Triceratops was a very common dinosaur which lived at the very end of the Cretaceous period. It had a huge frilled head with horns over each eye that could reach over 3 feet long. Triceratops had a third, smaller horn on its nose. These would be fearsome weapons against a predator.[1] Triceratops is one of the most common dinosaur fossils found. More than 50 skulls have been found. Within the genus, at least 7 species have been identified. The ceratopsian family is one of the most successful and varied of the Late Cretaceous. Triceratops is the largest member of this family, reaching the size of a school bus.[1] Triceratops was a herd animal; it is believed that large groups numbering into the hundreds roamed North America. Their large, horny beaks and long rows of teeth were well designed for chewing the tough, low-growing plants of the Late Cretaceous. It was likely the main predator of these animals was Tyrannosaurus rex. A number of skeletons show bite and chew marks that match the teeth of T. rex. Horns and frills seemed to vary among individuals within the species. Some frills were very broad, others narrow. The nasal horn shows the most variance among individual specimens, no two being the same. The material that covered its horns in life would have added significantly to the length of the fossilized bone.[1] In 2009 researchers under supervision of Jack Horner argued that Triceratops was actually a juvenile version of Torosaurus. The massive frill of Triceratops would grow longer and thinner in time, until it may have had the same holes as seen in Torosaurus (see Triceratops#Torosaurus as growth stage of Triceratops). However, this evidence is still open to criticism and is still yet to be fully accepted by the scientific community.