By Sid Perkins
Ancient marine reptiles, which cruised the seas while dinosaurs roamed the land, may have been warm-blooded, with some species able to maintain a body temperature as much as 20 degrees Celsius above that of the waters they swam in, a new study suggests.
The finding, reported in the June 11 Science, bolsters the notion that many marine reptiles had relatively high metabolic rates similar to those of modern-day tuna, says Christophe Lécuyer, an isotope geochemist at the University of Lyon 1 in Villeurbanne, France. And that warm-bloodedness played a big role in these creatures’ evolutionary success, he notes: As a group, marine reptiles such as ichthyosaurs and plesiosaurs were among the ocean’s top predators for more than 185 million years.
While the body temperature of a cold-blooded, or ectothermic, creature is the same as that of the animal’s environment, warm-blooded, or endothermic, creatures use internally generated heat to partially or fully regulate their body temps. Within warm-blooded animals, those that can maintain a constant body temperature in most environmental conditions, such as mammals, are dubbed homeotherms.
Warm-bloodedness typically leaves its mark in a creature’s teeth — specifically, in the ratios of various forms of oxygen, or isotopes, that are preserved in the long-lasting phosphate minerals of tooth enamel, says Lécuyer. So he and his colleagues compared the proportion of oxygen-18 isotope in the tooth enamel of specimens from all three major groups of ancient marine reptiles — ichthyosaurs, plesiosaurs and mosasaurs — with the proportion of that isotope found in tooth phosphates of the presumably cold-blooded fish unearthed from the same rocks.