Convergent morphology in small spiral worm tubes (‘<em>Spirorbis</em>’) and its palaeoenvironmental implications

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