During the 1970s a major debate erupted among paleontologists. On the basis of new evidence, from the anatomy of the recently-discovered dinosaur Deinonychus to the microscopic bone structure of dinosaurs, paleontologists such as John Ostrom and Bob Bakker proposed that dinosaurs may have been endotherms—animals able to internally regulate their own body temperature. The work generated waves of support and criticism and ultimately gave birth to the image of fast, dynamic dinosaurs that we know today. While things are not as vociferous as they used to be, paleontologists are still investigating what kind of metabolisms dinosaurs had, how they regulated their body temperatures, and other aspects of their physiology. Similar questions have been asked about many of the creatures that lived alongside the dinosaurs, as well, and a new study published last week in Science suggests that some of the great vertebrates that lived in the sea may also have had unique metabolisms which allowed them to carry out active lifestyles.
During the time of the dinosaurs there were various types of marine reptiles, but among the most successful were the ichthyosaurs, the plesiosaurs and the mosasaurs. As with dinosaurs, it was previously suggested that some of these marine reptiles might have been able to maintain high, constant body temperatures (meaning they were both endotherms and homeotherms), but a team of paleontologists led by Aurélien Bernard and Christophe Lécuyer has found another way to approach the same hypotheses.
To investigate what kind of metabolisms these animals had, the scientists sampled the oxygen isotopes contained inside the teeth of the marine reptiles and compared them to oxygen isotopes taken from fish that lived at the same time. Previous studies have determined that the values of these oxygen isotopes can serve as signals of body temperature and the makeup of the water taken in by the animal's body during the time its teeth were being developed, and so they provided scientists with a way to investigate the metabolism of these animals. Furthermore, since the body temperatures of most fish are dictated by the surrounding water, their body temperatures would provide a proxy for the temperature of the sea in each place marine reptile samples were taken. By looking at the correspondence between the body temperatures of the fish and the marine reptiles, the scientists could see whether the marine reptiles had body temperatures dictated by the surrounding environment or whether they had some other metabolic mechanism.
What the scientists found was that the body temperatures of ichthyosaurs and plesiosaurs did not appear to be tied to the seawater temperature around them—they maintained their body temperature of about 95° Fahrenheit (35° Celsius) and as high as 102° F (39° C). This is within the range of living whales. The body temperatures of mosasaurs, on the other hand, did appear to be influenced by the surrounding water. They could maintain body temperatures above that of the surrounding seawater, like some sharks can, but their body temperatures still dipped as water temperature fell.
Based upon the evolutionary history of the three types of marine reptiles—each having a different origin—the results of the study suggest that homeothermy evolved among marine reptiles at least twice and the ability to maintain a body temperature above that of the surrounding seawater evolved three times. But what could account for the difference between plesiosaurs/ichthyosaurs and the mosasaurs? As the scientists behind the study hypothesize, it may be a matter of feeding habits. Ichthyosaurs were pursuit predators which relied on speed to catch food, and plesiosaurs were probably long-distance ocean cruisers—both lifestyles would have required high metabolic rates and hence body temperatures maintained above that of the seawater. Mosasaurs, by constrast, were probably ambush predators who lay in wait for prey and then struck quickly. They would not have to engage in the same kind of sustained activity, and so it makes sense that they did not have the same kind of high, constant metabolism.
Unfortunately there are no living plesiosaurus, ichthyosaurs, or mosasaurs for us to examine, but the study of these oxygen isotopes allows scientists to test ideas about the biology of these animals. Through a bit of geochemistry paleontologists can gain insight into the physiology of long-extinct animals, and I look forward to seeing how the results of this new study will be reassessed and investigated as further research is carried out.
Bernard, A., Lecuyer, C., Vincent, P., Amiot, R., Bardet, N., Buffetaut, E., Cuny, G., Fourel, F., Martineau, F., Mazin, J., & Prieur, A. (2010). Regulation of Body Temperature by Some Mesozoic Marine Reptiles Science, 328 (5984), 1379-1382 DOI: 10.1126/science.1187443