First report of epithelial cell moulds with nuclei impressions preserved in lower Cambrian (Stage 3) brachiopods from South China

Epithelial tissue may represent the earliest distinctly differentiated tissue type, but the exact timing of its emergence remains unknown. Here we present the impressions of three different types of recognizable simple epithelial cells (squamous, cuboidal, columnar) with associated nuclei, found on internal valves of specimens of the acrotretoid brachiopod Eohadrotreta zhenbaensis from the Shuijingtuo Formation (ca. 518 Ma), Three Gorges area, South China. The morphology of these cell impressions are largely similar to cell impressions found on their modern counterparts, indicating that brachiopod epithelial tissues had already achieved substantial functional differentiation by the early Cambrian. Differences in the distribution of the different epithelial cell types identified herein may potentially be responsible for regulating the overall morphology of E. zhenbaensis, with columnar impressions concentrated on the highly conical ventral valve and squamous cells more prevalent on the generally flattened dorsal valve. Detailed examination of these cell impressions not only establishes that epithelial tissues were already well-developed in Cambrian animals, but also provides valuable insights into the mineralization mechanisms that produce exoskeletal shells.