Hyolitha

Abstract: Small shelly fossils are common in sediments of Early Cambrian age and include the earliest common representatives of metazoan animals with mineralized hard parts. The group includes fossils of very different morphology, composition and ultrastructure. They seem to represent skeletal remains of numerous animal groups, the biological affinities of which are largely unresolved. However, the wide geographic range of many forms has the potential to enhance biostratigraphic and palaeogeographic resolution in the Early Cambrian. The late Early Cambrian sequence of North-East Greenland has yielded an assemblage of more than 88 species of small shelly fossils, brachiopods and trilobites, indicative of a middle Dyeran age (Botoman equivalent). The recovered fossils include a number of species that are known from other Early Cambrian palaeocontinents, and particularly strong ties to late Early Cambrian faunas of Australia are documented. The many cosmopolitan taxa thus identified suggests a close juxtaposition of palaeocontinents at this time. The systematic affinity of many of these small shelly fossils is poorly understood, partly because of their fragmentary nature and poor preservation. However, new data from North-East Greenland improves our understanding of the function and biological affinity of certain taxa. Collections of the problematic fossil Mongolitubulus from North and North-East Greenland exhibit characters indicative of a defensive function as spines of bivalved arthropods, while species of the problematic genus Triplicatella represent the opercula of an unknown tubular shell, probably related to orthothecid hyoliths. The bivalved fossil Mickwitzia from North-East Greenland combines characters of linguliform brachiopods and sclerites of Micrina, a non-bivalved problematic form (halkieriid) from Australia. The combination suggests that Mickwitzia is a stem group brachiopod and strengthens arguments for a halkieriid ancestry of the brachiopod phylum.

Abstract: 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.

Abstract: The rare hyolith Alfaites romeo gen. et sp. nov. is described from the lower half of the middle Cambrian (Drumian) Buchava Formation of the Skryje-Týřovice Basin in the Barrandian area of the Czech Republic. This new taxon is based on excellently preserved external and internal moulds of three opercula and several conchs collected from carbonate nodules and shale. This rare species has been established at three separate outcrops in the lower part of the Biozone of Paradoxides ( Eccaparadoxides ) pusillus (Barrande 1846). Morphology of conch and both external and internal surface of operculum are well known. However, specific morphology excludes assignment to any predescribed hyolithid family.

Abstract: The hyoliths, an obscure group of tubular fossils having a closed apex, are confined to the Palaeozoic. They have been known for nearly two centuries, but their zoological placement remains uncertain. History of their assignment, particularly during the last five decades is summarised. The two principal current views are that either they represent an extinct class within the Mollusca or an extinct phylum. Based on the cross-lamellar structure of the hyolith shell, the writers' view is that a class within the Mollusca is a more appropriate placement than as a phylum.