National Geographic News
A sea monster hunting belemnites.
An ancient ichthyosaur hunts squid-like animals in an artist's rendering.

Image by Matte FX, National Geographic

Charles Q. Choi

for National Geographic News

Published June 10, 2010

Giant reptiles that ruled dinosaur-era seas might have been warm-blooded, a new study says.

Researchers found that ancient ocean predators possibly regulated their body temperatures, which allowed for aggressive hunting, deep diving, and fast swimming over long distances.

(See "Giant 'Sea Monster' Fossil Discovered in Arctic.")

"These marine reptiles were able to maintain a high body temperature independently of the water temperature where they lived, from tropical to cold-temperate oceanic domains," said study co-author Christophe Lécuyer, a paleontologist at Université Claude Bernand Lyon 1 in France.

The prehistoric reptiles may have had body temperatures as high as 95 to 102 degrees Fahrenheit (35 to 39 degrees Celsius)—comparable to those of modern dolphins and whales, Lécuyer noted. (See whale pictures.)

Most modern reptiles and fish are cold-blooded, which means their internal temperatures vary along with those of the surrounding water.

Since the modern oceans' top predators—such as tuna and swordfish—are to some degree warm-blooded, this made the team wonder if ancient marine reptiles might have been, too, Lécuyer said.

Tuna and swordfish are homeothermic, or capable of keeping their body temperatures relatively constant, despite changing environmental temperatures. The predators are also partially endothermic, which means they can generate and retain enough heat to raise their body temperatures to high but stable levels.

Most animals thought of as warm-blooded, including mammals and birds, are also both homeothermic and endothermic.

Fossil Teeth Provide Sea-Reptile Clues

While dinosaurs dominated land during the Mesozoic period (251 million to 65 million years ago), three kinds of large swimming reptiles reigned in the seas—the dolphin-like ichthyosaurs, the serpentine mosasaurs, and the Loch Ness monster-like plesiosaurs. (See a prehistoric time line.)

By studying fossil teeth of fish that would have lived alongside these creatures, Lécuyer and colleagues were able to determine the teeth's oxygen isotopes, or atomic structures.

The levels of oxygen isotopes in teeth reflect those of the blood, which in turn reflect animals' body temperatures.

The team compared these results with oxygen-isotope compositions in modern-day fish that live in a variety of hot and cold environments.

Since most modern fish are cold-blooded, this data helped the team figure out the ocean temperatures of the ancient species' habitat.

Then the researchers compared oxygen-isotope data from the fossilized fish teeth with those seen in fossil-reptile teeth from the same areas.

"Enthralling" Sea-Reptile Findings

Homeothermy and endothermy in ichthyosaurs and plesiosaurs would make sense, as past studies of their body plans suggested the creatures were pursuit predators that needed to keep active, according to the study, published tomorrow in the journal Science.

The new data for mososaurs, which scientists suspect hunted by ambush, were more ambiguous, but are consistent with the idea that these reptiles could control their body temperatures to some degree, the authors write.

(See prehistoric sea-monster pictures.)

"The studies presented are enthralling," said paleontologist Zulma Gasparini at Argentina's Universidad Nacional de La Plata, who did not take part in the study.

"From what is known in living vertebrates, they are trying to interpret what could happen in vertebrates of the past."

The ancient reptiles' higher body temperatures also suggest the animals may have possessed heat-conservation systems, such as blubber layers and specialized blood circulation, said Ryosuke Motani, a vertebrate paleontologist at the University of California, Davis.

"From here we can really begin to investigate how this might have evolved," said Motani, who was not involved in the new research.

(See picture: "New 'Sea Monster' Species Identified.")

"These [sea reptiles] all came from land reptiles, who we're pretty sure were so-called cold-blooded, and it was probably the same when they started swimming. But over time it looks like homeothermy evolved, and so we need to figure out when that happened and why," he said.

"Maybe it evolved as they became better at cruising, or [because] there were changes in average temperature or in sea level."

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