The placement of landmarks or points on three-dimensional (3D) digital fossils allows for the visualization and characterization of shape. Eublastoidea, an ancient (443–251 million years ago) echinoderm (e.g., sea urchins) group, is an ideal group for such analysis, because they are composed of skeletal elements whose connections are inherited and easily identifiable on all species. Herein, we use 3D landmarks on fossil echinoderms to investigate relationships, change through time, and whether the varying proportions of skeletal elements that produce the animals' overall shapes are distinguishable in morphospace. The plating around the animals' mouths (oral plates) shows visible patterns in morphospace, while other ratios examined show no disambiguation in morphospace. Applying modern analytical methods to previously explored questions allows for an updated understanding of this important echinoderm group and provides a framework for others to assess echinoderms in a similar manner.

Geometric morphometrics facilitates the quantification and visualization of variation in shape and proportion through the comparison of homologous features. Eublastoidea, a Paleozoic echinoderm clade with a conservative body plan, is an ideal group for morphometric analysis, because their plate junctions are homologous and identifiable on all species. Eublastoids have previously been grouped taxonomically by generalized shape types (e.g., globose). These shapes are often used in taxonomic descriptions and as characters in phylogenetic analyses. The underlying homology of these broad shape types has never been explored. Herein we apply the first comprehensive use of three-dimensional geometric morphometrics (3D GMM) on fossil echinoderms to investigate taxonomic assignments, temporal distribution, and whether the varying proportions of skeletal elements that produce the gross thecal morphology are distinguishable. Taxonomic assignments specifically at the ordinal and family levels show varying amounts of overlap in morphospace, suggesting that many assignments may not be reevaluated. Our results suggest that none of the generalized shape types are distinct in morphospace and, therefore, likely do not capture the homologous changes in taxa. The plate circlet ratios showed trends specifically relating to the deltoid plate circlet, which has the most variability. We reanalyzed previous work and subsetted our data to be more comparable and found that there are key differences between methodologies and landmarks that will require future evaluation. Applying modern technological methods to previously explored questions allows for an updated understanding of this important fossil clade and provides a framework for others to assess fossil clades in a similar manner.