The water vascular system and functional morphology of Paleozoic asteroids

Asteroids of all geologic ages share a single basic body form, surficial skeletal arrangement, and aspects of water vascular construction. In almost all described Paleozoic species, however, either podial pores to the interior of the arm were lacking, or they are directed laterally, above the adambulacrals. They are internal and above the ambulacrals in known post-Paleozoic species and the Pennsylvanian Calliasterella. Certain features of the ambulacral skeletal arrangement also differ. Calliasterella is the closest known Paleozoic relative of post-Paleozoic asteroids. Classifications of asteroids that stress only overall form and surticial skeletal arrangement erroneously include Paleozoic and Holocene species in common ordinal or even lower level groupings. Taxonomic revision is premature: however, most known Paleozoic asteroids represent primitive lineages. Transitional forms allow reconstruction of events leading to the modern arrangement. Ampullar and skeletal arrangements of post-Paleozoic asteroids appear to offer some functional advantages over those of their precursors, but as early as the Ordovician, diverse feeding habits had evolved and ecological roles paralleled those of Holocene species.