Ancient marine reptiles losing their cool

Study suggests creatures might have been warm-blooded

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.

In general, the marine reptiles had more oxygen-18 in their tooth enamel than fish that had lived in the same waters — a sign that the reptiles had some degree of warm-bloodedness, or endothermy. The larger the isotope-ratio difference between the reptile and fish, the larger the difference in body temperature, says Lécuyer.

In some cases, marine reptiles had a body temperature about 20 degrees Celsius warmer than the waters where they swam. The team’s data suggest that the marine reptiles probably had body temperatures ranging between 35° and 39° C.

How the marine reptiles generated and preserved their body heat isn’t clear, says Lécuyer. In addition to having high rates of cell metabolism, many warm-blooded creatures such as mammals and birds glean heat from their digestive processes. Also, some sharks and tuna maintain a body temperature higher than their surroundings by taking advantage of heat generated by muscles deep within their bodies. Many of today’s sea turtles have a layer of blubber that slows the loss of body heat and therefore helps maintain body temperature above that of the surrounding water.

Warm-bloodedness enabled marine reptiles to maintain high levels of activity needed to pursue prey or to migrate long distances, says Lécuyer. Previous studies have suggested that some species of ichthyosaurs needed a high rate of metabolism to swim at the speeds most suited to their dolphin-like shape (SN: 8/24/02, p. 122).

The new findings may help researchers determine when and how warm-bloodedness evolved in these three lineages of marine reptiles, says Ryosuke Motani, a paleontologist at the University of California, Davis. Because the land-dwelling ancestors of these creatures probably weren’t endothermic, the trait was likely to have evolved as the creatures adapted to life in the open ocean, he notes.

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