Thickest dinosaur skull

Pachycephalosaurus, a dome-headed herbivore that lived during the late Cretaceous period, about 76 to 65 million years ago, had a skull 65 cm (2 ft) long with a bone dome 20 cm (8 in) thick. Fragments of the skull have been found in Alberta, Canada, the western United States, the Isle of Wight, Mongolia and Madagascar. Pachycephalosaurus was discovered in 1938 by William Winkley on the family ranch outside of Ekalaka, Montana, USA. The dinosaur was eventually named in 1943 by Barnum Brown and Erich M. Schlaikjer.  

This compares to the thickest part of the human skull (the internal occipital protuberance) which is 20 mm. (0.7 in).