THE GREAT INSIGHT THAT JAMES SPRINKLE ADDED TO THE ECHINODERM CONVERSATION: 2-1-2 SYMMETRY

Few morphological insights have advanced the study of Paleozoic echinoderms as much as James Sprinkle’s fundamental recognition of the 2-1-2 symmetry that forms the plesiomorphic ambulacral arrangement found in pentaradiate echinoderms. The common wisdom that radial pentameral symmetry is a foundational aspect of echinoderms is erroneous because the fundamental symmetry of non-eleutherozoan, pentaradiate echinoderms is bilateral. The 2-1-2 ambulacral symmetry of Sprinkle identifies the symmetrical arrangement of ambulacral across the crinoidal plane and around the centrally located peristome resulting from developmental pattering. This pattern is characterized by three clusters of ambulacra - a centrally branching, singular ambulacrum along the crinoidal plane (the A ambulacrum) and two pairs of laterally branching ambulacra, B and C to the right and D and E to the left. The 2-1-2 pattern templates the bilateral symmetry of the peristome border seen in the arrangement of oral plates, primary peristomial cover plates, and oral frame plates where present. Many planes of symmetry have been described in echinoderms, that generally relate to the positioning of the hydropore, gonopore, and anus rather than the underlying developmental body axes of the organism. While these planes are significant in certain echinoderm subclades, such as Lovén’s plane in irregular echinoids, they are not integral to echinoderms as a whole. The ambulacral symmetry remains an underlying constant even as these body openings translate across the theca. The crinoidal plane was later identified as the A/P axis of echinoderms and provides the basis for uniquely identifying ambulacral homology. Because the 2-1-2 symmetry results from developmental patterning, changes early in development form the basis for understanding Paedomorphic Ambulacral Reduction that leads to echinoderm clades with unusual, yet predictable symmetries. Furthermore, because the 2-1-2 pattern forms the basis for interpreting ambularal homology, it also forms the foundation upon which Universal Elemental Homology is built.