Hyolitha: status of the phylum

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Abstract
Hyoliths are operculate calcareous shells found in Paleozoic rocks. Runnegar et al. (1975) suggested that they be referred to a new phylum (Hyolitha), but Marek and Yochelson (1976) and Dzik (1978) preferred to regard them as an extinct class of the Mollusca. Since the hypolith 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. Hyoliths are probably not mollusk. Previous reconstructions of articulated hyolithids suggest that left and right appendages (helens) curved dorsally. Crushed articulated specimens from the [Middle Cambrain] Burgess Shale indicate that this conclusion is incorrect; hyolithid helens may have curved ventrally when the animals were alive.

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