Pachycephalosaurus

Pachycephalosaurus was the largest and final known member of the dome-headed dinosaur lineage, living right up to the end-Cretaceous extinction event. Found in famous deposits such as the Hell Creek Formation, it reached over four metres in length and weighed roughly four hundred kilograms. The roof of its skull formed an immense dome of solid bone up to twenty-five centimetres thick, fringed by short spikes on its snout and cheeks. Scientists widely debate whether rival adults directly head-butted one another or engaged in flank-ramming during territorial contests; many fossil domes exhibit healed lesions consistent with physical trauma. Some palaeontologists also argue that the spiky flat-headed genus Dracorex and the dome-crested Stygimoloch merely represent younger growth stages of Pachycephalosaurus.

Its solid skull dome was up to twenty-five centimetres thick and surrounded by blunt bony spikes.

Where and when

Pachycephalosaurus lived in the Late Cretaceous, from 68 to 66 million years ago (Maastrichtian age). Its remains come from United States, in the Hell Creek Formation.

The numbers

Length 4.3 m (estimates run from 4 to 4.6 m). Mass about 420 kg. Hip height about 1.4 m. That is 2.5 human-lengths, or 0.38 of a double-decker bus. These are estimates from partial skeleton; fields marked estimated by the site are length, mass, hip height.

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