Did dome-headed dinosaurs sport bristly headgear?

Newswise — If you examine a sufficient number of dinosaur fossils, you will observe that their cranial structures display a remarkable assortment of osseous embellishments, spanning from the horns of Triceratops and the crest resembling a mohawk found in hadrosaurs to the protrusions and protuberances adorning the cranium of Tyrannosaurus rex.

However, researchers in paleontology are progressively uncovering indications that dinosaurs possessed even more intricate cranial adornments, which are not retained within the fossilized skulls. These structures, composed of keratin, the material found in fingernails, were probably utilized as visual cues or signals to communicate with others of their species.

An recently identified species of cranial-bulged dinosaur, belonging to the pachycephalosaur group and estimated to have lived approximately 68 million years ago, serves as the most recent illustration. Pachycephalosaurs thrived during the Cretaceous epoch, spanning from roughly 130 to 66 million years ago, and generally displayed a modest-to-intermediate stature while subsisting on plant matter. With lengths ranging from 3 to 15 feet, these dinosaurs traversed on two legs and possessed a lengthy, rigid tail crucial for maintaining equilibrium.

The novel species is established upon a fragmentary pachycephalosaur skull, encompassing its distinctive dome resembling a bowling ball, which was discovered in 2011 within the Hell Creek Formation situated in Montana. This geological formation comprises strata of Upper Cretaceous rock that has been a prominent site for paleontologists to excavate dinosaur fossils for several decades.

Through the utilization of CT scans and microscopic examinations conducted on cross-sectional slices of the fossilized dome, researchers Mark Goodwin from the University of California, Berkeley, and John "Jack" Horner from Chapman University in Orange, California, have deduced that the skull presumably possessed keratin bristles, evoking the resemblance of a brush cut.

"We do not possess precise knowledge regarding the precise form of the covering on the dome, but it exhibited a vertical element that we hypothesize was enveloped in keratin," stated Goodwin, highlighting that a flat-topped covering with bristles "aligns with biological plausibility." He further explained that animals often adapt or utilize specific features, especially on their skulls, for various purposes, such as display or engaging in social and biological interactions through visual communication."

"I would speculate that there existed something rather intricate atop the dome," commented Horner, who serves as a lecturer and presidential fellow at Chapman University, as well as a professor emeritus at Montana State University in Bozeman and an emeritus curator at the Museum of the Rockies.

Interestingly, the skull exhibited a peculiar gouge at its peak, which had healed over time, suggesting that the creature had experienced a significant accident. However, it had managed to survive long enough for new bone tissue to regenerate and fill the wound.

"We are observing what is likely the initial conclusive proof of head trauma in any pachycephalosaur, where the bone was actually expelled from the dome in some manner and partially healed while the creature was still alive," stated Goodwin, who holds the position of emeritus assistant director and paleontologist at the UC Museum of Paleontology. "The exact cause of this trauma remains unknown. It could potentially be a result of head-butting, and we do not discount that possibility."

Goodwin and Horner advise against jumping to conclusions regarding the hypothesis that these dinosaurs engaged in head-butting as a social behavior, similar to the head clashes seen in modern-day bighorn rams. They emphasize that the head lesion, which reached a depth of approximately half an inch, cannot be considered definitive evidence supporting this theory. Instead, the injury could have been the result of various factors, such as a falling rock, an accidental encounter with a tree, or an interaction with another dinosaur.

"That's the initial assumption everyone tends to make — let's envision them colliding. However, when we thoroughly examine the tissues beneath the dome, both in this specimen and in other pachycephalosaur skulls, we simply do not observe any evidence supporting it histologically," explained Horner. "It appears that something struck the top of this dinosaur's head, causing significant damage. However, having a defensive mechanism on the head may not be advantageous for any species. I believe that any features or adornments found on dinosaur skulls are primarily for display purposes. It's all about visual display."

The presence of such adornments is a common characteristic observed in the reptilian ancestors of dinosaurs as well as their avian descendants, serving both to attract mates and intimidate competitors. However, Horner and Goodwin have long advocated the notion that the internal structure of pachycephalosaur skulls lacks the necessary cushioning to withstand head-butting without causing severe brain damage. They contend that head-butting is predominantly observed in mammals and is rare among reptiles and birds. Specifically, pachycephalosaur skulls lack specialized features, such as a pneumatic chamber above the braincase as seen in bighorn sheep, or other characteristics typically found in mammals that engage in aggressive head-butting behavior.

"I see no reason to transform dinosaurs into mammals instead of exploring their behaviors as bird-like reptiles," expressed Horner.

Horner, Goodwin, and David Evans from the University of Toronto and the Royal Ontario Museum in Canada recently published their detailed account of the newly discovered pachycephalosaur in the Journal of Vertebrate Paleontology. The team bestowed the name Platytholus clemensi upon the new species, as a tribute to the late paleontologist William Clemens from UC Berkeley, who extensively collected numerous fossils, including mammalian fossils, within the Hell Creek Formation where the new species was unearthed.

'A bowling ball in the fossil record'

As per Horner and Goodwin's analysis, pachycephalosaur cranial remains are relatively prevalent in various dinosaur deposits, albeit slightly less abundant within the Hell Creek Formation—a geological stratum from the late Cretaceous Period, occurring a few million years prior to the cataclysmic impact of an asteroid or comet that brought about the demise of dinosaurs and significantly altered the course of life on our planet. The prominent factor contributing to the widespread occurrence of these skulls is the substantial size of their bony domes.

"When it comes to pachycephalosaurs, envision a bowling ball within the fossil record," remarked Goodwin. "Their skulls tend to roll around, become buried, and when eventually exposed on the surface, they exhibit remarkable durability, enabling them to endure extensive weathering and erosion. On numerous occasions, individuals have traversed an area throughout the entire summer, only to realize that what they initially dismissed as a mere rock, resembling a glacial cobble, was actually an excellently preserved dome."

Despite having discovered numerous fossils, such as Triceratops, T. rex, and duckbilled hadrosaurs, from the Hell Creek Formation throughout the last 45 to 50 years, Goodwin and Horner harbor a specific fascination with pachycephalosaurs. They are particularly interested in studying the evolution of these dinosaurs and the transition they undergo from juvenile to adult stages. Through extensive skull dissections, they have examined the developmental changes over time and have sought to test the hypothesis that these creatures engaged in head-butting behavior, possibly limited to the male individuals.

Their findings indicate that there is no evidence, as suggested by the bone structure, to support the notion that the skull or neck of pachycephalosaurs could endure direct head-to-head collisions. Furthermore, the newly described partial skull, which was discovered independent of other skeletal elements, exhibits a bone structure that does not align with the characteristics typically associated with head-butting behavior.

Their research has revealed that the bone structure of pachycephalosaurs provides no evidence to support the hypothesis of their skulls or necks being capable of withstanding direct head-to-head collisions. Additionally, the analysis of the newly described partial skull, found separate from other skeletal remains, demonstrates a bone structure that contradicts the features commonly associated with head-butting behavior.

Due to the dissimilarity between the skull bones of the newly discovered specimen and those of other juvenile, sub-adult, or adult pachycephalosaurs coexisting in the same region during that period—such as Pachycephalosaurus, the namesake of these dinosaurs, and Sphaerotholus—Horner and Goodwin assigned the creature to a distinct genus and species. As a result, the animal was classified as Platytholus clemensi.

The skull of the newly discovered pachycephalosaur did exhibit certain characteristics observed in other pachycephalosaurs, including Sphaerotholus. These characteristics involved the presence of blood vessels within the skull that terminated abruptly at the surface of the dome. This suggests that the blood supply originally nourished some form of tissue that resided on top of the dome. If this covering was composed of keratin, similar to a sheath, the blood vessels would have spread out and left impressions or grooves on the domed surface, analogous to what is observed beneath the beaks of birds or on the skulls of Triceratops and other ceratopsians or horned dinosaurs. However, in this case, the blood vessels were perpendicular to the surface, implying that they supplied a vertical structure instead.

"The presence of these vertical canals reaching the surface implies the potential presence of vertically oriented keratin," explained Horner. "I believe that these pachycephalosaurs possessed some form of head ornamentation or display that remains unknown to us. They were more than just domes; I think there was an intricate and elaborate adornment on top of their heads."

Goodwin pointed out that the shape of pachycephalosaur domed heads underwent changes during the maturation process, becoming more pronounced and intricate as the animals approached adulthood. This observation supports the hypothesis that these structures were employed for sexual display and courtship purposes, although they may have been used to strike the flanks rather than the heads of male rivals. Goodwin speculates that dinosaurs likely differentiated between genders through coloration, similar to many contemporary birds like cassowaries, peafowls, and toucans, which possess vibrant colors around their face and head as a means of visual communication.

"It is reasonable to propose that the covering atop the dome may have also exhibited vivid colors or potentially undergone seasonal color changes," he stated.

The paleontologists are utilizing CT scans and conducting thin-section histology on other pachycephalosaur domes to investigate whether additional dome-headed dinosaurs exhibited intricate vertical head adornments, in addition to the already documented assortment of bumps, nodes, and horns.

"The integration of cranial histology through thin-sectioning of the skull, along with CT scanning, has provided us with a considerably more extensive dataset and serves as the foundation for our hypothesis regarding the presence of a keratinous covering over the dome," Goodwin explained. "We are aware that the dome was indeed covered by a substance, and in this particular taxon, we propose a hypothesis that it possessed a vertical structural element, in contrast to dinosaurs such as Triceratops, T. rex, and others, which had either thick skin or keratin directly encasing the bone."

Goodwin and Horner chose to name the newly discovered species after Clemens as a tribute to their close relationship and shared experiences. Throughout numerous summers, the three paleontologists extensively explored Montana together in pursuit of Cretaceous fossils, often collaborating and working alongside each other in the field.

"Bill Clemens held a significant role in Mark's life, but his impact on my life may have been even more profound. In 1978, he informed me about a woman in Bynum, Montana, who had discovered a substantial dinosaur specimen and sought identification," Horner recounted. Intrigued by the news, Horner and his colleague Bob Makela visited the woman's rock shop. To their surprise, she also presented them with several smaller fossils that turned out to be "the first baby dinosaur bones ever discovered in the world."

The groundbreaking discovery of the baby dinosaur bones not only marked the first conclusive proof of parental care among dinosaurs but also served as a catalyst for Horner's exploration of family dynamics among duckbilled dinosaurs. Inspired by this revelation, Horner authored several books on the subject, including Digging Dinosaurs in 1988 and its sequel, Dinosaur Lives: Unearthing an Evolutionary Saga, in 1997. Additionally, he penned a number of children's books, such as Maia: A Dinosaur Grows Up in 1995, which revolved around a young duckbill of the species named Maiasaura by Horner. Furthermore, he recently released a sequel in 2023 titled Lily and Maia...a Dinosaur Adventure.

"I owe a great amount of gratitude to Bill Clemens for sending me on that little trip," said Horner.

The work was funded by a Smithsonian Institution grant to Horner and grants from the National Science Foundation (EAR-1053370) to Goodwin.