New Study Suggest Pachycephalosaurs may have Clashed Heads After All

The pachycephalosaurs (the name means “thick-headed lizards”) are often referred to as the “Bone-heads” as the most striking feature about these bird-hipped dinosaurs was their incredibly thick skulls, which in many cases were adorned with bony nodules.  Related to the ceratopsians, scientists believe that these bipeds evolved in the Early Cretaceous and survived up to the very end of the Age of Reptiles.

Pachycephalosaurs

Most of what we know about this particular group of dinosaurs comes from skull material as many genera have been named and described on the basis of the discovery of skull bones.  This is in stark contrast to other types of dinosaur, the sauropods for example, where bones relating to the head are exceptionally rare.  The thickened skulls with their solid bone domes had excellent fossil preservation potential.  Such thick bones could withstand the stresses of the fossilisation process.

Perhaps the most famous of the pachycephalosaurids is Pachycephalosaurus wyomingensis, (from which this group of dinosaurs was named).  P. wyomingensis is known from just skull material but it has been estimated to have reached lengths in excess of 8 metres, making it the largest pachycephalosaur dinosaur discovered to date.

An Illustration of Pachycephalosaurus (P. wyomingensis)

Picture credit: Everything Dinosaur

To read more about Pachycephalosaurus: Pachycephalosaurus – The “Bone-headed” Dinosaur.

Why did these dinosaurs have such thick skulls?  It was not to protect a particularly large brain, these dinosaurs had brains no bigger than any other comparable sized ornithischian, perhaps these animals evolved well-protected heads as they engaged in head-butting contests just like some sheep and antelope do today.  The ideas of pachycephalosaurs indulging in such behaviour was first put forward by the American palaeontologist Ed Colbert in 1954.

Debating the Theory

This theory has been debated ever since.  Perhaps the most famous challenge to the theory of intra-specific conflict amongst pachycephalosaurs came in 2004 when the head-butting activities of these dinosaurs was examined by two American researchers, Mark Goodwin and John “Jack” Horner.  These researchers discovered that the radiating bone structure that was thought to provide strength to the dome of the skull was only present in juvenile specimens, and not in mature adults.  It was assumed, just like in extant animals the adults would have indulged in any head-butting, not young dinosaurs.

However, new research from the University of Calgary published in the online scientific journal “PloS One” suggests that these Cretaceous dinosaurs may indeed have used their thick heads for head butting contests.  The Canadian based research team compared the fossilised skulls of two types of pachycephalosaur with the skulls of modern herbivores some of which were known to be “head-bangers”.

Bio-mechanical Studies

They concluded that previous bio-mechanical studies may have suggested that these dinosaurs butted heads, but this had been challenged by studies on how the skull bones grew and developed as the animals matured.  However, new computer analysis and modelling using advanced statistical methodologies do support the theory first put forward by Colbert, back in 1954 – that the skulls of these dinosaurs could have withstood the impact from a clash of heads.  These dinosaurs could have been head-butters after all.

The two pachycephalosaurs involved in the study were Stegoceras validum and Prenocephale prenes.  The fossil skulls of these two dinosaurs were compared to ten skulls of artiodactyls (even-toed ungulates), known to indulge in various forms of intra-specific combat including head butting.  The team found that the bony anatomy of some pachycephalosaur domes are better at protecting the brain than in any modern head-butter.

Co-author Dr Eric Snively commented:

“Pachycephalosaur domes are weird structures not exactly like anything in modern animals. We wanted to test the controversial idea that the domes were good for head-butting.  Finding out brings us closer to their social lives: were pachycephalosaurs more likely just showing off their domes like peacocks with their tails, or were they also cracking their heads together like musk oxen?”

Using CT scanning and the new statistical method for diagnosing behaviour in fossil animals, the researchers compared the bony-headed dinosaurs with modern ungulates (hoofed animals) that engage in different kinds of combat.

Dr Snively stated:

“Our analyses are the closest we can get to observing their behaviour.  In a way, we can get inside their heads by colliding them together virtually.  We combined anatomical and engineering analyses of all these animals for a pretty thorough approach.  We looked at the actual tissue types in the skulls and heads of the animals.”

Co-author and fellow researcher Dr Jessica Theodor (Associate Professor in the Biological Sciences Dept. at the University of Calgary said:

“Head-butting is a form of male-to-male competition for access to females.  It’s pretty clear that although the bones are arranged differently in the Stegoceras, it could easily withstand the kinds of forces that have been measured for the living animals that engage in head-butting.”

Studying the Skulls

Describing the skulls of animals known to crack heads as “like a good motorcycle helmet”, the team stated that the skull of a typical head-butter was hard on the outside with a sort of spongy impact absorbing material just beneath the outer surface and then a stiff, really dense coat of hardened material to protect the brain.

Images show sections through the skulls of the dinosaur Stegoceras validum, the small African antelope known as a Duiker (Cephalophus leucogaster), regarded as morphologically close to this type of pachycephalosaur and a Bighorn sheep (Ovis canadensis).

The CT scans reveal similar dome structure. A. In the Stegoceras specimen, compact bone (z1 and z3: zones 1 and 3), occurs deep and superficial to a cancellous region (z2: zone 2:). Moderately dense compact bone shows as a green band at the base of zone 3 (white line); note cancellous bone (blue) above the line in the anterior portion of this zone. B. Cephalophus. C. Similar stratification is evident in the sectioned Ovis cranium, with nearly identical zones of cancellous and compact bone broken by a ventral sinus.

The Stegoceras had an extra layer of dense bone in the middle.  Stegoceras was a small pachycephalosaur approximately three metres long, that lived in north America during the Late Cretaceous.

The researchers concluded that Llamas would crack their skulls if they indulged in head-butting.  Giraffes would not be very good at head-banging contests – their skulls could not withstand the force of too many collisions.  Musk ox and Big Horn Sheep have the sort of adaptations to help them cope with bouts of head-butting.  In this way they have similar skulls to Stegoceras, so this could be evidence to support head-banging pachycephalosaurs.

Perhaps, Colbert was onto something after all.

To view models and replicas of pachycephalosaurs and other dinosaurs: Papo Dinosaur and Prehistoric Animal Models.