The Class Asteroidea (Echinodermata): Fossils and the Base of the Crown Group

Because of limited research, a generally accepted hypothesishas not emerged for the phylogeny of the Asteroidea. The fossilrecord is a potential source of needed data, although fossilasteroids are rare, and they tend to be poorly preserved. Emphasis in the taxonomy of both recent and fossil asteroidshas been on characters visible from the exterior, and paleontologistshave sought to fit even the most ancient (i.e., Ordovician)specimens into taxonomic ordinal schemes devised for recentasteroids. Animal form and arrangement of body wall ossiclesof Paleozoic asteroids can be similar to those of younger species,thereby suggesting close affinities, yet ambulacral arrangementsindicate clear separation of Paleozoic stem groups from thecrown group. Traits taken from the ambulacral column that mark crown-groupasteroids include presence of dorsal podial pores (which allowedtransfer of the ampullae to the arm interior), an offset arrangementof ambulacrals on the adambulacrals, and increased complexityof the articulation structures between ambulacrals and adambulacrals.Transfer of ampullae to the arm interior provided protectionand more space for ampullae within the arm, as well as spacewithin the furrow and between the ambulacral and adambulacralossicles for elaboration of the soft tissues that enhance armmotion.