On the nature and development of graptoblasts

Two specimens of graptoblasts, which provisionally may be assigned to 'Grap toblastoides' sp. and 'Graptoblastus' sp., are described from the early Lla ndovery of Anticosti Island (Quebec, Canada). Previous reports of Silurian graptoblasts were restricted to a single specimen from Poland. One of the n ew specimens is morphologically distinct from all known graptoblasts - both chambers of the inner cavity contain numerous incomplete septae, here refe rred to as hemiseptae. SEM observations show distinct two-layered character of the transverse septum, whereas the hemiseptae show no trace of layering . Hemiseptae and the transverse septum are derivatives of the inner lining of the graptoblast. The 'genera' Graptoblastus Kozlowski, 1949 and Graptobl astoides Kozlowski, 1949 are interpreted as stages in graptoblast developme nt, and are consequently replaced by the purely descriptive terms 'graptobl astus' and 'graptoblastoides'. The following stages of graptoblast developm ent are recognized: (1) pre-graptoblastoides stage, (2) early graptoblastoi des stage, (3) late graptoblastoides stage, (4) early graptoblastus stage, and (5) late graptoblastus stage. A graptoblast s.s. (i.e., devoid of a bla stotheca) and its developmental stages strikingly resemble the earliest sta ges of the astogeny of the modem pterobranch Rhabdopleura. These observatio ns support A. Urbanek's hypothesis that zooids became encysted within grapt oblasts and rejuvenated by a process of metamorphosis.