Abundant trilobite remains were recovered from late mid-Famennian marlstones from various sites in Eastern Tafilalet, southeast Morocco. All belong to a single taxon previously identified as Cyrtosymbole (Waribole) prima. This taxon is designated the type species of Osmolskabole gen. nov. A redefinition of this species, including the description of newly discovered, disarticulated exuviae both in limestone and silicified state of preservation, is given. In particular, silicified sclerites of various sizes allow the first complete growth series of a cyrtosymboline proetid to be presented. The close morphological resemblance of its protaspid stages to known proetoid larvae emphasizes the homogeneity of the early ontogeny in this superfamily. The Famennian proetoid anaprotaspis is also of comparable size to that of other Devonian proetoid larvae. However, their size-range is much less than that observed in Carboniferous larvae. This suggests that the survival of proetoid trilobites at the Frasnian-Famennian Kellwasser crisis did not result from a modification of the developmental strategy, as it might have been the case at the terminal Devonian extinction event. Moreover, O. prima possesses a plectrum from the metaprotaspid to the mid meraspid periods. This implies that the natant hypostomal condition is not steadily acquired early in the ontogeny of the Proetida. Thus we preclude the use of this character in the diagnosis of this order.

Key words: Trilobita, Proetoidea, Cyrtosymbolinae, ontogeny, protaspis, Famennian, Morocco.

Rudy Lerosey−Aubril [lerosey@isem.univ−montp2.fr] and Raimund Feist [rfeist@isem.univ−montp2.fr], Laboratoire de Paléontologie, Paléobiologie et Phylogénie, Institut des Sciences de l’Evolution, Université Montpellier II, 34095 Montpellier Cedex 05, France.