Hyolitha: status of the phylum

Hyoliths are operculate calcareous shells found in Palaeozoic rocks. Runnegar et al. (1975) suggested that they be referred to a new phylum (Hyolitha) but Marek & Yochelson (1976) and Dzik (1978) preferred to regard them as an extinct class of the Mollusca. Since the hyolith cone is not easily homologized with the monoplacophoran shell, the exoskeletons of the shelled Mollusca and the Hyolitha appear to have developed independently. Reconstructions of the anatomy of hyoliths indicate that it is unlikely that both groups shared a common molluscan ancestor. Therefore, hyoliths are probably not molluscs. Previous reconstructions of articulated hyolithids have suggested that left and right appendages (helens) curved dorsally. Crushed articulated specimens from the Burgess Shale indicate that this conclusion is incorrect; hyoltthid helens seem to have curved ventrally when the animals were alive.