No fossil has revealed two dinosaurs caught in the act -- the only known vertebrates to be unequivocally preserved mating are a pair of 47 million-year-old turtles
that were attached by their genitals as they got buried alive.
It's also not possible to easily determine whether a dinosaur is male or female from fossilized bones.
Fossils that preserve elements of dinosaur behavior are very rare. However, with close analysis and insights from what we know about living animals, particularly birds, paleontologists are beginning to piece together the sex lives of dinosaurs.
Many species of animals show a difference in appearance between the sexes -- what's called sexual dimorphism. Think of a lion's mane, a peacock's feathers or a stag's antlers. Such features are surprisingly difficult to determine in extinct species.
Despite many earlier claims, including that female T. rexes were bigger than males, such findings are now thought inconclusive.
Differences in anatomy could point to a young and old individual or two separate species, or just variations that have nothing to do with sex.
"We really don't know, 100%. I could not confidently hold my hands up and say you know what, this T. rex is male, this T. rex is female. It's unfortunate because as a paleontologist it's a fascinating and fun area to explore," said paleontologist Dean Lomax, a visiting scientist at The University of Manchester's department of Earth and environmental sciences.
One exception to this is Confuciusornis, a 125 million-year-old dinosaur that has many features in common with modern bird species and shows a remarkable difference in plumage between male and female specimens.
Some fossils show body-length ribbonlike tail feathers -- a feature that had been interpreted as being used for sexual display. Scientists were able to find indisputable proof that females did not have this ornamental plumage.
Researchers identified evidence of the medullary bone -
- calcium-rich tissue present during a short period of time in a reproductively active female bird used to make eggshells
-- in the ancient birds that did not sport the long plumage.
Work in the past decade on the cells that contain color pigments in the exquisitely preserved fossils of feathered dinosaurs have revealed that some dinosaurs were brightly colored -- perhaps surprisingly so, given how popular culture historically portrayed them as grayish green. Lomax believes it's possible that in the future we'll find a fossil that shows clear evidence of sexual dimorphism.
"In the future, probably from China, I imagine you'll find two distinct dinosaurs found with color, their anatomies will match, but they'll be very different in their coloration," said Lomax, who is also
the author of "Locked in Time: Animal Behavior Unearthed in 50 Extraordinary Fossils