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The name Pachycephalosaurus means "thick-headed lizard", and describes the creature's most noteworthy feature: the huge 10 inch (25 centimeter) thick bone dome on its head. At the rear of the dome were a number of bony knobs, and the animal's snout also possessed several short bony spikes. This has led most scientists to conclude that the animal was probably related to the Ceratopsians (which include Protoceratops, Psittacosaurus, Styracosaurus and Triceratops), although unlike most Ceratopsians, Pachycephalosaurus is believed to have been bipedal.
It was once thought that Pachycephalosaurus would have used the dome on its head either to engage in butting contests for mates (similar to modern big-horned sheep), or to defend against predators. However, it is now believed that this could not have been the case: the head bones do not appear to be strong enough for such impacts, the skulls lack shock absorbing features, and there no evidence of scars or other damage have been found on fossilized Pachycephalosaurus skulls.
Another somewhat unusual feature of Pachycephalosaurus is its teeth. The animal had very small ridged teeth which would have been far less capable of chewing tough, fibrous plant material than other herbivorous dinosaurs of the late Cretaceous. It is therefore thought that perhaps Pachycephalosaurus lived on a diet of leaves, seeds, fruit and perhaps insects.
So far, only the skull of Pachycephalosaurus has been discovered. Scientists estimate that the fully grown creature was 15 feet (4.6 meters) long, and weighed about 950 pounds (430 kilograms). They have come up with these figures by examining the skull and extrapolating the likely size on the creature based on the head-to-body ratios of its close relatives.
Pachycephalosaurus was a herbivore (plant-eater) that lived from 76 to 65 million years ago
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