Pachycephalosaurus

Chapter • Meet the dinosaur

Overview

Pachycephalosaurus was the largest known member of the dome-headed dinosaurs. It reached about four to four and a half metres in length and is estimated to have weighed 370–450 kilograms. Its defining feature was the skull roof: a solid dome of compact bone more than 20 centimetres thick, ringed by bony knobs and short spikes across the back of the skull and along the snout. The body was built for two-legged walking, with short arms, powerful hind limbs and a stiff tail held horizontally to counterbalance the forward-leaning torso. Lattices of ossified tendons braced the spine, so the tail never dragged along the ground. Exactly what the dome was for remains one of palaeontology's liveliest debates. Early reconstructions imagined bighorn-sheep-style head-on collisions, but computer modelling and studies of bone microstructure suggest the domes were more likely used in flank-butting and shoving contests, or as visual signals of age and status. Crucially, several specimens preserve lesions and healed bone tissue on the dome, which is hard to explain without genuine physical combat. The dome grew and changed shape dramatically with age, which is why juveniles look so different from adults. The teeth of Pachycephalosaurus were small, leaf-shaped and rather weak, with slightly differentiated front teeth. That points to a diet of soft plant material — leaves, shoots, fruits and seeds — and quite possibly some insects or other small animals as well, making it functionally closer to an omnivore than a bulk browser. It lacked the grinding tooth batteries of hadrosaurs, so it must have been a selective feeder rather than a mass processor of tough vegetation. Barnum Brown and Erich Maren Schlaikjer formally named the genus in 1943, based on a skull from the Hell Creek Formation of Montana. A fragment collected far earlier, described by Joseph Leidy in 1872 as Tylosteus, was later recognised as part of a pachycephalosaur skull. In 2009 John Horner and Mark Goodwin argued that the spiky, flat-headed Dracorex and the horn-fringed Stygimoloch are simply juvenile and subadult growth stages of Pachycephalosaurus — a proposal that is widely, though not universally, accepted.

Chapter • Where it lived

Habitat

Pachycephalosaurus lived during the very end of the Cretaceous, in the Maastrichtian stage roughly 72–66 million years ago, in what is now the western United States. The setting was a warm, humid coastal lowland east of the rising Rocky Mountains, threaded with meandering rivers and covered in conifer forest, flowering plants, ferns and palms. Fossils come from the Hell Creek Formation of Montana and North Dakota, the Lance Formation of Wyoming and the Denver Formation of Colorado. These beds represent floodplains and river channels, and the tough domes are typically found isolated and water-worn, having survived transport that destroyed the rest of the skeleton.

Chapter • Wild behavior

Behavior & Ecology

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Chapter • Scientific place

Taxonomy