Craniofacial Ontogeny In Centrosaurus apertus

Abstract

Centrosaurus apertus, a large bodied ceratopsid from the Late Cretaceous of North America, is one of the most common fossils recovered from the Belly River Group of Canada. This fossil record shows a wide diversity in morphology and size, with specimens ranging from putative juveniles to fully-grown individuals. The goal of this study was to reconstruct the ontogenetic changes that occur in the craniofacial skeleton of C. apertus through a quantitative cladistic analysis. Forty-seven cranial specimens were independently coded in separate data matrices for 80 hypothetical multistate growth characters and 130 binary growth characters. Analyses were executed under heuristic searches with all characters unordered and equally weighted. Both analyses yielded the max-limit of 100,000 most parsimonious saved trees and the strict consensus collapsed into large polytomies, so a 50% majority rule consensus was obtained to recover structure in the data. In order to reduce conflict resulting from missing data, fragmentary individuals were removed from the data matrices and the analyses were rerun under a branch and bound search for both multistate and binary data sets. The multistate analysis yielded a single most parsimonious tree, while the binary analysis yielded thirteen equally most parsimonious trees. A strict consensus of the thirteen trees collapsed into a polytomy in the most mature individuals, but the resolved portion is consistent with the tree recovered in the multistate analysis. Among both the complete and the reduced data sets the multistate analyses recovered a shorter tree with a higher consistency index (CI) than the additive binary data sets. The arrangement within the trees show a progression of specimens with a recurved nasal horn in the least mature individuals, followed by specimens with straight nasal horns in relatively more mature individuals, and finally specimens with procurved nasal horns in the most mature individuals. The supraorbital unit, however, shows no consistent pattern of development. The parietal horns develop relatively early, becoming long and curved in some of the least mature skulls. In relatively mature individuals these structures resorb, leaving the horns with a withered appearance. This resorption continues in the most mature individuals until much of the horn is gone. The development of the parietal and nasal horns may represent a heterochronic process (i.e. peramorphosis) in centrosaurine evolution, where juvenile morphology is similar to that of basal neoceratopsians, whereas the adult condition is comparable to that of derived centrosaurines. Bone textural changes were found to be sufficient proxies for relative maturity in individuals that have not reached adult size. Additionally, frill size is congruent with relative maturity status and makes an acceptable proxy for ontogenetic status, especially in smaller individuals. In adult-sized individuals, the fusion of the epoccipitals and the orientation of the nasal horn are the best indicators of relative maturity. There is no clear evidence for sexually specific characters or sexual size dimorphism in C. apertus.