GEOMETRIC MORPHOMETRIC ONTOGENY OF CALYPTAULAX: PATTERNS OF PHYSIOLOGICAL CHANGE AT THE MERASPID-HOLASPID TRANSITION IN PHACOPIDS

Study of trilobite ontogeny has historically focused on the number and distribution of trunk segments. A landmark-based two-dimensional morphometric analysis was performed on specimens of the phacopid trilobite Calyptaulax strasburgensis (Ulrich and Delo, 1940) to study trends in shape change across the transition from meraspid to holaspid growth phases. The data-set consisted of 100 silicified cranidia and pygidia collected from the mid-Ordovician Edinburg Formation of Virginia. Ontogenetic trajectories of centroid size against Procrustes distance for both cranidial and pygidial data sets suggested disproportionate changes in shape with size. Least-squares segmented linear regression suggested breakpoints in both cranidial and pygidial trajectories indicating a reduction or cessation of net shape change during growth, corroborating qualitative observations from previous studies of trilobite ontogeny. Multivariate regressions of partial warp scores from a mean reference form against centroid size indicated 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 demonstrated the overall changes in shape during this period of ontogeny, including glabellar expansion through lateral deflection of the axial furrow, S1, S2, and S3, the development of a more angular appearance to the anterior portion of the glabella, and the anterior migration of the pygidial anterior margin. These results are consistent with previous work on phacopid ontogeny and maturation finding not only a decrease in the rate of shape change corresponding with the meraspid-holaspid transition but also allometric lateral expansion of the glabella during the meraspid stage. This observed broadening of the glabella has potential implications regarding ontogenetic resource partitioning in phacopids, and may relate to a shift in reproductive strategy associated with maturation.