A reassessment of the Lower Cambrian psammocoral Spatangopsis costata

The sand button of the psammocoral (vendobiont) Spatangopsis costata from the Early Cambrian Mickwitzia Sandstone of Sweden possessed radial rays on its uppermost surface that extended outward, in the living organism, farther than previously recognized. With intact rays, the button reached a diameter three or more times larger than its normally preserved central portion. The rays were likely housed in softtissue slots in close proximity to the external environment, and may have become directly exposed and easily broken off if the soft tissues contracted during storm-induced transportation, thus producing the normally observed short rays. A large individual, exceptionally preserved in situ, shows the true length of rays, as well as a major portion of a circular, radially ribbed mechanical impression of a soft-tissue surface underneath the button. Back-filling of this impression suggests a slight, active upwards movement to counter sedimentation. Small individuals are more commonly preserved with complete rays. A reconstruction of the S. costata organism as a solitary polyp appears unlikely, since the above evidence leaves insufficient space for an actinian-like gut above the sand button. More likely alternatives are a colonial organism in which a large, central polyp/zooid was specialized as a sand-weighted anchor and supported a number of smaller feeding individuals, or a solitary or colonial descendant of frond-bearing Ediacaran vendobionts that replaced the basal holdfast with a weighted anchor, as an evolutionary response to the Early Paleozoic decrease in biomats suitable for the disc type of attachment, and probably carried multiple fronds. Fragmentary impressions of suitable candidates for these fronds are preserved in the same sediments.