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

The Extraxial-Axial Theory (EAT) is applied to the body wallhomologies of asteroids. Attempts to characterize major platesystems of asteroids as axial or extraxial, particularly thosethat are highly organized into series, can be problematic. However,the Optical Plate Rule (OPR) is instrumental in establishingthat ambulacrals and terminals are axial. It is equally clearthat the region aboral to the marginal frame is a part of theperforate extraxial body wall (with the possible exception ofthe centrodorsal, which is likely imperforate extraxial). Previouslyestablished EAT criteria, particularly those strongly rootedin the embryologically expressed boundary between axial andextraxial body wall in larvae, suggest that marginals, and perhapsadambulacrals, are extraxial in origin. We also explore theextraxial nature and phylogenetic significance of the odontophore.Our data from both juveniles and adults show that plate andtube foot addition sequences occur according to the OPR, andshed light on poorly known homologies of the asteroid mouthframe. These data indicate that the mouth angle ossicle mustat least contain the first ambulacral, although we cannot ruleout the possibility that the first adambulacral also contributesto the construction of this ossicle. The interpretations providedby the EAT for all ossicles suggest a synapomorphy scheme forsomasteroids, ophiuroids, and asteroids.