Feeding mechanics It is uncertain what pachycephalosaurs ate; having very small, ridged teeth they could not have chewed tough, fibrous plants as effectively as other dinosaurs of the same period. It is assumed that their sharp, serrated teeth were ideally suited for a mixed diet of
leaves,
seeds,
fruit and
insects. Sues and Galton suggested in 1987 that
Stegoceras may have had an entirely herbivorous diet, as the tooth crowns were similar to those of
iguanid lizards. The premaxillary teeth show wear facets from contact with the predentary bone, and the maxillary teeth have double wear facets similar to those seen in other ornithischian dinosaurs. Every third maxillary tooth of UALVP 2 are erupting
replacement teeth, and tooth replacement happened in backwards progression in sequential threes. The occipital region of
Stegoceras was well-demarcated for muscle-attachment and it is believed that the jaw movement of
Stegoceras and other pachycephalosaurs was mostly limited to up-and-down motions with only a slight capability for jaw rotation. This is based on the structure of the jaw and dental microwear and wear facets of the teeth indicate that the bite-force was used more for shearing than for crushing. In 2021, palaeontologist Michael N. Hudgins and colleagues examined the teeth of
Stegoceras and
Thescelosaurus and found that while both had heterodont teeth, they could be statistically distinguished from each other. Due to its broad rostrum and more uniform teeth,
Stegoceras was an indiscriminate bulk-feeder that cropped large amounts of vegetation, while the teeth and narrow rostrum of
Thescelosaurus indicates it was a selective feeder. Pachycephalosaurs and Thescelosaurids occur in the same North American formations, and it appears that their coexistence was made possible by them occupying different
ecomorphospaces (though
Stegoceras and
Thescelosaurus themselves were not contemporaries). In 2026, palaeontologists Louis-Philippe Bateman and Hans C.E. Larsson examined the feeding performance in pachycephalosaurs using
Stegoceras as a model, and found that the presence of their domes limited the space and movement of their jaw muscles. Compared with other orhithischians, the feeding performance of
Stegoceras was closer to basal ornithiscians and ornithopods than would be expected of a derived taxon from the Late Cretaceous. They concluded that the dome constrained pachycephalosaurs to a more
plesiomorphic (ancestral) dentition with a low-fiber herbivorous niche, and that it was an example of a sociosecual display structure constraining evolution of feeding traits.
Nasal passages In 1989, Emily B. Griffin found that
Stegoceras and other pachycephalosaurs had a good sense of smell (olfaction), based on the study of cranial
endocasts that showed large
olfactory bulbs in the brain. In 2014, Jason M. Bourke and colleagues found that
Stegoceras would have needed cartilaginous
nasal turbinates in the front of the nasal passages for airflow to reach the olfactory region. Evidence for the presence of this structure is a bony ridge to which it could have attached. The size of the olfactory region also indicates that
Stegoceras had a keen sense of smell. The researchers found that the dinosaur could have had either a scroll-shaped turbinate (like in a
turkey) or a branched one (as in an
ostrich) as both could have directed air to the olfactory region. The blood vessel system in the passages also suggest that the turbinates served to cool down warm arterial blood from the body that was heading to the brain. The skull of
S. validum specimen UALVP 2 was suited for a study of this kind due to its exceptional preservation; it has ossified soft tissue in the nasal cavity, which would otherwise be
cartilaginous and therefore not preserved through mineralization.
Ontogenetic changes Several explanations have historically been proposed for the variation seen in the skulls of
Stegoceras and other pachycephalosaurs. Brown and Schlaikjer suggested that there was
sexual dimorphism in the degree of doming, and hypothesized that flat-headed specimens such as AMNH 5450 (
Ornatotholus) represented the female morph of
Stegoceras. This idea was supported by a 1981
morphometric study by Champan and colleagues, which found that males had larger and thicker domes. After other flat-headed pachycephalosaurs were discovered, the degree of doming was proposed to be a feature with taxonomic importance, and AMNH 5450 was therefore considered a distinct taxon from 1979 onwards. In 1998, Goodwin and colleagues instead proposed that the inflation of the dome was an ontogenetic feature that changed with age, based on a
histological study of an
S. validum skull that showed the dome consisted of vascular, fast-growing bone, consistent with an increase in doming through age. These authors found that the supposedly distinct features of
Ornatotholus could easily be the results of ontogeny. A 2012 study by Schott and Evans found that the number and shape of the individual nodes on the squamosal shelf of the examined
S. validum skulls varied considerably, and that this variability does not seem to correlate with ontogenic changes, but was due to individual variation. These researchers found no correlation between the width of supratemporal fenestrae and the size of the squamosal.
Dome function The function of pachycephalosaur domes has been debated, and
Stegoceras has been used as a model for experimentation in various studies. The dome has mainly been interpreted as a weapon used in
intra-specific combat, a
sexual display structure, or a means for
species recognition. In 1997, palaeontologist
Kenneth Carpenter pointed out that the dorsal vertebrae from the back of the pachycephalosaur
Homalocephale show that the back curved downwards just before the neck (which was not preserved), and unless the neck curved upwards, the head would point to the ground. He therefore inferred that the necks of
Stegoceras and other pachycephalosaurs were held in a curved posture (as is the norm in dinosaurs), and that they would therefore not have been able to align their head, neck, and body horizontally straight, which would be needed to transmit stress. Their necks would have to be held below the level of the back, which would have risked damaging the spinal cord on impact. Modern bighorn sheep and
bison overcome this problem by having strong ligaments from the neck to the tall neural spines over the shoulders (which absorb the force of impact), but such features are not known in pachycephalosaurs. These animals also absorb the force of impact through sinus chambers at the base of their horns, and their foreheads and horns form a broad contact surface, unlike the narrow surface of pachycephalosaur domes. Because the dome of
Stegoceras was rounded, it would have given a very small area for potential impact, and the domes would have glanced off each other (unless the impact was perfectly centred). Combating pachycephalosaurs would have had difficulty seeing each other while their heads were lowered, due to the bony ridges above the eyes. In 2011, Snively and Jessica M. Theodor conducted a finite element analysis by simulating head-impacts with
CT scanned skulls of
S. validum (UALVP 2),
Prenocephale prenes and several extant head-butting
artiodactyls. They found that the correlations between head-striking and skull morphologies found in the living animals also existed in the studied pachycephalosaurs.
Stegoceras and
Prenocephale both had skull shapes similar to the bighorn sheep with
cancellous bone protecting the brain. They also shared similarities in the distribution of compact and cancellous regions with the bighorn sheep,
white-bellied duiker and the
giraffe. The white-bellied duiker was found to be the closest morphological analogue to
Stegoceras; this head-butting species has a dome which is smaller but similarly rounded.
Stegoceras was better capable of dissipating force than artiodactyls that butt heads at high forces, but the less vascularized domes of older pachycephalosaurs, and possibly diminished ability to heal from injuries, argued against such combat in older individuals. The study also tested the effects of a keratinous covering of the dome, and found it to aid in performance. Though
Stegoceras lacked the
pneumatic sinuses that are found below the point of impact in the skulls of head-striking artiodactyls, it instead had vascular struts which could have similarly acted as braces, as well as conduits to feed the development of a keratin covering. In 2012, Caleb M. Brown and Anthony P. Russell suggested that the stiffened tails were probably not used as defence against flank-butting, but may have enabled the animals to take a tripodal stance during intra-specific combat, with the tail as support. Brown and Russell found that the tail could thereby help in resisting compressive, tensile, and torsional loading when the animal delivered or received blows with the dome. Bryan R. S. Moore and colleagues examined and reconstructed the limb musculature of
Stegoceras in 3D in 2022, using the very complete UALVP 2 specimen as basis. They found that the musculature of the forelimbs was conservative, particularly compared to those of early bipedal
saurischian dinosaurs, but the pelvic and hindlimb musculature was instead more derived, due to peculiarities of the skeleton. These areas had large muscles, and combined with the wide pelvis and stout hind limbs (and possibly enlarged ligaments), this resulted in a strong, stable pelvic structure that would have helped during head-butting between individuals. Since the skull domes of pachycephalosaurs grew with
positive allometry, and may have been used in combat, these researchers suggested it may have been the case for the hindlimb muscles as well, if they were used to propel the body forwards during head-butting. They cautioned that while UALVP 2 is very complete for a pachycephalosaur, their study was limited by it missing large portions of its vertebral column and outer limb elements.
Other suggested functions In 1987, J. Keith Rigby and colleagues suggested that pachycephalosaur domes were
heat-exchange organs used for
thermoregulation, based on their internal "radiating structures" (
trabeculae). This idea was supported by a few other writers in the mid-1990s. In 2011, palaeontologists
Kevin Padian and
John R. Horner proposed that "bizarre structures" in dinosaurs in general (including domes, frills, horns, and crests) were primarily used for species recognition, and dismissed other explanations as unsupported by evidence. Among other studies, these authors cited Goodwin et al.'s 2004 paper on pachycephalosaur domes as support of this idea, and they pointed out that such structures did not appear to be sexually dimorphic. In a response to Padian and Horner the same year, Rob J. Knell and
Scott D. Sampson argued that species recognition was not unlikely as a secondary function for "bizarre structures" in dinosaurs, but that
sexual selection (used in display or combat to compete for mates) was a more likely explanation, due to the high cost of developing them, and because such structures appear to be highly variable within species. In 2013, the British palaeontologists David E. Hone and
Darren Naish criticized the "species recognition hypothesis", and argued that no extant animals use such structures primarily for species recognition, and that Padian and Horner had ignored the possibility of mutual sexual selection (where both sexes are ornamented). In 2012, Schott and Evans suggested that the regularity in squamosal ornamentation throughout the ontogeny of
Stegoceras was consistent with species recognition, but the change from flat to domed frontoparietals in late age suggests that the function of this feature changed through ontogeny, and was perhaps sexually selected, possibly for intra-specific combat. ==Palaeoenvironment==