A hundred million years ago, the seas were filled with swimming reptiles - some of them 70 feet long. A new study of their teeth suggests these exotic giants kept a constant body temperature - a step toward the kind of warm-bloodedness that keeps us at 98.6 degrees Fahrenheit.

The temperature inside an animal is reflected in the ratio of two forms of oxygen - the far more common oxygen-16 and the slightly heavier oxygen-18 (which has two extra neutrons). The colder the temperature, the fewer atoms of oxygen-18 get incorporated into the teeth of fish.

Scientists from the University of Lyon in France measured this ratio in teeth from three giant marine reptiles: the paddle-finned plesiosaur, the fish-like ichthyosaur, and the giant lizard-like mosasaur. They also examined presumably cold-blooded fish from the same era - whose teeth should reflect what the temperature was in the seas.

They concluded that these reptiles were running temperatures over 100 degrees. They published their work last week in the journal Science.

But those creatures probably weren't warm-blooded the same way mammals and birds are, said Ryosuke Motani of the University of California, Davis, who wrote a commentary piece in the same issue. He used the more general term homeothermic.

Homeothermic animals today include leatherback turtles and tuna. The latter combine good insulation, a heat exchange system and a slightly revved-up metabolism to stay near 80 degrees.

Motani said he agrees with the use of oxygen isotopes to read ancient temperatures, but he quibbles with the way they calibrated the scale to get such fevered readings. He estimates the old reptiles' temperatures at closer to 80 degrees.

How and why we became warm-blooded remains a mystery. Some say it evolved to give animals more strength and endurance.

That may be true of the carinvorous reptiles, too. These creatures probably cruised around 24/7, said Motani. Otherwise they would drown. - Faye Flam