Microconchids are a group of tiny, encrusting tubeworms. They appeared
in the Late Ordovician, some 450 Myr ago, flourished during the rest of
Palaeozoic and Triassic, and became extinct in the Middle Jurassic. Their
morphological resemblance to the sedentary polychaete genus Spirorbis,
very common in marine environments, misled various authors for decades.
This mistake originally gave the genus Spirorbis an enormous stratigraphical
range, from Ordovician to Recent. Indeed, microconchids provide an excellent
example of evolutionary convergence with respect to both their morphology
and ecology. In the late 1970s these ‘spirorbids’ were interpreted as vermetid
gastropods on the basis of the inner architecture and microstructure of
their tubes. This idea, however, was challenged in the 1990s when detailed
microstructural investigation showed them to be neither polychaetes nor
gastropods, but closely related to an extinct Palaeozoic enigmatic group of
organisms called tentaculitids. No thorough investigation of their origin,
phylogeny and ecology has ever been conducted, but new data concerning
their palaeobiology has come to the light during the last few years.