Newly-discovered pachycephalosaur had elaborate head ornaments

Gazing upon the fossilized skulls of dinosaurs, we marvel at the plethora of bony ornaments. Horns adorn the head of the mighty Triceratops. Crests, akin to a mohawk, are noticeable on hadrosaurs. Tyrannosaurus rex sports an assortment of bumps and knobs.

But the evidence does not stop there

Paleontologists have begun to unearth clues pointing towards additional decorative features not directly seen in these fossils. They propose these ornamentations, made of keratin – the same material found in our fingernails, served as visual signals within their kind.

New discoveries about pachycephalosaurs

Recent revelations around a new species of dome-headed dinosaur, a pachycephalosaur from approximately 68 million years ago, strengthen this hypothesis. These creatures thrived during the Cretaceous period, between 130 and 66 million years ago. As plant-eating dinosaurs of small to medium size, they ambled on two legs, brandishing long, rigid tails for balance, and measured anywhere between 3 to 15 feet in length.

This new species identification arises from a partial pachycephalosaur skull discovered in 2011. The discovery took place within the Hell Creek Formation in Montana, a region rich in Upper Cretaceous rock that has provided paleontologists with a treasure trove of dinosaur fossils for decades.

Microscopic analyses and CT scans of the fossilized dome, conducted by paleontologists Mark Goodwin of the University of California, Berkeley, and John “Jack” Horner of Chapman University in Orange, California, hint at an unusual feature. The skull seemingly once showcased keratin bristles, much like a brush cut.

“We don’t know the exact shape of what was covering the dome, but it had this vertical component that we interpret as covered with keratin,” said Goodwin. He explained that the bristly covering would be biologically plausible. “Animals change or use certain features, particularly on the skull, for multiple functions – it could be for display or for social and biological interactions involving visual communication.”

Horner, also professor emeritus at Montana State University in Bozeman and emeritus curator at the Museum of the Rockies, suggests that these dinosaurs had an elaborate structure on their heads, saying, “I would guess that there was something pretty elaborate up there.”

Interestingly, the skull also bore a healed gouge, a vivid remnant of a past trauma. “We see probably the first unequivocal evidence of trauma in the head of any pachycephalosaur, where the bone was actually ejected from the dome somehow and healed partially in life,” Goodwin notes, suggesting the cause of the injury could range from head-butting to an accidental encounter with a falling rock or another dinosaur.

What was the benefit of these elaborate head ornaments?

The researchers caution against leaping to the conclusion that these dinosaurs engaged in head-butting as part of their social interactions, like bighorn rams do today. Horner, referring to detailed studies of the tissues underlying the dome, states, “That’s the first place everybody wants to go — let’s crash them together. And, you know, we just don’t see any evidence of it, histologically.”

Instead, Horner suggests the display was key to these features. He extends this concept to the entire dinosaur family, arguing that the primary purpose of the diverse array of features on dinosaur heads was display. This behavior aligns with the practices seen in their reptile ancestors and bird descendants, used for attracting mates and intimidating rivals.

Horner and Goodwin have continually advocated that the internal structure of pachycephalosaur skulls lacks the necessary cushioning to withstand head-butting without causing severe brain damage. 

They point out this behavior is more typical of mammals and rarely seen in reptiles or birds. Horner stresses, “I don’t see any reason to turn dinosaurs into mammals, rather than just trying to figure out what they might be doing as bird-like reptiles.”

New study takes a closer look

Last month, Horner, Goodwin, and David Evans of the University of Toronto and the Royal Ontario Museum published their research in the Journal of Vertebrate Paleontology. The newly discovered pachycephalosaur received the name Platytholus clemensi, honoring the late UC Berkeley paleontologist William Clemens, renowned for his fossil collections from the Hell Creek Formation.

Pachycephalosaur skulls appear frequently in many dinosaur beds. Their presence in the Hell Creek Formation, dating from the late Cretaceous Period, is less common. Goodwin suggests the large bony dome might explain the abundance of these skulls in the fossil record. 

“With pachycephalosaurs, think about a bowling ball in the fossil record. Their skulls roll around, get buried, and when exposed on the surface, they’re very robust, so they can withstand a lot of weathering and erosion sitting out there,” explained Goodwin.

Despite finding a variety of fossils from the Hell Creek Formation over the past half-century, Goodwin and Horner maintain a keen interest in pachycephalosaurs. They’ve intensively studied the evolution and development of these dinosaurs, slicing through numerous skulls to track their changes over time and scrutinize the theory of head-butting behavior. 

What the researchers concluded

Their research draws a consistent conclusion: there’s no evidence, based on bone structure, that the skull or neck could withstand a head-to-head collision.

Goodwin explained that as pachycephalosaurs matured, the domed heads’ shape became increasingly elaborate, which could suggest a role in sexual display and courting. He notes, “It’s reasonable to suggest that the covering over the dome may also have been brightly colored or subject to color change seasonally.”

The team is currently conducting more CT scans and histological studies on other pachycephalosaur domes, investigating if other dome-headed dinosaurs showcased elaborate vertical headgear, extending the known array of bumps, nodes, and horns.

“The combination of cranial histology after thin-sectioning the skull and CT scanning gave us a much richer body of data and forms a basis for our hypothesis that there was a keratinous covering over the dome,” said Goodwin.

Finally, paying tribute to the late William Clemens, Horner noted: “Bill Clemens was a very important person in Mark’s life, but he may have been more important in my life because he was the person who, back in 1978, said, ‘You know, Jack, there’s this woman in Bynum, Montana, who found a large dinosaur, and she needs it identified.'” 

This simple suggestion led to the monumental discovery of the world’s first baby dinosaur bones, shifting the understanding of dinosaur family structures and leading to Horner’s influential books on the subject. “I owe a great amount of gratitude to Bill Clemens for sending me on that little trip,” concludes Horner.

More about pachycephalosaurs

Pachycephalosaurs, meaning “thick-headed lizards,” were a group of dinosaurs known for their distinctive dome-shaped skulls. They lived during the Late Cretaceous period, roughly 85 to 65 million years ago, predominantly in what is now North America and Asia.

Physical Characteristics

Pachycephalosaurs had small, slender bodies and were likely bipedal, meaning they walked on two legs. They had short arms and long, stiff tails that may have aided in balance. Size estimates vary widely among different species, but most are believed to have been about the size of a human, ranging from about 2 to 5 meters (6.5 to 16.4 feet) in length.

Distinctive Skulls

The most recognizable feature of pachycephalosaurs is their thick, domed skulls, which could be up to 25 cm (10 inches) thick. These skulls were studded with various knobs, bumps, or spikes, and the thick dome was made of dense bone tissue. The function of this dome is still debated among scientists, with some suggesting it may have been used in head-butting behavior for defense or competition, while others dispute this due to the potential for brain damage.

Diet

Pachycephalosaurs are often considered herbivores, likely feeding on a variety of plants. However, some evidence suggests that they may have been omnivores, supplementing their diet with small insects or other animals.

Behavior and Lifestyle

The behavior of pachycephalosaurs is not well-known due to limited fossil evidence. Some theories, based on the dome-shaped skulls, suggest these dinosaurs engaged in head-butting or flank-butting behaviors, either as a display of dominance or for mating purposes. However, as mentioned earlier, these theories remain disputed.

Notable Species

The most famous member of this group is probably the Pachycephalosaurus, the largest known pachycephalosaur. Other notable species include Stegoceras, Stygimoloch, and Dracorex.

Fossil Record

Pachycephalosaur fossils have been discovered in various parts of the world, including North America and Asia, with a particularly high concentration in the Hell Creek Formation in the United States.

Evolution and Extinction

Pachycephalosaurs belong to the group of dinosaurs known as Ornithischia, or “bird-hipped” dinosaurs. However, they are not ancestors of modern birds, which evolved from a different group of dinosaurs called theropods. Like all non-avian dinosaurs, pachycephalosaurs went extinct in the Cretaceous-Paleogene (K-Pg) extinction event around 66 million years ago.

As with any group of dinosaurs, our knowledge about pachycephalosaurs is limited to what we’ve gleaned from the fossil record, and new discoveries continue to refine our understanding of these fascinating prehistoric creatures.

Image Credit: Graphic courtesy of Jack Horner

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