Periostracum and ﬁbrous shell microstructure in the unusual Cambrian hyolith Cupitheca
MICHAEL J. VENDRASCO
Departamento de Estratigrafía y Paleontología, Universidad de Granada, 18071 Granada, Spain.
Department of Geology, Pasadena City College, 1570 E. Colorado Blvd., Pasadena, California 91106-2003, USA. firstname.lastname@example.org
ANTONIO G. CHECA
Departamento de Estratigrafía y Paleontología, Universidad de Granada, 18071 Granada, Spain. email@example.com
SUSANNAH M. PORTER
Earth Research Institute and Department of Earth Science, University of California, Santa Barbara, CA 93106, USA. firstname.lastname@example.org
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.
Key words: Lamello-fibrillar, crossed lamellar, Parara Limestone, Hyolitha, Problematica, Brachiopoda.
How to cite: Vendrasco, M.J., Checa, A.G. & Porter, S.M. 2017. Periostracum and ﬁ brous shell microstructure in the unusual Cambrian hyolith Cupitheca. Spanish Journal of Palaeontology, 32 (1), 95-108.
Received 3 December 2016, Accepted 11 March 2017, Published 30 June 2017