Pattern and transition of surficial bone texture of the centrosaurine frill and their ontogenetic and taxonomic implications
ABSTRACT Distinct textures have been observed on the surface of the parietals and squamosals of centrosaurine dinosaurs. It is widely held that they exhibit an ontogenetic succession and can be used as an index of progressive age-related changes. To date, the pattern of texture expression across the parietals and squamosals has not been traced. The pattern of transition from long-grained to subsequent textures on centrosaurine parietals is investigated by mapping their distribution on a size series hypothesized to represent an ontogenetic series. These data reveal a distinct and conservative pattern of texture occurrence and transition across all examined centrosaurine taxa. Small (putatively juvenile) specimens exhibit striated or long-grained texture, whereas the largest elements (presumed adult) demonstrate the complete absence of this texture. The transition from exclusively long-grained to total absence of long-grained texture is characterized by surficial changes in two regions: (1) the long-grained texture regresses along a front situated lateral and posterior to the parietal fenestrae, and (2) an anterior to posterior series of point source ?islands? forms along the dorsal midline of the parietal, associated with the midline undulations. Our findings are consistent with the interpretation that the surface texture change is associated with growth and aging, and have implications for the taxonomy of both immature individuals and small-bodied taxa. Investigation of texture transitions subsequent to the long-grained to mottled transition (the mottled to adult transition) await the assembly of suitable series of nearly full grown conspecific individuals and cannot be reliably assessed employing the methodology used herein.