A New Homology Model for Edrioasteroid Ambulacra and Its Phylogenetic Implications for the Origin of Asteroid Ambulacra

At present, there is no synapomorphy for Edrioasteroidea, and the two primary clades, edrioasterids and isorophids, are dramatically different in the construction of their ambulacra. Traditionally, edrioasterids were interpreted to bear biserial, pore-bearing, ambulacral floor plates, whereas isorophids bear uniserial floor plates lacking podial pores. New fossil material from the Middle Cambrian of China and the Ordovician of Morocco demonstrates that edrioasteroid floor plates do not follow this simple dichotomy. The Chinese species bears quadraserial floor plates with both biserial adradial and biserial abradial floor plate series perforated by adradial and abradial podial pores. Edrioasterids retain biserial abradial floor plates separated by podial pores as seen by the broad floor plate exposure beyond the cover plate articular surface, whereas the adradial floor plate series is lost. A new species from Morocco that preserves thecal interior views demonstrates that pyrgocystid isorophids fused the adradial floor plates into a uniseries and enlarged the abradial series into hood plates, which articulate directly to single cover plates. This arrangement is seen in related taxa such as Streptaster, Belochthus, and probably Cystaster. In all other isorophids, the abradial floor plate series is lost while retaining the fused, uniserial, adradial floor plate series. Thus, edrioasterid and non-pyrgocystid isorophid floor plates cannot be reconciled because they are not homologous.

Researchers have long searched for an edrioasteroid/asteroid connection. The main difficulty has been reconciling the either uniserial or biserial floor plates of edrioasteroids to the apparent asteroid floor plate homologues, adradially positioned ambulacrals and abradially positioned adambulacrals. Some have suggested that ambulacrals are simply modified floor plates. These new edrioasteroid data suggest that asteroid ambulacrals are homologues of adradial floor plates and adambulacrals are homologues of abradial floor plates. This also suggests that the edrioasteroid/asteroid split occurred prior to the edrioasterid/isorophid split.