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Pachycephalosaurus was a dome-headed dinosaur. Its huge head housed an incredibly thick skull, a tiny brain, and large eyes. Its rounded skull was up to 10 inches thick.

Pachycephalosaurus grew to be about 15 feet long and may have weighed roughly 950 pounds. Pachycephalosaurus probably had a good sense of smell. It had bumpy knobs on its snout and along the rear of its skull. This plant-eater had short forelimbs and a stiff tail (which had a distinctive mesh of interwoven tendons surrounding its rear portion).

Pachycephalosaurus lived during the late Cretaceous period, about 76 to 65 million years ago, toward the end of the Mesozoic, the Age of Reptiles. Among the contemporaries of Pachycephalosaurus were Albertosaurus and Troodon, Maiasaura, Tyrannosaurus rex, Ankylosaurus (an armored herbivore), Parasaurolophus, Corythosaurus (a crested dinosaur), and Dryptosaurus (a leaping dinosaur).

William Winkley discovered Pachycephalosaurus in 1938 on the family ranch outside of Ekalaka, Montana, USA. It was named Pachycephalosaurus in 1943 by Barnum Brown and Erich M. Schlaikjer. Although complete fossils are rare, many thick skull fragments have been found. Pachycephalosaurus fossils have been found in Alberta, Canada, the western United States, the Isle of Wight, Mongolia and Madagascar.