Traditionally, dinosaurs have been lumped together with and depicted alongside reptiles, leading to the assumption that they shared the latter’s traits and lifestyle. But evidence has been mounting over the last three decades that dinosaurs were quite bird-like, and the ancestors of birds, a new question arose among paleontologists: were dinosaurs cold-blooded?

A new study from the California Institute of Technology brings fresh evidence that the metabolism of these ancient beasts likely functioned to produce warm-blooded creatures, judging from the biochemical waste compounds they produced.

Telltale traces

“This is really exciting for us as paleontologists — the question of whether dinosaurs were warm- or cold-blooded is one of the oldest questions in paleontology, and now we think we have a consensus, that most dinosaurs were warm-blooded,” said lead study author Jasmina Wiemann, a postdoctoral researcher at the California Institute of Technology.

For the study, the team analyzed the waste products resulting from the interaction between oxygen and the proteins, sugars, and lipids within the bodies of dinosaurs. Wiemann and her team analyzed samples of femur — thigh bone — from 55 different creatures, including 30 extinct and 25 modern animals, to chart how the quantity of such molecules varies across different organisms.

The quantity of such waste molecules produced by an organism scales with the amount of oxygen it takes in from the environment, and can therefore tell us whether it was warm- or cold-blooded. These molecules appear as dark-colored patches in fossils; they are very stable and do not dissolve in water, meaning they weather the fossilization process extremely well and give us reliable information about any fossil they are found in.

Among the samples the team analyzed were bones belonging to dinosaurs, pterosaurs (giant, flying reptiles), and marine reptiles such as plesiosaurus, alongside modern birds, lizards, and mammals. They used a technique known as infrared spectroscopy to measure waste molecules in these samples and then compared these figures with the known metabolic rates of modern animals. Based on these datasets, the team could then estimate the metabolic rates of the extinct species.

The results suggest that dinosaurs’ metabolic rates were typically high. Some species’ rates were more similar to that of modern birds that have average body temperatures of around 107.6 degrees Fahrenheit / 42 Celsius, and higher than those of the average mammal, with typical body temperatures of around 98.6 degrees Fahrenheit / 37 Celsius.

Some notable exceptions were identified in the study, however. Ornithischians, an order of animals characterized by their lizard-like hips that includes triceratops and stegosaurus, seemed to have evolved lower metabolic rates, comparable to those of cold-blooded modern species.

“Lizards and turtles sit in the sun and bask, and we may have to consider similar ‘behavioral’ thermoregulation in ornithischians with exceptionally low metabolic rates. Cold-blooded dinosaurs also might have had to migrate to warmer climates during the cold season, and climate may have been a selective factor for where some of these dinosaurs could live,” Wieman said.

Although it has been hypothesized in the past that birds survived the mass extinction that wiped out the dinosaurs 66 million years ago, the team’s findings do not support this view. Many species that were identified to have very high metabolic rates went extinct during that time.

Having a high metabolic rate has been proposed as one reason why birds survived the mass extinction that wiped out dinosaurs 66 million years ago. However, Wiemann said this study indicated that this wasn’t true: Many dinosaurs with bird-like, exceptional metabolic capacities went extinct.

Warm-blooded animals, those with high metabolic rates, need lots of oxygen and calories to maintain their body temperature — whereas cold-blooded animals can coast along on much less food. The current paper answers a long-standing question in paleontology, and will better inform our understanding of how these ancient beasts lived.

The paper “Fossil biomolecules reveal an avian metabolism in the ancestral dinosaur” has been published in the journal Nature.