Pachycephalosaurus

Pachycephalosaurus was the largest of the bone-headed dinosaurs, a bipedal herbivore that roamed western North America during the final age of the dinosaurs. Its most distinctive feature was an enormous domed skull roof up to 25 centimeters (10 inches) thick, surrounded by a crown of bony knobs and short spikes. This remarkable cranial architecture has made it one of the most recognizable dinosaurs, though its actual function remains hotly debated.

For decades, paleontologists assumed Pachycephalosaurus used its reinforced skull for head-butting combat, similar to modern bighorn sheep. However, recent biomechanical studies have raised doubts — the dome's rounded shape might have caused glancing blows rather than solid impacts, and the neck vertebrae may not have been aligned to absorb such forces. Alternative hypotheses suggest the dome served for flank-butting, species recognition, or sexual display.

The genus was first described in 1943 by Barnum Brown and Erich Schlaikjer based on fragmentary skull material from Montana. For many years, Pachycephalosaurus was known almost entirely from skull fragments, making body size estimates uncertain. The possibly synonymous genera Stygimoloch and Dracorex may actually represent juvenile growth stages of Pachycephalosaurus — a hypothesis championed by Jack Horner that suggests the dramatic skull ornamentation changed substantially as individuals matured.

Pachycephalosaurus was among the last non-avian dinosaurs to exist, living alongside Tyrannosaurus and Triceratops right up until the Cretaceous-Paleogene extinction event 66 million years ago. Its fossils have been found in the Hell Creek and Lance Formations of Montana, South Dakota, Wyoming, and Alberta, Canada.