On the nature and development of graptoblasts

Two specimens of graptoblasts, which provisionally may be assigned to "Graptoblastoides" sp. and "Graptoblastus" sp., are described from the early Llandovery of Anticosti Island (Québec, Canada). Previous reports of Silurian graptoblasts were restricted to a single specimen from Poland. One of the new specimens is morphologically distinct from all known graptoblasts - both chambers of the inner cavity contain numerous incomplete septae, here referred 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 Kozłowski, 1949 and Graptoblastoides Kozłowski, 1949 are interpreted as stages in graptoblast development, and are consequently replaced by the purely descriptive terms "graptoblastus"and "graptoblastoides". The following stages of graptoblast development are recognized: (1) pre-graptoblastoides stage, (2) early graptoblastoides stage, (3) late graptoblastoides stage, (4) early graptoblastus stage, and (5) late graptoblastus stage. A graptoblast s.s. (i.e., devoid of a blastotheca) and its developmental stages strikingly resemble the earliest stages of the astogeny of the modern pterobranch Rhabdopleura. These observations support A. Urbanek's hypothesis that zooids became encysted within graptoblasts and rejuvenated by a process of metamorphosis.