Cyclostomatous Bryozoa from the Silurian of Gotland

Silurian cyclostomatous Bryoza are described from Gotland, Sweden. In addition several species from the U.S.A. and Estonia, which are important for the taxonomical discussions, are described. Totally, twenty-one species, six of them new, are described. The interzooidal walls of the investigated Paleozoic Cyclostomata are tripartite structures composed of a middle granular layer flanked on both sides by laminar layers or, in species with thin interzooidal walls, of a double laminar layer. The laminae are orientated either parallel to the growing direction of the wall or in an orally divergent pattern. The exterior walls of the zooecia consist of a single laminar layer. The crystals in the laminar layers consist of long, flat laths, approximately 1 μ thick and up to 50 μ long. They probably grew by edgewise additions to the terminal ends. The skeletal structure agrees with that of the single-walled cyclostomes of Borg (1926). It is probable that the soft tissues were arranged in the same manner. It seems certain that the interzooidal walls were covered by a secreting epithelium (the inner epithelium) on both sides, whereas the exterior zooidal wall consisted of a cuticle, calcareous layer and a secreting epithelium (the outer epithelium ) on its inner side. Grow ing tips and thin sections indicate that the budding of new zooecia took place in a common bud, sensu Borg. The interzooidal walls of the investigated species lack interzooidal pores. In addition, pseudopores are absent in the exterior walls. An exception to this is, however, the genus Kukersella Toots, which exhibits pseudopore-like structures. Calcareous rods resembling the rods in other groups of Paleozoic tubular Bryozoa have been found in one species (Clonopora gotlandica n. sp.). Terminal diaphragms with aborally flexed laminae were found as well as subterminal diaphragms with orally or aborally flexed laminae in several species. In one species, Sagenella consimilis (Lonsdale), numerous gonozooecia were found. The Paleozoic cyclostomes here investigated are placed in a special suborder, Paleotubuliporina Brood (1973). The species and genera are grouped in four separate families, namely Corynotrypidae n. fam., Clonoporidae n. fam., Kukersellidae n. fam., and Phaceloporidae Ulrich.