ABSTRACT
All known post-Paleozoic sea stars differ from all known Paleozoic representatives in the structure of the ambulacral column; differences include placement of the water vascular system and nature of articular arrangements. These ambulacral differences are complex enough to imply a strictly monophyletic origin for the younger fauna from some as yet unrecognized Paleozoic lineage. Many major sea star constructional types originated in the Ordovician and Jurassic, and survived through long intervals of Phanerozoic time. The fossil record, with its variety of constructional types in the Ordovician, suggests an early Paleozoic establishment of diverse life habits, then continuity until widespread extinction around the end of the Paleozoic. The post-Paleozoic has been a time of evolutionary stability, perhaps engendered by the broad range of environmental tolerances and feeding types present in the group. Sea star constructional patterns appear unaffected by changes around them, although differences in relative importance of different life habits would remain undetected from the sketchy fossil record.
