Hyolith-type microstructure in a mollusc-like fossil from the Early Cambrian of Yunnan, China

A phosphatized bilaterally symmetrical, spirally coiled conch from the Lowe r Cambrian phosphates of the Meishucunian Stage of eastern Yunnan, China, i s composed of two well-preserved layers. The outer one is built of longitud inal mineralized fibres not strictly parallel to each other, but may anasto mose or branch. The fibres form comarginal ribs with transversal grooves be tween them, corresponding to growth stages. In the ribs, the fibres are arr anged closely and are steeply inclined towards the apex, while in the groov es the fibres are subparallel to the conch surface and more loosely packed. Small elliptic and triangular holes are situated within and between the fi bres of the outer layer. The inner layer consists of transversal fibres run ning around the conch. They are almost parallel to each other but partly se parated by narrow discontinuous slits. Despite overall morphological simila rity of the conch to shells of some early Cambrian molluscs, the observed o rthogonal configuration of longitudinally oriented fibrous structures of th e outer layer and transversal fibres of the inner layer is found in orthoth ecid hyoliths, phosphatized microstuctures of which have been described fro m the Lower Cambrian of the Siberian Platform. The new material from the Ya ngtze Platform supports a conclusion of a distinctive type of microstructur e available in hyoliths different from molluscan microstructures available from the Lower Cambrian.