BioEd Online

Palaeontologists have found a way to pin down the sex of fossils, thanks to a similarity between the bones of dinosaurs and birds.

Mary Schweitzer of North Carolina State University in Raleigh and her colleagues developed the method by looking at the remains of MOR 1125, a Tyrannosaurus rex unearthed a few years ago in the Montana mountains.

By cutting up the dinosaur's skeleton, Schweitzer and her team found a substance that looked like medullary bone. This is a tissue that coats the inside of the hollow bones of modern female birds: it is dense and shot through with many blood vessels, which help to transport calcium to a developing egg.

The therapods, the group to which T. rex belongs, are thought to be more closely related to birds than to present-day reptiles. Although Tyrannosaurus rex was too cumbersome even to run, it did have hollow bones: the lightweight feature that makes it easier for birds to fly. Finding medullary bone in MOR 1125 implies that the specimen was female.

"We can say now that these animals produced their eggs like birds, and probably very differently to crocodiles," adds Schweitzer.

Egg hunt

Determining the sex of dinosaur specimens is usually tricky. "The only way is to find one with eggs in its body cavity, or this medullary material, everything else is just conjecture," Schweitzer says. Some experts have suggested that females may have more ornamented heads or skeletons that differ from male ones, but such theories have been impossible to prove.

"It's pretty difficult," agrees Paul Barrett, a dinosaur expert at the Natural History Museum in London. "You're dealing with extinct animals. Sometimes you might be lucky enough to find a population of dinosaurs that fall into two distinct groups, but even then deciding which is which can be difficult."

Schweitzer's technique has some disadvantages, however. Although the presence of medullary bone can prove that an animal was female, the absence of such bone does not prove that it was male. It may be that medullary bone is very poorly preserved, Barrett says. Finding it involves cutting the bone in cross-section, he adds. "There are a lot of bones that museum curators wouldn't want you to cut up."

As a female bird ages, her amount of medullary bone dwindles. The thinness of MOR 1125's medullary bone hints that she may have been nearing the end of her egg-laying career, Schweitzer says. "She had probably laid most of her eggs and didn't have many left."