Abstract
A new species of trilobite, Cyrtosymbole rectifrons, from Famennian marlstones in the south-eastern vicinity of Erfoud (Tafilalt, Morocco), is described. Silicified sclerites of various sizes allow the description of its late growth sequence, which demonstrates that some characteristic features of the Cyrtosymbolinae are acquired during late ontogeny. Precise examination of exceptionally well-preserved material enables reconstruction of the in situ position of the adult hypostome, together with a reconsideration of its relation to the dorsal exoskeleton and its morphofunctional capacity for movement. The natant hypostomal condition inferred for C. rectifrons suggests that it is not always possible to deduce the nature of hypostomal attachment from the dorsal morphology alone.