CONVERGENT MORPHOLOGY IN SMALL SPIRAL WORM TUBES ('SPIRORBIS') AND ITS PALAEOENVIRONMENTAL IMPLICATIONS

Abstract

Calcareous tube-worms generally identified as Spirorbis range from Ordovician to Recent, often profusely encrusting shells and other substrates. Whereas Recent Spirorbis is a polychaete annelid, details of tube structure in pre-Cretaceous 'Spirorbis' suggest affinities with the Microconchida, an extinct order of possible lophophorates. Although characteristically Palaeonzoic, microconchid tube-worms survived the Permian mass extinction before being replaced in late Mesozoic ecosystems by true Spirorbis. Recent Spirorbis is stenohaline but spirorbiform microconchids also colonized freshwater, brackish and hyper-saline environments during the Devonian-Triassic. Anomalies in the palaeoenvironmental distributions of fossils 'Spirorbis' are explained with the recognition of this striking convergence between microconchids and true Spirorbis.