"SAND CORALS" FROM THE EARLY CAMBRIAN OF SWEDEN

Protolyellia and Spatangopsis from the Lower Cambrian Mickwitzia Sandstone of Sweden have been interpreted by Seilacher as sediment accumulated in the internal cavities of “sand corals” or Psammocorallia, an extinct group of organisms of actinian cnidarian affinities possessing unique adaptations to sessile life on soft sediments.

A re-examination of these genera reveals previously unrecognized morphologic and preservational features, which suggest that these genera were neither closely related to each other, nor to actinians. Although cnidarian affinities cannot be disproved, neither can affinities with Ediacara-type organisms (rare fragments of which do occur in the same beds). A picture emerges of organisms morphologically and adaptively more exotic than previously assumed.

It is suggested that these two genera, together with the tool-mark Eophyton from the same beds, represent Cambrian survivors of earlier groups of sessile soft-bottom forms that convergently evolved sediment-filled or –encrusted holdfasts in order to cope with the gradual disappearance of microbial mats that made non-weighted superficial holdfasts, common in the Precambrian, no longer effective.