Some prehistoric reptiles may have been warm-blooded

Some of those enormous marine reptiles prowling the Earth’s prehistoric seas may have had a surprising edge in their search for prey, researchers say. They may have been warm-blooded.

In a study published online Thursday in the journal Science, French scientists explored whether three types of marine predators — ichthyosaurs, plesiosaurs and mosasaurs — might have been able to maintain their body temperature internally, much the way that mammals and birds do. Two of them appear able to have done so.

The findings add to growing evidence that so-called warm-blooded animals and cold-blooded animals coexisted much further back on the evolutionary tree than scientists had believed — breaking some long-held ideas about evolution.

“Warm-blooded did not replace cold-blooded — it’s just something parallel,” said Christophe Lecuyer, an isotopic geochemist at the University of Lyon and one of the study’s authors.


Reptiles are generally considered to be cold-blooded, or exothermic, meaning they rely on the temperature of their environment to determine body temperature. Thus, they have less energy and move more slowly when it’s cold, and they become more active when the temperature rises.

Scientists had long thought that held true for prehistoric reptiles as well. But that distinction between warm-blooded (or endothermic) and cold-blooded has become increasingly less clear-cut. Some nonmammalian predators in today’s oceans — tuna and certain sharks, for example — exhibit some ability to regulate their body temperature.

The researchers wondered whether the same might hold true for ancient ocean reptiles. To answer that question, they compared the aforementioned types of large marine reptiles. Ichthyosaurs sported a dolphin-like body with a fish-like tail; plesiosaurs had long necks and paddle-like limbs. Both were known as “cruisers,” meaning they moved frequently and actively searched for prey. That’s typical of warm-blooded animals.

Mosasaurs shared the long neck and paddle-like limbs of plesiosaurs but were known as “accelerators,” meaning they lay in wait and grabbed prey as the opportunity arose. The ability to regulate body temperature would not have been as useful to them.


To determine whether the animals maintained their body temperature in spite of their environments, the scientists looked at the mineral composition of fossilized teeth, specifically the proportion of two types of oxygen isotopes. The ratio between oxygen-16 and the slightly heavier oxygen-18 changes depending on the temperature. Thus, the proportion of heavier oxygen to lighter oxygen would reveal how warm the animals’ bodies were at the time the teeth were growing.

The scientists found that the plesiosaurs and ichthyosaurs — the cruisers who swam constantly in search of prey — appeared to maintain their body temperature, while the mosasaurs’ internal heat appeared to vary slightly with the water temperature at the time.

Lecuyer said an animal that could maintain its temperature must have been able to generate its own heat, and that the next logical course of study would be to determine which animal was the first to evolve the ability to regulate body temperature.

But Ryosuke Motani, a paleobiologist at UC Davis who wrote a commentary on the paper, pointed out that the fact that ichthyosaurs’ and plesiosaurs’ body temperatures appear to have been constant does not necessarily mean that the animals generated their own heat. Perhaps they simply retained heat extremely well.


Still, he said, “this opens up [a line of inquiry for] many new future studies. My suspicion is the evolution of this kind of creature may be in correlation with a change in environment — it’ll be a long way to test that, but it’s doable now, thanks to the paper.”