Dinosaur fossils reveal a fascinating array of bony ornaments on their skulls, showcasing a wide range of features such as horns, crests, and bumps. However, scientists are now uncovering evidence that suggests dinosaurs possessed even more intricate head ornaments made of keratin, similar to fingernails, which were likely used as visual signals or communication tools among their own species.
A recently identified species of dome-headed dinosaur, belonging to the pachycephalosaur group and dating back approximately 68 million years, provides further support for this idea. Pachycephalosaurs were plant-eating dinosaurs that lived during the Cretaceous period. They were typically small to medium-sized, walked on two legs, and had long, rigid tails for balance.
The discovery of this new species is based on a partial pachycephalosaur skull, including its characteristic bowling-ball-shaped dome, which was excavated in 2011 from the Hell Creek Formation in Montana. The Hell Creek Formation is composed of Upper Cretaceous rock layers that have yielded numerous dinosaur fossils over the years.
Using CT scans and microscopic analyses of cross-sections of the fossilized dome, paleontologists Mark Goodwin from the University of California, Berkeley, and John “Jack” Horner from Chapman University in California have determined that the skull likely had keratin bristles, similar to a brush cut, covering the dome.
While the exact shape of the covering remains uncertain, Goodwin explains that the presence of a vertical component indicates a keratinous structure. He suggests that a bristly, flat-topped covering would make biological sense, as animals often employ skull features for various functions, including display and communication.
Horner, a lecturer and presidential fellow at Chapman University, professor emeritus at Montana State University, and emeritus curator at the Museum of the Rockies, speculates that the head ornamentation could have been quite elaborate.
Interestingly, the skull also exhibits a noticeable gouge at its apex, which shows signs of healing. This suggests that the dinosaur experienced a significant injury but managed to survive long enough for new bone tissue to form and partially mend the wound.
Goodwin describes this as the first unequivocal evidence of head trauma in any pachycephalosaur. However, he and Horner caution against immediately linking this injury to the theory that these dinosaurs engaged in head-butting behavior as part of their social interactions, akin to the way modern-day bighorn rams clash heads. The skull injury could have resulted from a variety of causes, such as a falling rock or an accidental encounter with a tree or another dinosaur.
Regarding the hypothesis of head-butting behavior in pachycephalosaurs, Horner expresses skepticism, stating that there is no histological evidence to support it. He refers to detailed studies of the underlying tissues of pachycephalosaur skulls, including the specimen under examination and others, which do not indicate adaptations for cushioning during head-butting. According to Horner, any features or ornaments found on dinosaur skulls are likely for display purposes rather than for head-butting.
He points out that such ornamentation is common in reptile ancestors of dinosaurs and their bird descendants, serving as visual displays to attract mates or intimidate rivals. However, Horner and Goodwin argue that pachycephalosaur skulls lack the necessary structural adaptations, such as pneumatic chambers or other features found in mammals like bighorn sheep, which engage in aggressive head-butting behavior.
Horner emphasizes the importance of understanding dinosaurs as bird-like reptiles rather than trying to attribute mammalian behaviors to them. He believes it is unnecessary to turn dinosaurs into mammals and suggests focusing on their potential behaviors within their reptilian and avian context.
The team of Horner, Goodwin, and David Evans from the University of Toronto and the Royal Ontario Museum published their findings on the new pachycephalosaur species, naming it Platytholus clemensi. They dedicated the species name to the late paleontologist William Clemens from UC Berkeley, who made significant contributions to fossil collection, particularly in the Hell Creek Formation where the new species was discovered. Their research was published in the Journal of Vertebrate Paleontology.
‘A bowling ball in the fossil record'
According to Horner and Goodwin, pachycephalosaur skulls are relatively common in many dinosaur fossil sites, although they are slightly less prevalent in the Hell Creek Formation, which dates from the late Cretaceous Period. The large bony dome of pachycephalosaurs contributes to the abundance of their skulls in the fossil record. These skulls have the ability to endure weathering and erosion, making them resilient when exposed on the surface.
Over the course of 45 to 50 years, Goodwin and Horner have discovered numerous fossils from the Hell Creek Formation, including bones from Triceratops, T. rex, and hadrosaurs. However, they have a particular interest in pachycephalosaurs and have focused on studying their evolution and development from juvenile to adult stages. They have conducted extensive studies on sliced skulls to examine how they change over time and to investigate the theory of head-butting behavior, particularly among males.
Their research has led them to conclude that there is no evidence, based on the bone structure, to support the notion that pachycephalosaurs engaged in head-to-head collisions. The partial skull they described in their study, which was not found in association with other skeletal elements, also exhibits a bone structure that is inconsistent with head-butting behavior.
Due to the distinct appearance of the skull bones, which differ from specimens of the two other known types of pachycephalosaurs in the area, namely Pachycephalosaurus and Sphaerotholus, the paleontologists classified the newly described partial skull as a new genus and species, naming it Platytholus clemensi.
The paleontologists observed that the skull of the newly discovered pachycephalosaur exhibited characteristics similar to those seen in other pachycephalosaurs, including Sphaerotholus. They noted the presence of blood vessels in the skull that terminated abruptly at the surface of the dome, indicating that there was likely some tissue or covering sitting atop the dome. If this covering were made of keratin, like in birds or ceratopsians such as Triceratops, the blood vessels would have spread out, leaving grooves or indentations on the domed surface. However, the vessels were perpendicular to the surface, suggesting that they fed a vertical structure.
Horner suggested that pachycephalosaurs may have had an unknown feature or elaborate display on top of their heads, rather than just simple domes. Goodwin added that the shape of the domed heads changed as the pachycephalosaurs matured, becoming more prominent and intricate as they approached adulthood. This suggests that the domes were likely used for sexual display and courtship, possibly involving interactions where males butt the flanks of their rivals instead of their heads. Goodwin also proposed that pachycephalosaurs may have used color to distinguish gender, similar to modern birds like cassowaries, peafowls, and toucans, which have vibrant facial and head colors for visual communication.
The paleontologists plan to conduct CT scans and histology on other pachycephalosaur domes to investigate whether additional dome-headed dinosaurs also possessed elaborate vertical headgear in addition to the known array of bumps, nodes, and horns.
Based on their research, Goodwin and Horner named the new species after paleontologist William Clemens. The three scientists had developed close relationships during their extensive fieldwork in Montana, searching for Cretaceous fossils. Clemens played a pivotal role in Horner's career by directing him to examine a large dinosaur discovered by a woman in Bynum, Montana. This discovery provided evidence of parental care among dinosaurs and inspired Horner to write several books on family structures in duckbilled dinosaurs, including works for children.
Horner expressed gratitude to Clemens for the guidance that led to his significant contributions to the field of paleontology.