The class Asteroidea (Echinodermata): Fossils and the base of the crown group

Because of limited research, a generally accepted hypothesis has not emerge d for the phylogeny of the Asteroidea, The fossil record is a potential sou rce of needed data, although fossil asteroids are rare, and they tend to be poorly preserved. Emphasis in the taxonomy of both recent and fossil asteroids has been on ch aracters visible from the exterior, and paleontologists have sought to fit even the most ancient (i.e., Ordovician) specimens into taxonomic ordinal s chemes devised for recent asteroids, Animal form and arrangement of body wa ll ossicles of Paleozoic asteroids can he similar to those of younger speci es, thereby suggesting close affinities, yet ambulacral arrangements indica te clear separation of Paleozoic stem groups from the crown group. Traits taken from the ambulacral column that mark crown-group asteroids inc lude presence of dorsal podial pores (which allowed transfer of the ampulla e to the arm interior), an offset arrangement of ambulacrals on the adambul acrals, and increased complexity of the articulation structures between amb ulacrals and adambulacrals, Transfer of ampullae to the arm interior provid ed protection and more space for ampullae within the arm, as well as space within the furrow and between the ambulacral and adambulacral ossicles for elaboration of the soft tissues that enhance arm motion.