The “thickhead reptile” was described by Gilmore in 1931. Pachycephalosaurus lived during the Late Cretaceous Period about 67 to 65.5 million years ago. It was about 4 meters (13 ft) in length, 1 and half meters (5 ft) in height and 800 pounds (365 kilos) in weight.
Howard King saw a herd of Pachycephalosaurus on Isla Sorna when he was driving to the dock. They were described as easily irritated and their behaviour resembled that of buffaloes. Using their reinforced skulls, the dinosaurs encountered by King managed to inflict moderate damage upon the Jeep Wrangler he was driving in the form of a deep dent and a shattered plastic taillight.
Sources: The Lost World, by Michael Crichton, pages 290-91 (Arrow Books 1997 edition)