Abstract

Rhabdopleura shows several features of skeleton growth that are also seen in graptoloids. The similarities between the growth patterns, in terms of the plan towards which the zooids aimed and of their response to environmental disturbance, are profound. Both demonstrate a high degree of genetic control. The excution of the "blueprint' is facilitated by spatial awareness in the zooids of both groups. An important new observation is that mortality is common amongst zooids in both Rhabdopleura and graptoloids, with new animals taking over tube or thecal building from where the previous zooid left off. Several generations of zooids can inhabit a rhabdopleuran tube and can be demonstrated to have inhabited a graptolite theca. This means that innate senescence was not a major cause of death for graptolite colonies. It also means that all thecae might have been continuously occupied and that the colony could have survived significantly bleak environmental conditions by large-scale zooid mortality followed by regeneration. -from Author