MIKKEL JUUL JENSEN / SCIENCE PHOTO LIBRARY MIKKEL JUUL JENSEN / SCIENCE PHOTO LIBRARY
Pachycephalosaurus dinosaur. Artwork showing a series of types of head found in Pachycephalosaurus dinosaurs. It is thought that the variation from no dome to a domed skull is a progression from young to old, or alternatively that the anatomy at left is that of a female. Some identify these variations as distinct species (from left to right): Dracorex, Stygimoloch and Pachycephalosaurus. These dinosaurs lived in the Late Cretaceous, around 70 to 66 million years ago.
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