What a New Model of Skeletal Homologies Tells Us About Asteroid Evolution

Abstract

The Extraxial-Axial Theory (EAT) is applied to the body wall homologies of asteroids. Attempts to characterize major plate systems of asteroids as axial or extraxial, particularly those that are highly organized into series, can be problematic. However, the Optical Plate Rule (OPR) is instrumental in establishing that ambulacrals and terminals are axial. It is equally clear that the region aboral to the marginal frame is a part of the perforate extraxial body wall (with the possible exception of the centrodorsal, which is likely imperforate extraxial). Previously established EAT criteria, particularly those strongly rooted in the embryologically expressed boundary between axial and extraxial body wall in larvae, suggest that marginals, and perhaps adambulacrals, are extraxial in origin. We also explore the extraxial nature and phylogenetic significance of the odontophore. Our data from both juveniles and adults show that plate and tube foot addition sequences occur according to the OPR, and shed light on poorly known homologies of the asteroid mouth frame. These data indicate that the mouth angle ossicle must at least contain the first ambulacral, although we cannot rule out the possibility that the first adambulacral also contributes to the construction of this ossicle. The interpretations provided by the EAT for all ossicles suggest a synapomorphy scheme for somasteroids, ophiuroids, and asteroids.