Colony life then and now: Lower Paleozoic trepostomes (500-350 mya) and living cyclostomes: a review

Nearly all zooidal budding in stenolaemates of all ages is done by interior walls of free-walled zooids under outermost cuticular membranes of colonies. Interior walls apparently grow continuously and walls of neighboring zooids grow outwardly at equal rates to establish basic growth habits of colonics. The controlling difference in skeletal growth between trepostomes and living cyclostomes is the presence of interzooidal communication pores in the interior walls of living colonies. Lack of pores in walls of trepostomes required that skeletons above basal walls of colonies were entirely interior, nutrients were distributed through outermost confluent colony-wide body cavities so that growth occurred only on outermost skeletal surfaces, and orificai walls of autozooids were free-walled. As a result, trepostome colonies developed few growth habits but could continue outward growth to form colonies that were typically robust and “stony”’. In contrast, interzooidal communication pores in interior walls of living cyclostomes distribute nutrients throughout colonies permitting skeletal growth on all interior skeletal surfaces and inner surfaces of cuticular membranes to form exterior skeletal walls. Exterior skeletons of living colonies make possible numerous growth habits and commonly halt outward growth of interior walls early so that living colonies typically are relatively delicate. Polypide anatomy of living colonies is unexpectedly varied, complex in function, and seems often to be uncorrelated with skeletal characteristics. Polypides range from relatively short to extremely long in different taxa, grow faster than their skeletal walls, undergo relatively few cycles, and start feeding early and steadily during regeneration so that inward growth from stationary attachment organs requires inward shifting of retractor muscle connections during a cycle of constant feeding. Undersized regenerating polypides in Lower Paleozoic trepostomes also must have started feeding early in each cycle, but in contrast, grew’ away from basal diaphragms at roughly the same rate as their living chamber walls to a uniformly short size, and underwent many cycles indicated by ontogenetic sequences of basal diaphragms in long autozooids. In some trepostomes,. closely spaced skeletal diaphragms in many autozooids were structural diaphragms that were too close to colony surfaces or to each other to have accommodated feeding polypides. Temporarily at least, they were nonfeeding and out-of-phase with actively feeding neighbors, just as many autozooids in living cyclostomes stop feeding when they develop skeletal terminal diaphragms.