Which is the she rex and which the he rex?
Researchers think they have found the answer.
A team led by Mary H. Schweitzer of North Carolina State University reports finding a layer of medullary bone inside the leg bones of a Tyrannosaurus rex discovered in Montana.
Medullary bone is a calcium-rich layer that develops in the long bones of birds during the egg-laying process. It provides a ready supply of calcium to form eggshells.
Its presence inside this T. rex's legs indicates that the dinosaur was a female, Schweitzer said. The finding will enable researchers to determine the sex of at least some dinosaurs.
It also adds weight to the widespread belief that today's birds descended from dinosaurs.
Her findings are reported in Friday's issue of the journal Science.
"The discovery of medullary bone in a specimen of T. rex is hugely exciting," said Chris Dacke of the department of pharmacy and biomedical sciences at the University of Portsmouth in England. "It has never previously been reported in any other class of animal than birds."
Dacke, who was not part of Schweitzer's research team, said the finding "certainly helps cement the notion that birds are directly descended from dinosaurs." He said it also suggests that "dinosaurs, like birds, developed similar reproductive strategies for providing an adequate supply of calcium for the eggshell."
This discovery will not enable paleontologists to determine the sex of all dinosaurs because medullary bone is present only during the egg-laying cycle. But when present, it at least enables scientists to say that a particular example is female.