Graptolite-like fibril pattern in the fusellar tissue of Palaeozoic rhabdopleurid pterobranchs

The fusellar tissue of Palaeozoic rhabdopleurid pterobranchs Ls been studie d using the SEM techniques. The fibrillar material of Ordovician Kystodendr on ex gr. longicarpus and Rhabdopleurites primaevus exhibits a distinct dim orphism, comprising: (1) thinner, wavy and anastomosing/branching fusellar fibrils proper, producing a tight three-dimensional meshwork; and (2) long, more or less straight and unbranched cortical fibrils, sometimes beaded, a nd arranged in parallel, These fibrils are similar to the fusellar and cort ical fibrils of graptolites, respectively. Until now, dimorphic fibrils and their arrangement within fusellar tissue were regarded as unique character s of the Graptolithina. In general, the fibrillar material of these fossils is partially preserved in the form of flaky material (new term) composed o f flakes (new term). Flakes are interpreted as flattened structures origina ting from the fusion of several neighbouring tightly packed fibrils. A Perm ian rhabdopleurid, referred to as Diplohydra sp., reveals a fabric and patt ern of fusellar tissue similar to that of both Ordovician rhabdopleurids bu t devoid (?) of cortical fibrils. The results presented here question views that: (1) substantial differences in fabric and pattern of fusellar tissue exist between fossil pterobranchs and graptolites; and (2) the ultrastruct ure of pterobranch periderm has remained unchanged at least since the Ordov ician. The Palaeozoic rhabdopleurids investigated are closer ultrastructura lly to graptolites than to contemporary pterobranchs. The pterobranchs and the graptolites should be treated as members of one class - the Graptolitho idea.