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

Abstract

A phosphatized bilaterally symmetrical, spirally coiled conch from the Lower Cambrian phosphates of the Meishucunian Stage of eastern Yunnan, China, is composed of two well-preserved layers. The outer one is built of longitudinal mineralized fibres not strictly parallel to each other, but may anastomose or branch. The fibres form comarginal ribs with transversal grooves between them, corresponding to growth stages. In the ribs, the fibres are arranged closely and are steeply inclined towards the apex, while in the grooves the fibres are subparallel to the conch surface and more loosely packed. Small elliptic and triangular holes are situated within and between the fibres of the outer layer. The inner layer consists of transversal fibres running around the conch. They are almost parallel to each other but partly separated by narrow discontinuous slits. Despite overall morphological similarity of the conch to shells of some early Cambrian molluscs, the observed orthogonal configuration of longitudinally oriented fibrous structures of the outer layer and transversal fibres of the inner layer is found in orthothecid hyoliths, phosphatized microstuctures of which have been described from the Lower Cambrian of the Siberian Platform. The new material from the Yangtze Platform supports a conclusion of a distinctive type of microstructure available in hyoliths different from molluscan microstructures available from the Lower Cambrian.