ACTUALISTIC APPROACH TO CERATOPSIAN CRANIAL ONTOGENY AND SCARS

Bone ontogeny is partially related to applied stresses that determine shape. Bone reshaping seems to represent a process of stress (e.g., muscle)-induced plastic deformation. Stresses result only in plastic deformation (to adult state), if rate of osteoblastic activity is greater than osteolysis. If repair rate does not keep pace, stress fractures result. Minor bone structure disruption is part of the normal growth process. Linear patterns of periosteal reaction on external surfaces of jugal, squamosal and parietal bones of Triceratops are typical of bone changes previously noted with stress fractures in humans and other animals. The en face macroscopic appearance and standard x-ray appearance are indistinguishable from previously reported Triceratops phalangeal stress fractures. Occasional presence of a linear surface cleft further substantiates that actualistic analysis. The veterinary term for the associated linear sclerosis is “puppy line,” attributed to plastic deformation that resolves as the canine ages. Absence of peri-lesional disorganized bone reaction rules out infectious disease, the only major differential diagnostic consideration. Stress fractures result from repetitive stresses, not single traumatic events. Intraspecific conflict-related horn injuries would be expected to cause bone scoring, not linear periosteal reaction. Actualistic analysis eliminates even that remote possibility. Attributing linear alterations to effects of conspecific horn might have credibility, if damage was a gouge rather than periosteal reaction and if a reasonable behavior were presented that would explain the direction of the linear damage. It is unclear how a horn attack would cause such vertical damage, unless the victim was already lying on its side, a position offering a much more vulnerable target. It is hypothesized that the observed linear reaction in Triceratops skull elements may represent part (although exaggerated) of normal growth or at least that the process of repair could not keep up with the stress-induced changes of remodeling. This is parsimonious with the variation in general frill morphology that characterizes ceratopsian ontogeny.