Fate Mapping the Ontogenetic Series of Oral Cover Plates In the Edrioasteroid Neoisorophusella Lanei

Edrioasteroids are an extinct clade of sessile, suspension-feeding Paleozoic echinoderms (Lower Cambrian-Upper Permian). In situ populations are commonly preserved by obrution deposits, providing excellent ontogenetic series. Previous studies of ontogeny utilized inconsistent plate naming schemes that do not allow tracing homologous elements through the ontogenetic sequence, i.e. fate mapping. For this study we investigated the Lower Mississippian taxon Neoisorophusella lanei as a model for ontogenetic fate mapping of the primary and secondary oral cover plate series. This research has necessitated a new labeling system for the oral cover plates, based on time of ontogenetic expression and relative position in the oral framework. The five primary orals (PO) present at the earliest stage of development are labeled Left and Right Lateral Bifurcation Plates (LLBP, RLBP), Left and Right Anterior Primary Oral (LAPO, RAPO) and Posterior Primary Oral (PPO). Remaining orals are labeled Hydropore Oral (HO), Right Anterior (R), Left Anterior (L), Right Posterior (r), Left Posterior (l). Secondary plates are indicated by (S,s). Plates are numbered according to their relative order of expression. The earliest preserved ontogenetic stages (diameter of ~1mm) bear only the five POO with the HO inserted next. At this stage, the shared and distal ambulacra are undeveloped. Shared ambulacra develop by first adding the distal primary oral cover plates L1,l1,R1,r1 by 1.5mm, followed by the more proximal L2,R2,r2, also by 1.5mm; l2 is inserted distal to l1. Ambulacral cover plates appear at this time followed by expression of l3. No new oral plates appear until LS4, ls4, RS4, rs4 express at 4.0mm. Secondary plates LS5 and RS5 express at 5.0mm. Later growth is marked by the addition of new plates only in the distal ambulacra, and increase in size of preexisting plates. Only minor variation is seen in plate expression, particularly ls4, and occasionally RS5 or RS4.