TOMMOTIIDS FROM THE EARLY CAMBRIAN (SERIES 2, STAGE 3) OF MOROCCO AND THE EVOLUTION OF THE TANNUOLINID SCLERITOME AND SETIGEROUS SHELL STRUCTURES IN STEM GROUP BRACHIOPODS

摘要

An assemblage of tannuolinid sclerites is described from the Amouslek Formation (Souss Basin) of the Anti-Atlas Mountains in Morocco. The assemblage contains two species, Tannuolina maroccana n. sp., which is represented by a small number of mitral and sellate sclerites, and Micrina sp., represented by a single mitral sclerite. Tannuolina maroccana differs from other species of the genus in the presence of both bilaterally symmetrical and strongly asymmetrical sellate sclerites. This observation suggests that the scleritome of Tannuolina was more complex than previously thought and that this tommotiid may have held a more basal position in the brachiopod stem group than previously assumed. The shell structure of both T. maroccana and Micrina sp. is well preserved and exhibits two fundamentally different sets of tubular structures, only one of which was likely to contain shell-penetrating setae. Based on these observations, the structure of the tannuolinid shell is discussed and its implications for the evolution of tubular microstructures in stem and crown group brachiopods are analysed.