Skeletal microstructures of hyoliths from the Early Cambrian of Siberia

Phosphatised conchs and opercula of various hyoliths, 1–2 mm in size, retaining original skeletal microstructures have been extracted with dilute acetic acid from Lower Cambrian carbonates of northern Siberia. The conchs grew by deposition of new lamellae on the inner side of the aperture. Two microstructural types are recognised among the conchs assigned to the Allathecidae, namely with a simple or composite inner layers. The conchs are built of inner and outer layers of mineralised fibre bundles. In the outer layer the bundles are longitudinally oriented. Their external ends are inclined toward the apex of conch. Each bundle contains a channel. Transverse bundles of the inner layer commonly run around the conch but may branch toward the aperture and produce an orthogonal network. The inner layer in this case is composite and consists of two sublayers. In the outer sublayer the bundles are situated transversely around the conch, and in the inner sublayer the bundles are directed longitudinally. Channels between the bundles of the inner layer were possibly connected to those of the outer layer and reflect a system of tubules. The latter possibly participated in biomineralisation and provided a framework, which reinforced the skeleton. The tubules do not penetrate the outermost portion of the outer layer. Septa display radially arranged fibrous microfabric with the evidence of a centripetal growth from the inner surface of conch. The opercula consisted of series of growth lamellae each containing fibres, oriented radially, and tubules. The tubules were almost normal to the growth surfaces and concentrated in the opercular limb. The hyolith skeleton probably contained organic fibres mineralised by aragonite and commonly fused into bundles. The affinity of the Hyolitha to a separate phylum may be supported by this microstructural investigation.