Annual growth lines seen in the femurs of 115 wild warm-blooded mammals such as giraffes, reindeer and gazelles are similar to those previously seen in the bones of reptiles and dinosaurs and cited by some as evidence of their cold-blooded nature, study lead author Meike Kohler, a paleontologist at the University of Barcelona in Spain, said.
"People always said that mammals do not show these lines," Kohler said.
In dinosaurs and reptiles, yearly cycles of growth and nutrition are seen in the bones like the rings of a tree as animals pack on blood vessel-rich bone tissue in times of bounty and in lean months they skimp, laying down only thin sheets.
Because these "lines of arrested growth" or "rest lines" are seen in the bones of both dinosaurs and reptiles, some scientists had assumed that dinosaurs, like reptiles, were cold-blooded.
But Kohler's study, reported in the journal Nature, shows warm-blooded mammals have the same bands of lines in their bones.
"[Line of arrested growth] cannot be used as an argument that dinosaurs could not have been endothermic [warm-blooded," Kohler said.
Dinosaur bone tissue is indistinguishable from that of today's warm-blooded ruminant mammals, she said, meaning that dinosaurs were endothermic, too.
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