Pachycephalosaurus wyomingensis was the last and largest of the so-called “dome-headed” dinosaurs. It measured fifteen feet long and lived 68-65.5 million years ago, right at the very end of the age of dinosaurs. Its fossils have been found in Wyoming (hence its name) and Montana, USA. The latter of the two localities is especially important, because this area is home to the Hell Creek Formation, a geologic formation which dates to the last days of the Mesozoic. It is within the Hell Creek Formation that most fossils of Tyrannosaurus have been found. It is possible that Pachycephalosaurus might have occasionally been prey for T. rex, but the presence of its foot-thick dome of solid bone and its sharp teeth-lined beak make this idea somewhat suspect.

The first remains of Pachycephalosaurus wyomingensis, which means “thick-headed lizard from Wyoming” were discovered during the 1850s in Montana. it consisted of pieces of the back of the skull (the squamosal bone) where there were several bony bumps. The eminent American paleontologist Joseph Leidy described this fossil, identifying it as armor, but he wasn’t sure what sort of animal it might belong to. He christened it Tylosteus (ancient Greek: tylos = “bump”; osteon = “bone”). In the first half of the 20th Century, two other fragmentary specimens were found, and in 1943, the name Pachycephalosaurus was created.

In the 1980s, it was realized that Tylosteus and Pachycephalosaurus were the same animal. According to the rules of the International Code of Zoological Nomenclature, the original name (Tylosteus) takes precedence, and any secondary names (such as Pachycephalosaurus) would have to be declared invalid.  In 1985, paleontologist Donald Baird was able to convince the ICZN to make an exception – to keep Pachycephalosaurus as its legitimate name and to discard the original name. He stated that the name Tylosteus hadn’t been used in over fifty years, and therefore shouldn’t be used. The ICZN granted his request, something that hardly ever happens! Usually, they are very “by the book”.

Pachycephalosaurus and its kind have generated a great deal of confusion and controversy since they were discovered. Most of the debating has pertained to their most obvious feature, their massive thick skulls. For a long time, it was assumed that these thick skulls were used for head-butting, like modern-day bighorn sheep. For years, paleontologists had an academic showdown over this idea, with some claiming that this idea was the only one that made sense, and others claiming that it was physically impossible for the animal to engage in head-butting behavior. For more information regarding this debate, you should watch the following two television programs:

Paleoworld, season 3, episode 3 – “Boneheads”. TLC, 1996.

Bizzare Dinosaurs. National Geographic Channel, 2009.

Now, the issue appears to be settled. Scientists at the University of Wisconsin recently conducted a study of various pachycephalosaur skulls from a number of different genera. If these dinosaurs did engage in some serious head-to-head combat, then their skulls would surely show some damage. After looking at over a hundred skulls, about 20% showed bone damage, specifically a condition called “osteomyelitis”, which is most commonly caused by trauma or injury to the bone. In other words, something hit it. For decades, the hypothesis that pachycephalosaurs engaged in head-butting has been conjecture. Now, we have the first hard evidence to support this idea. You can read more about their research and findings at the website article listed below.

“Study Confirms Head-Butting Behavior in Dome-Headed Dinosaurs”, by Natali Anderson (July 22, 2013).

The identity problem of Tylosteus and Pachycephalosaurus has resurfaced in recent years, but relating to different species. In the past few years, it has been proposed that two other pachycephalosaurs – Dracorex and Stygimoloch – are actually juvenile forms of Pachycephalosaurus. Watch the following videos for more information:

Dinosaurs Decoded. National Geographic Channel, 2009.

Bizarre Dinosaurs. National Geographic Channel, 2009.

“TEDxVancouver – Jack Horner – The Shape-Shifting Skulls of Dinosaurs”.

Lastly, it had been believed for many years that pachycephalosaurs belonged to a group of dinosaurs called the ornithopods. This is a very large group within Dinosauria, compsising hypsilophodonts, iguanodonts, and hadrosaurs (commonly known as “duck-billed” dinosaurs). This was because pachycephalosaurs appeared similar to these animals, especially the first on the list, the hypsilophodonts. These animals were bipedal, had beaks, and had front teeth – traits that were also present in pachycephalosaurs. However, recent re-examination of pachycephalosaur skeletons and cladistics have shown that they are not ornithopods at all; they are actually more closely related to the ceratopsians, the “horned” dinosaurs like Triceratops and Styracosaurus.

The drawing that you see here was made using a combination of shading and stippling, similar to my earlier drawing of Albertosaurus, but on a much smaller scale.

Categories: Paleontology, Uncategorized

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1 reply


  1. Pachycephalosaur discovery in Canada | Dear Kitty. Some blog

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