Sexing a dinosaur isn’t easy.

As far as gross skeletal anatomy is concerned, male and female dinosaurs are practically identical. And the shape of saurian bones provides no help. So far as anyone has been able to tell, the skeletons of dinosaurs were not sexually dimorphic (or, in other words, different between males and females). Even in highly-ornamented species of horned dinosaurs, armored dinosaurs, and others, all the gnarly spikes and plates and crests don’t show a definitive split in form that can be taken as a marker of different sexes.

But the evolutionary architecture of bones isn’t everything. A pair of surprises provided opportunities for paleontologists to identify some female dinosaurs, at least. In 2005 paleontologist Tamaki Sato and coauthors reported on