Periostracum and fibrous shell microstructure in the unusual Cambrian hyolith Cupitheca

Resumen

Cupitheca is an enigmatic tubular fossil common in early Cambrian deposits worldwide. It has recently been argued to be a hyolith, probably orthothecid. Cupitheca had a dense network of mantle-filled tubules that connected to what we interpret as a continuous organic periostracum. The innermost shell layer consists of horizontal or slightly inclined bundles of fibres elongated along the a-axis and offset from other bundles at aragonitic twin angles, confirming aragonite as the original mineralogy for the shell of Cupitheca. This is a similar Shell microstructure to that inferred for Cambrian hyoliths, strengthening the claim that Cupitheca is a hyolith. This shell microstructure of bundled aragonite fibres and the tubule systems can also be seen in many Cambrian molluscs and other lophotrochozoans. In some lineages this shell texture evolved into fracture-resistant crossed lamellar microstructure and in others nacre. These transitions began to occur sometime between the mid-Cambrian and Ordovician, and nacre and crossed lamellar microstructure were the most common constituents of the inner shell layer of molluscs by the middle or late Palaeozoic Era.