Dinosaurs kept warm in the polar chill.

Studies of Australian dinosaurs' bones have added fire to the debate about whether the extinct creatures were warm-blooded or cold-blooded. Unlike almost all other species examined so far, the Australian fossils show evidence of sustained, rapid growth, suggesting that these dinosaurs kept their bodies warm even during frigid winter conditions, according to paleontologists who described their finding last month at the Dinofest symposium in Philadelphia.

Anusuya Chinsamy of the University of Cape Town in South Africa and her colleagues cut through bones of small herbivorous dinosaurs in search of lines of arrested growth, which resemble the growth rings in trees. Such lines, seen in all modern reptiles, indicate that the bones periodically go through episodes of little or no growth, says Chinsamy. The lines do not appear in the bones of endothermic animals, such as mammals and modern birds, which generate their own heat to maintain a constant body temperature.

The group found no growth lines when they examined the bones of these dinosaurs, which lived roughly 100 million years ago along the southern coast of Australia. The animals belonged to the hypsilophodontid family and lived south of the Antarctic circle, where winters were frozen and dark during the Cretaceous period. The average temperature of the coldest month could have ranged between -24 [degrees] C and -32 [degrees] C, reports Patricia Vickers-Rich of Monash University in Clayton, Australia.

In past studies, Chinsamy has found lines of growth in the bones of all dinosaurs except other hypsilophodontids from sites in Texas, the Isle of Wight in England, and Africa. Coupled with the evidence of low winter temperatures in Australia, the bone data suggest that hypsilophodontids could maintain an elevated body temperature and that their relatively advanced physiology approached that of modern mammals and birds.

"If any dinosaurs were endothermic, it would be these ones," says Chinsamy, who describes the new study in the June Journal Of Vertebrate Paleontology. Other dinosaurs had physiologies somewhere between those of modern reptiles and modern mammals, she says.

The Australian hypsilophodontids had much larger optic lobes than did the species living closer to the equator, a possible adaptation that would have helped the polar creatures forage throughout the dark winter. Other Australian dinosaurs from this time had lines of growth in their bones, which suggests they may have hibernated through the cold, suggests Vickers-Rich.