Asexual colony multiplication by fragmentation: an important mode of genet longevity in the Carboniferous bryozoan Archimedes

Abstract

Bases of attachment of Archimedes colonies are very rare, but many colonies show evidence of origination from a pre-existing set of branches. The evidence consists of spiralled supports (axial screws) originating from fragments of pre-existing sets of branches, proximal taper of colonies or axial screws to a diameter smaller than that of bases of attachment, early stages of new colonies originating from the whorl margins of older colonies, and sets of laterally fused colonies. In the most favorable environments Archimedes populations were dominated by a few genets that consisted of hundreds to perhaps thousands of genetically identical colonies.The ability to reproduce by colony fragmentation was probably an important cause of Archimedes' abundance in the Chesterian seas of eastern North America, especially where it occurred in dense populations in the lee of submarine carbonate shoals. Colony fragmentation, the importance of which is becoming increasingly recognized in modern corals and bryozoans, was also of importance in Paleozoic seas.