In ancient Mesozoic seas, the biggest predators might not have been entirely cold-blooded killers. Rather, a new study suggests some of these rapacious reptiles might have been able to regulate their own body temperature, thereby expanding their hunting ranges. 

Some modern aquatic reptiles, including leatherback turtles, as well as some sharks and tuna are able to keep their body temperatures relatively stable compared to the fluctuating water temperatures around them. But just when and how many times this ability emerged in the evolutionary past has remained up for debate, shrouding many questions about the impact of ancient environmental changes on adaptation and hunting strategies.

The new study, published online June 10 in Science, proposes that ichthyosaurs, plesiosaurs and, perhaps to a lesser extent, mosasaurs, were all able to maintain relatively stable body temperatures in tropical and frigid waters alike some 251 to 65 million years ago.

By analyzing oxygen isotopes in the fossil teeth of these extinct animals, researchers were able to assess the creature's body temperature when the teeth were growing—even for teeth that protruded from the mouth. And comparing isotope ratios among animals that presumably lived in the same time and place, the team determined the reptiles' different internal temps—and different temperature regulation systems.

The researchers, led by Aurélien Bernard, of Paléoenvironnements et Paléobiosphère at the University of Lyon, found that these extinct marine behemoths were able to keep their internal temperatures between about 35 degrees Celsius and 39 degrees C (normal human body temperature is 37 C) regardless of ambient water temperature. And smaller fish that were found nearby in the same fossil beds, whose body temperatures were assumed to be closer to the water temperature, had much more variable stats.

This finding does not mean that the ichthyosaurs, plesiosaurs and mosasaurs were entirely endothermic (able to control their body temperature via internal mechanisms such as shivering or fat burning). Rather, the reptiles, unlike today's lizards and snakes, might have been homeothermic, another type of "warm-blooded" thermoregulation which entails maintenance of a relatively consistent body temperature. Leatherback turtles accomplish this through gigantothermy (a type of homeothermy), relying on their large enough body mass to keep a stable internal temperature.

Similarly, ichthyosaurs and plesiosaurs were large enough to have been gigantotherms, Ryosuke Motani, of the Department of Geology at the University of California, Davis, noted in an essay published in the same issue of Science. And these two reptiles are thought to have been "cruisers" that were constantly on the move looking for prey, and thus could have benefited from the higher metabolic rates—which would also have helped them extend their realms to cooler seas. Mosasaurs, however, were slightly smaller and hunted by ambush. Although they seem to have had warmer body temperatures than the nearby fish, Motani proposed that they might have been poikilotherms, whose body temperature does vary with the ambient temperature.

The authors note that reference fish might have different isotope readings from the large predators because they swam throughout the water column, or because they might have been deposited in the sediment at a slightly different time. The trends in temperature that the researchers found, they contend in their paper, are strong enough to indicate these variables do not explain away the consistent difference of temperature in the bigger reptiles.

Motani also cautions that some of the temperature readings are likely colored by "time-dependent depletion" of the oxygen isotopes, which means that "the older the fossil, the higher the estimated temperature unless the bias is removed." By his calculations, this adjustment would turn down the average body temperatures of the animals to 24 degrees C to 26 degrees C—closer to the realm of modern homeothermic sea creatures, such as tuna and leatherbacks, which range from 20 degrees C to 25 degrees C, than to the body temperatures of modern-day marine mammals.

Regardless of the exact body temperature of Jurassic ichthyosaurs and plesiosaurs, the new insight is a step toward determining when these animals' ancestors might have developed the capability to modulate  their body temperature. And that will present researchers a rare "opportunity to clarify environmental effects on evolution," Motani noted.

Image of elasmosaurid plesiosaur tooth from the late Cretaceous of Morocco courtesy of Christophe Lecuyer