Two Ordovician asterozoans (Echinodermata) of problematic affinities

Abstract

Phragmactis grayae Spencer and Swataria derstleri new genus new species are early (Ordovician) asterozoans (Echinodermata) that comprise the Phragmactinidae. Asterozoans are complexly varied, but as is true for other echinoderms, ambulacral construction is critical to interpretation. Phragmactinids share plesiomorphic aspects of ambulacral form and articulation with basal somasteroids and stenuroids whereas the apomorphic ambulacral expressions of asteroids and ophiuroids are lacking. Phragmactinids, like asteroids and ophiuroids, have only one virgal-series ossicle associated with each ambulacral, unlike the multiple ossicles of somasteroids and stenuroids. Virgal morphology of phragmactinids is reminiscent of expressions in somasteroids and stenuroids. Aspects of phragmactinid mouth frame construction are apomorphic. Morphologies of other ossicular series are similarly varied, and as a result, the family cannot be easily fitted into a recently proposed class-level taxonomy of early asterozoans; it is left in open nomenclature. Phragmactinid morphology does not indicate behavior significantly different from that of other early asterozaons. Asterozoan diversity suggests an early period of rapid evolutionary radiation.