Cortical developments in the Graptolithina (Pterobranchia) under the scanning electron microscope: A review and new clues

The cortical bandages of the Graptolithina are involved in the construction of a number of structures of the tubaria. Cortical material differs from fusellar tissue in the type of fibrils and their arrangement. The fibrils of the bandages, forming the cortex, are densely packed, running parallel to the length of the bandage and creating strong and flexible material. Cortical tissue is present in all graptolites as a secondary development, overlying the fusellum and in some cases forming additional features. In the Retiolitinae, the main constructional role of the tubaria was taken over by a network of lists, made of cortical bandages, not the fusellum. Among the meshwork of cortical lists, there are thicker ones (clathrium), in various configurations, which possibly strengthened the tubaria. For example, the cortical structures are the sicular annuli, virga, virgella, nema, ancora umbrella and ancora sleeve of retiolitines. A scanning-electron-microscope study of the sicular annuli, specific for some monograptids, shows for the first time the ultrastructure of their cortical bandages. The details of thickened cortical construction may be useful for the interpretation of flattened material but also may provide a basis for misleading interpretations.