Paleontologists have long wrangled with the question of dinosaur metabolisms — whether they ran hot, like modern birds and mammals do, or resembled the slower metabolisms of modern reptiles. In a surprise, the answer seems to be both.
“While we had assumed that most dinosaurs were warm-blooded, there was just no way to measure the underpinning metabolic capacities,” said Jasmina Wiemann, a paleontologist at the California Institute of Technology. In the absence of available dinosaurs, she said, paleontologists grappling with questions about prehistoric metabolisms — whether a given beast was warm-blooded or coldblooded, for example — have had to rely on indirect evidence, like isotopic evidence or growth rates from slices of bone.
Now, Dr. Wiemann and her colleagues have pioneered a new method for directly measuring the metabolic rate of extinct animals. Their conclusions, published on 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 evolving a coldblooded metabolism more like those of contemporary and ancient reptiles.
The team analyzed over 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 the fossils and modern bones — one that directly correlates with how much oxygen the animal breathed. That, in turn, is a direct indicator of its 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, seem 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 having high metabolisms is 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 any dinosaurs had lower metabolisms, it would have been the giant, lumbering herbivores.
“Just 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 around another group of dinosaurs — the diverse superfamily of herbivores called ornithischians — were more surprising still. While ancestral ornithischians shared the hot-blooded metabolisms of other dinosaurs, Dr. Wiemann said, their larger 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 might have needed to maintain their core temperature via behavior — basking in the sun or seasonally migrating to warmer climates.
“The evolution of decreased metabolic rates in some ornithischians is surprising, especially given that the same is not true of 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 how we interpret the lifestyles and behaviors of these animals.”
Further research — and many more fossil samples — will be necessary to take the temperature of all the limbs on the ornithischian family tree. But they would not be the first members of the broader family dinosaurs were members of, the archosaurs, to potentially make the switch. Dr. Wiemann said growth rates of certain extinct crocodile groups suggested they also may have been warm-blooded, while their modern relatives evolved 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 high metabolisms.
“That seems counterintuitive because we cherish warmbloodedness in ourselves as 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 sustain it may have been “too much of a liability for some dinosaurs.”