GEOMETRIC MORPHOMETRIC ONTOGENY OF THE TRILOBITE CALYPTAULAX: SHAPE CHANGE DURING THE MERASPID-HOLASPID TRANSITION

A combined landmark and semi-landmark based study of the ontogeny of the trilobite Calyptaulax demonstrates the extent to which shape is dependent on size as meraspids develop into holaspids. Silicified individuals from the Edinburg Formation of Virginia form the raw material for the dataset, and vary in cranidial length from approximately 1 mm to 1 cm. Landmarks are primarily intersections and end points of cranidial furrows, and a semi-landmark curve summarizes cranidial shape along the facial suture. A bivariate plot of procrustes distance and log centroid size demonstrates the linear aspects of the ontogenetic trajectory across the meraspid-holapsid boundary. To test if growth is allometric in Calyptaulax, we conducted a multivariate regression of partial warp scores from a mean reference form against log centroid size. Results of the regression indicate statistically significant allometric growth across the meraspid-holaspid transition in Calyptaulax. Thin plate spline deformation grids based on the regression analysis summarize a series of morphological changes during ontogeny, most notably the lateral deflection of furrows S1, S2, S3, and axial furrow. In addition, the anterior portion of the glabella becomes more angular during ontogeny and the meraspids lose a prominent occipital tubercle.