explain why some families of dinosaurs gave up on high metabolisms.

“It seems inconsequential because we cherish the warm blood in ourselves as this great evolutionary innovation, which it was,” said Dr. Brusatte. But a high metabolism is expensive in terms of diet and energy, he notes, adding that what they need to maintain may have been “too much of a burden for some dinosaurs.”