© 2000 by The Society for Integrative and Comparative Biology
The Class Asteroidea (Echinodermata): Fossils and the Base of the Crown Group 1
1 Department of Geology, University of Illinois at Urbana-Champaign, 1301 W. Green St., Urbana, Illinois 61801
Because of limited research, a generally accepted hypothesis has not emerged for the phylogeny of the Asteroidea. The fossil record is a potential source 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 characters visible from the exterior, and paleontologists have sought to fit even the most ancient (i.e., Ordovician) specimens into taxonomic ordinal schemes devised for recent asteroids. Animal form and arrangement of body wall ossicles of Paleozoic asteroids can be similar to those of younger species, thereby suggesting close affinities, yet ambulacral arrangements indicate clear separation of Paleozoic stem groups from the crown group.
Traits taken from the ambulacral column that mark crown-group asteroids include presence of dorsal podial pores (which allowed transfer of the ampullae to the arm interior), an offset arrangement of ambulacrals on the adambulacrals, and increased complexity of the articulation structures between ambulacrals and adambulacrals. Transfer of ampullae to the arm interior provided 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.