Pachycephalosaurus means “thick-headed lizard”. With a skull that was up to 10 inches thick, we believe it used its head as a defensive weapon. This herbivore walked upright and lived in, what is today, North America. Fossil remains have been found showing it reaching 15 feet long and weighing up to 1,000 pounds! As with many other dinosaurs, Pachycephalosaurus became extinct 66 million years ago.