ONTOGENETIC ANALYSIS OF THE PHACOPID TRILOBITE CALYPTAULAX STRASBURGENSIS

Study of trilobite ontogeny has historically focused on the number and distribution of trunk segments. With modern geometric morphometric and statistical methods, changes in shape and size can be rigorously quantified and compared. A landmark-based two-dimensional morphometric analysis was performed on specimens of the phacopid trilobite Calyptaulax strasburgensis to determine the extent to which shape is dependent on size as meraspids develop into holaspids. The data-set consists of over 100 silicified cranidia and pygidia collected from the mid-Ordovician Edinburg Formation of Virginia. Digital landmarks include the intersections and termini of furrows, with semi-landmarks used to improve geometric coverage along the facial suture of the cranidium and the axial furrow of the pygidium. Ontogenetic trajectories of centroid size against Procrustes distance for both cranidial and pygidial data sets suggest disproportionate changes in shape with size. Multivariate regressions of partial warp scores from a mean reference form against centroid size indicate statistically significant allometric change in growth during the transition from the meraspid to the holaspid stage. Thin plate spline deformation grids based on the regression analyses demonstrate the overall changes in shape during this period of ontogeny, including lateral deflection of the axial furrow, S1, S2, and S3, and the development of a more angular appearance to the anterior portion of the glabella.