ABSTRACT
The Blastoidea are a class of Paleozoic echinoderms containing about 100 genera distributed world-wide. Blastoids are currently divided into two orders, the Fissiculata and the Spiraculata, which have fundamentally different patterns of diversity and biogeographic range. This chapter examines overall thecal morphology of the blastoids as a beginning to understanding these patterns. Variations in thecal design in blastoids range from elongate conical to spherical resulting from differential sizes of these three plate types rather than the loss or gain of plates. Several Late Paleozoic genera have thecae that are not minimal surface design suggesting increased niche partitioning and more extensive experimentation in thecal form. Because the majority of blastoid thecal designs can be modeled as minimal surfaces, the variety of geometric forms seen in blastoid thecae can be viewed as constructional morphologic pathways rather than a functionally adaptive complex.
