Paleontologists have long debated the question of dinosaur metabolism: whether they warmed up, as modern birds and mammals do, or whether they resembled the slower metabolisms of modern reptiles. In a surprise, the answer seems to be both.
“Although we had assumed that most dinosaurs were warm-blooded, there was no way to measure the underlying metabolic capabilities,” said Jasmina Wiemann, a paleontologist at the California Institute of Technology. In the absence of available dinosaurs, he said, paleontologists facing questions about prehistoric metabolism (whether a particular beast was warm-blooded or cold-blooded, for example) have had to rely on indirect evidence, such as evidence isotopic or growth rates of bone slices. .
Now, Dr. Wiemann and his colleagues have pioneered a new method of directly measuring the metabolic rate of extinct animals. His findings, published in the journal Nature on Wednesday, confirmed that many dinosaurs, as well as their winged relatives, the pterosaurs, were ancestral warm-blooded. But in a twist, research also suggests that some herbivorous dinosaurs spent tens of millions of years developing a cold-blooded metabolism more similar to that of contemporary and ancient reptiles.
The team analyzed more than 50 extinct, modern vertebrates from the Yale Peabody Museum of Natural History collections, including mammals, lizards, birds, and 11 different non-avian dinosaurs. Using laser microscopy, they identified a specific molecular marker of metabolic stress in both fossils and modern bones, one that correlates directly with the amount of oxygen the animal breathed. This, in turn, is a direct indicator of your metabolism.
The team found that both mammals and plesiosaurs (long-necked marine reptiles) had independently developed their high metabolisms. Pterosaurs and dinosaurs, which together form a group called Ornithodira, appear to be descended from warm-blooded ancestors, a condition that persisted in long-necked sauropods, predatory theropods such as Tyrannosaurus rex and their surviving feathered offspring, such as chickens. .
Sauropods with a high metabolism are unexpected, says Stephen Brusatte, a paleontologist at the University of Edinburgh who did not participate in the study. Researchers in the past had suggested that if dinosaurs had lower metabolisms, they would have been giant, heavy herbivores.
“Imagine the hundreds or thousands of pounds of plants that would have to be eaten every day to feed such a fast metabolism,” Dr. Brusatte said.
The team’s findings around another group of dinosaurs – the diverse herbivorous superfamily called ornithischians – were even more startling. While ancestral ornithischians shared the warm-blooded metabolisms of other dinosaurs, Dr. Wiemann, its larger descendants such as Stegosaurus and Triceratops actually reduced their metabolism over time, ending metabolic rates closer to those of modern reptiles. And like modern reptiles, they may need to maintain their core temperature through their behavior: sunbathing or migrating seasonally to warmer climates.
“The evolution of declining metabolic rates in some ornithischians is striking, especially considering that this is not the case with giant sauropods,” said Jingmai O’Connor, associate curator of fossil reptiles at the Field Museum in Chicago. who also did not participate in the study. “This work will drastically 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 the limbs of the ornithischian family tree. But they would not be the first members of the larger family of which the dinosaurs, the arcosaurs, were members to make the change. Dr. Wiemann said the growth rates of certain groups of extinct crocodiles suggest that they may also have been warm-blooded, while their modern relatives evolved with slower metabolisms.
Now that they have demonstrated the potential of this technique, Dr. Wiemann said more detailed studies could help clarify why certain dinosaur families abandoned their high metabolisms.
“This seems counterintuitive because we love hot blood on ourselves like this great evolutionary innovation, which it was,” Dr. Brusatte said. But high metabolisms are expensive in terms of diet and energy, he notes, adding that what they needed to maintain it may have been “too much of a responsibility for some dinosaurs.”