DISCRIMINATING BLASTOID SPECIES USING 3D MORPHOMETRICS

To date, blastoid ontogeny and phenotypic plasticity have only been studied using linear measurements and scatter plots. New techniques involving 3D data collection and landmark analysis provide an opportunity to make breakthroughs in the understanding of blastoid morphology. Blastoids are an ideal organism for morphometrics because they have a highly variable gross morphology superimposed upon an extremely conservative body design. Homologous morphological traits can thus be identified throughout the group and throughout ontogeny. This provides an opportunity to utilize 3D quantitative methods to discriminate between species, explore ontogeny, examine phenotypic plasticity, and describe evolutionary change. This study examined the ontogenetic trajectories of three species, Pentremites pyriformis, Pentremites gondoni and one previously undescribed species, collected from the Upper Mississippian Glen Dean Formation. All specimens used in this study have a fully inflated theca and were collected from the same bed. Consequently, all specimens are coeval and have been subjected to the same environmental pressures suggesting that the variation seen is largely genetic. For this quantitative study, laser images were acquired for all specimens using a Nextengine 3D Laser Scanner. From these images, X, Y, Z coordinates for a series of homologous landmarks that fully describe the morphology of the specimens in 3 dimensions were collected. These data were analyzed with morphometric and landmark analysis software packages. Analysis involved bivariate and multivariate regression of all biometric data to examine shape changes that occur with size increase. Slope and intercept values helped identify patterns of isometric versus allometric (positive or negative) growth. Of special interest was the examination of variance around the regression lines in order to assess the ontogenetic variation in shape within each species. This variation was then compared between the two species to identify differences between them. This project will be the foundation for all future work using 3D morphometrics and landmark analysis at the University of Tennessee.