Dinosaurs Started Out Hot, Then Some of Them Turned Cold

Dinosaur metabolisms have been a topic of debate for a long time, whether they ran hot, like modern birds and mammals do, or slower, like modern reptiles. The answer seems to be both.

While we had assumed that most dinosaurs were warm-blooded, there was no way to measure their underpinning metabolism. She said that in the absence of dinosaurs, paleontologists have had to rely on indirect evidence to answer questions about prehistoric metabolisms.

A new method for measuring the metabolism of extinct animals has been pioneered by Dr. Wiemann and her colleagues. 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. The research suggests that some herbivorous dinosaurs spent tens of millions of years evolving a coldblooded metabolism like those of modern 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. They found a specific marker of metabolism in the fossils and modern bones that correlated with how much oxygen the animal breathed. That is a direct indicator of its metabolism.

The team found that both mammals and long-necked marine reptile had independently evolved their high metabolisms. The Pterosaurs and dinosaurs seem to have descended from warm-blooded ancestors, like sauropods and theropods.

Stephen Brusatte, a paleontologist at the University of Edinburgh, did not participate in the study. If dinosaurs had lower metabolisms, it would have been the giant, lumbering herbivores.

Imagine the amount of plants they would have to eat a day to fuel their metabolism.

The team's findings around another group of dinosaurs were more surprising. Stegosaurus and Triceratops, the larger descendants of ornithischians, reduced their metabolisms over time and ended up being closer to modern reptiles. They might have needed to maintain their core temperature by basking in the sun or moving to warmer climates.

The evolution of decreased metabolic rates in some ornithischians is surprising, especially given that giant sauropods do not.

The temperature of the limbs on the ornithischian family tree will need to be taken. The archosaurs, members of the broader family dinosaurs, could potentially make the switch. The growth rates of certain extinct crocodile groups suggested they may have been warm-blooded, while their modern relatives evolved slower metabolisms.

They have demonstrated the potential of this technique, and Dr. Wiemann said more detailed studies could help clarify why certain dinosaur families abandoned high metabolisms.

It seems counterintuitive because we like warmbloodedness in ourselves as a great evolutionary innovation. He notes that high metabolisms are expensive and that it may have been too much of a liability for some dinosaurs.