Pachycephalosaurus, the thick-headed lizard
Pachycephalosaurus was one of the last dinosaurs that ever lived. It is a two-legged, plant-eating dinosaur that reached 15’ in length and 1,000 lbs. It has the thickest dinosaur skull, spiky, knobby, and the dome on top of the head was 10” thick! The 10” thickness has long been interpreted as being used for ramming other pachycephalosaurs similar to bighorn sheep of today. Recent studies of damaged pachycephalosaur bones suggest that, rather than ramming head-on, they stood slightly past one another and whipped their heads into the chests of their rivals like giraffes do today.
The beak at the end of its mouth allowed it to snip the toughest, most fibrous plants. Their teeth are very small, leaf-shaped, and oddly sharp and serrated. These sharp serrations finished crushing whatever the beak snipped off, making short work of cycads and conifers. Interestingly, with such serrated teeth it is possible they ate small reptiles, birds, mammals, and dinosaurs.
All of the small pachycephalosaurids like Dracorex, Stygimoloch, Goyocephale, and Homalocephale, which lived at the same time and place as Pachycephalosaurus, are now recognized to represent different gender and growth stages of Pachycephalosaurus! These “new” dinosaurs were named because of their small size and wildly different skull spikes and knobs. As it turns out the spikes and knobs change as the animal gets older. Juveniles start out very “spikey” but, as they age, the spikes turn to knobs and the knobs ultimately end up forming the giant dome that reaches 10” thick. As only the larger, dome-headed specimens have skull injuries, paleontologists believe they were males that fought one another for the rights to mate or lead a herd.
Pachycephalosaurus had binocular vision which it needed to keep a sharp eye out for the many predators of the day, including Tyrannosaurus, Acheroraptor, and Dakotaraptor. It relied primarily on speed to escape. Notice how the toe claw is blunt and slightly curved downwards. In life, it would have been covered in a large toe-nail and its size and shape helped it take off quickly and run at a high speed. However, with its powerful legs, the claw would have been a useful deterrent against predators that it couldn’t run from, turning and delivering a powerful kick like flightless birds do today.
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