Geometric morphometrics as a tool for evaluating Eublastoidea morphological variation

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 3D geometric morphometrics 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 level 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 subset 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.