Paleontologists have devised a new method to distinguish male from female dinosaurs, according to a paper in the latest issue of the Journal of Vertebrate Paleontology.
The technique, which involves analyzing upper-hind limb bones, may help to solve this long-standing male or female anatomical mystery, since the remains of dinosaurs today are not fleshed out with soft-tissue genitals.
"Bones are shaped by the muscles that attach to them, so difference in the shape or size of muscle attachments on the leg bones suggests differences in the muscle mass of the animal that the leg belonged to," co-author Susannah Maidment explained to Discovery News. Maidment is a postdoctoral researcher in the Department of Paleontology at the Natural History Museum in London.
For the study, she and colleague Holly Barden focused on stegosaur remains found in the Tendaguru Formation of Tanzania. Paleontologists may often just be able to unearth the fossils for a single dinosaur, making comparisons with other members of its species impossible.
In this case, however, numerous scattered bones all belonging to the armored stegosaurian dinosaur Kentrosaurus aethiopicus were found. These included 50 preserved femora (upper hind limb bones) dating to about 150 million years ago.
"We used a method that examines shape differences in the leg bones, and we were able to show that at the top end of the bone, where some of the hip muscles attach, there are shape differences," Maidment said. "We were able to group the adult bones into two statistically significant groups: that is all adult bones had one morphology or the other morphology."
Fossils for juvenile dinosaurs did not fall into the groups, indicating that the bone shape differences didn't occur until adulthood when the young dinosaurs likely evolved their secondary sexual characteristics.
Although the researchers now have the fossil groups separated by sex, they cannot be sure which group belonged to females and which belonged to males.
"We’ll probably never know that unless a complete, spectacularly well preserved specimen of Kentrosaurus is found with an egg in its oviduct," Maidment said.
Nevertheless, the fossil analysis technique provides researchers with a new tool that can be combined with other observed differences among members of the same dinosaur species.
"It is generally true in the natural world today that bizarre body parts or decorations are associated with some form of display, usually sexual," Barden, who conducted the study while at the University of Sheffield, told Discovery News. She is now a PhD student at the University of Manchester.
The scientists therefore now suspect that the unusual spikes and armored plates found on stegosaurs may have been had unique particular shapes, depending on whether the individual was a male or female. Since there are only two specimens of Stegosaurus with complete rows of armor, the researchers cannot yet assess those probable sexual differences.
Paleontologist Ralph Chapman, an owner/partner at New Mexico Virtualization, has also studied dinosaur fossils with sexual dimorphism in mind.
Chapman told Discovery News that the new study "pretty much achieves what they say it does and I think it is a good paper." He pointed out that the ends of femora fossils might be particularly revealing, in terms of identifying differences among dinosaurs.
Answers to present questions remaining about sex identification in dinosaurs require more fossils, Maidment said.
"The only way we will ever be able to definitively answer questions about sexual dimorphism and other behaviors in dinosaurs is through exceptionally preserved specimens," she concluded.