Devonian hyoliths in South Africa, and their palaeoenvironmental significance

Hyoliths, an extinct group of molluscan or mollusc-like invertebrates that secreted elongate conical shells, are relatively uncommon and often overlooked members of Middle Palaeozoic marine fossil assemblages. They have been recorded from a number of widespread localities in the argillaceous Gydo and Voorstehoek Formations of the Bokkeveld Group. Most of the specimens are crushed or are without opercula and other taxonomically important features of the aperture and therefore must remain indeterminate. The hyoliths occur in bioturbated mudstones along with brachiopods, trilobites, bivalves, gastropods, crinoids, cephalopods and conulariids. This assemblage is interpreted as representing an inner shelf community that occupied quiet waters below wave base seaward of the prograding Bokkeveld deltas. Unlike many of the other elements of the shelf community, the epifaunal, deposit-feeding hyoliths are not found in the shallower water communities and can be taken as diagnostic of the offshore environments