What a new model of skeletal homologies tells us about asteroid evolution

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