WAS it Tyrannosaurus rex or Tyrannosaurus regina? Until now, telling the tyrant-lizard kings from their queens has been pure guesswork. But the discovery of an unusual type of bone in a T. rex femur has given palaeontologists a reliable way of identifying the females.
Discerning the gender of fossilised animals is notoriously difficult, as the obvious differences are all in soft tissues that are not preserved. Some sex-specific traits do remain in bones, such as differences in size or skull adornment, but without any living animals for comparison, it is hard to be sure which sex is which. What’s more, it is sometimes uncertain whether two different fossil types are from different sexes of the same species or from different species altogether.
Fossils of the duck-billed dinosaur known as parasaurolophus, for example, have been found in two forms which differ mainly in the size of a flamboyant hollow bony crest. It is usually assumed that the showy individuals are males, which perhaps used the crest in courtship displays, but no one can be certain. So far the only foolproof way of identifying a dinosaur’s sex has been to find unlaid eggs inside the body, and only two such fossils are known (91av, 23 April, p 21).
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Now a structure called medullary bone, which is otherwise found only inside the leg bones of female birds, has been identified inside the femur of a 68-million-year-old T. rex fossil named B-rex. In birds, medullary bone is densely mineralised and rich in blood vessels. It acts as a store of calcium that can be quickly mobilised when it is needed to form eggshell, says Mary Schweitzer of North Carolina State University in Raleigh. It forms inside the outer cortex of the hollow bones, around the marrow.
Without it, the calcium needed to form eggshells would have to come from the birds’ bones, leaving them vulnerable to osteoporosis. As their eggs form, the shell formation depletes the medullary bone, which remains depleted during brooding and until the next ovulation. So finding medullary bone means you can be sure you have a female, but not finding it does not necessarily mean you have a male.
Schweitzer says she recognised the distinctive structure of medullary bone almost as soon as she put the first samples under the microscope. The T. rex medullary bone is similar to that in today’s emus and ostriches, which are close to the evolutionary roots of modern birds (Science, vol 308, p 1456).
The similarity bolsters the idea that birds evolved from theropod dinosaurs, a group whose members were largely, but not entirely, predators. Crocodiles, the closest living relatives of birds, do not have medullary bone, and there is no evidence that it was present in other types of dinosaurs.
“We have never seen anything like this in another dinosaur,” says team member Jack Horner of Montana State University in Bozeman. Schweitzer suspects that medullary bone evolved in early theropods – small, speedy animals with hollow bones – as a way to maintain strong bones while depositing eggshell.
So while medullary bone may help scientists tell she-rex from he-rex, it will be no use for most dinosaurs, including the ostentatious duck-bills.
