Dinosaurs Started Out Hot, Then Some of Them Got Cold

Paleontologists have long debated the question of dinosaur metabolism: whether they heated up, as modern birds and mammals do, or whether they resembled the slower metabolisms of modern reptiles. Surprisingly, the answer seems to be both.

“While we assumed most dinosaurs were warm-blooded, there was simply no way to measure the underlying metabolic capabilities,” said Jasmina Wiemann, a paleontologist at the California Institute of Technology. In the absence of readily available dinosaurs, he said, paleontologists grappling with questions about prehistoric metabolisms — for example, whether a given beast was warm-blooded or cold-blooded — have had to rely on indirect evidence, such as isotopic evidence or slice growth rates. of bone .

Now, Dr. Wiemann and her colleagues have pioneered a new method to directly measure the metabolic rate of extinct animals. Their findings, published Wednesday in the journal Nature, confirmed that many dinosaurs, as well as their winged relatives, the pterosaurs, were ancestrally warm-blooded. But in a twist, the research also suggests that some herbivorous dinosaurs spent tens of millions of years developing a cold-blooded metabolism more akin to that of contemporary 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-avian dinosaurs. Using laser microspectroscopy, they identified a specific molecular marker of metabolic stress in both fossils and modern bones, one that directly correlates with the amount of oxygen the animal was breathing. That, in turn, is a direct indicator of your metabolism.

The team found that both mammals and plesiosaurs (long-necked marine reptiles) had independently evolved their high metabolisms. Pterosaurs and dinosaurs, which together form a group called Ornithodira, appear to have descended from warm-blooded ancestors, a state that persisted in long-necked sauropods, predatory theropods like Tyrannosaurus rex, and their surviving feathered descendants like chickens. .

Sauropods that have high metabolisms are unexpected, says Stephen Brusatte, a paleontologist at the University of Edinburgh who was not involved in the study. Researchers in the past had suggested that if any dinosaur had a lower metabolism, it would have been the giant, lumbering herbivore.

“Imagine the hundreds or thousands of pounds of plants they would have to eat each day to fuel such a fast metabolism,” Dr. Brusatte said.

The team’s findings about another group of dinosaurs, the diverse superfamily of herbivores called ornithischians, were even more surprising. While ancestral ornithischians shared the warm-blooded metabolisms of other dinosaurs, Dr. Wiemann said, their larger descendants like Stegosaurus and Triceratops actually slowed their metabolisms over time, ending up with metabolic rates closer to those of modern reptiles. . And like modern reptiles, they may have needed to maintain their core temperature through behavior: basking or seasonally migrating to warmer climates.

“The evolution of reduced metabolic rates in some ornithischians is surprising, especially since the same is not true for giant sauropods,” said Jingmai O’Connor, associate curator of fossil reptiles at Chicago’s Field Museum, who was also not involved in the study. study. the study. “This work will dramatically change the way we interpret the lifestyles and behaviors of these animals.”

More research, and many more fossil samples, will be needed to take the temperature of all limbs of the ornithischian family tree. But they wouldn’t be the first members of the larger family that dinosaurs were members of, the archosaurs, to potentially make the switch. Dr Wiemann said the growth rates of certain groups of extinct crocodilians suggested they might also have been warm-blooded, while their modern relatives developed slower metabolisms.

Now that they have shown the potential of this technique, Dr. Wiemann said that more detailed studies could help clarify why certain families of dinosaurs abandoned high metabolisms.

“That seems counterintuitive because we appreciate the warm blood in ourselves as this great evolutionary innovation, which it was,” Dr. Brusatte said. But high metabolisms are costly in terms of diet and energy, he notes, adding that what they needed to maintain it may have been “too much responsibility for some dinosaurs.”

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