The same T-rex that preserved soft tissue was found in has been used to come up with a methodology to determine gender.
"Unlike mammals, we don't see [gender] differentiation in the skeletons of
reptiles," she added (this refers to Angela Milner at the Natural History Museum
in London). "From the skeleton itself, it's impossible to tell whether an animal
was a male or a female. Reptiles don't give birth to live young, so they don't
have broad or narrow pelvises, for example."
The top picture is of a hen's femur. The red colored inner bone is medullary bone (MB in the picture). The white outer layer is called cortical bone. In chickens the only gender that has medullary bone in their femurs are females - and then only prior to egg laying. The calcium in the medullary bone is used to create the egg shell. The bottom two pictures are of a T-rex femur. Note it also has medullary bone.
Medullary bone is rich in calcium, providing female birds a ready source of
material for making eggshells. So this particular T. rex, the researchers say,
must have been an egg layer, too.
That said, medullary bone might not be present in female dinosaurs that died when they weren't breeding. The tissue's formation in the leg bones of female birds is triggered by increasing levels of hormones associated with egg production. This calcium supply is then drawn down as it's converted into eggshells.
The team's analysis showed that the flesh-eating predator's medullary bone is more like that of female ratites, such as ostriches and emus, than that of other living birds. Schweitzer speculates that this could be because, like T. rex, these birds are relatively large animals.
"Their bones are much larger, relative to the size of their eggs, so the calcium draw would be significantly less," she said. "Also, ratites retain more primitive traits than other birds, and so more likely share more characteristics with dinosaurs."
The researchers say their findings strengthen the link between dinosaurs and birds by suggesting both share similar egg- laying processes.
I posted here on the similarity between egg laying in dinosaurs and birds. This new finding relating another aspect of the egg laying process in dinosaurs to birds only strengthens the evolutionary relationship between the two.
Added several hours later: I have just discovered that The Panda's Thumb has a post on this as does Palaeoblog and the press release (contains more pictures) is here.
From the Press Release:
Schweitzer viewed the tissues under both a light and an electron microscope, and found that the dinosaur tissues were virtually identical to those of the modern birds in form, location and distribution. Demineralization – the chemical removal of a bone’s minerals in order to obtain organic material that is much easier to work with in a lab environment – of the samples revealed that the medullary bone from the ostrich and emu was virtually identical in structure, orientation and even color, with that seen in the T. rex.