Abstract

In many species of lower Paleozoic trepostomes (Bryozoa; class Stenolaemata) transverse partitions called skeletal diaphragms differentiated feeding from non-feeding regions of colonies. It has been thought that each diaphragm floored the living chamber of a feeding polypide. However, analysis of skeletal growth patterns has shown that many diaphragms were too close to colony surfaces or too closely spaced in ontogenetic sequences to have accommodated feeding polypides at any given life horizon. Apparently colonies were capable of maintenance and even robust growth with reduced numbers of active polypides, an interpretation supported by comparison with living stenolaemates.

A synthesis of the inferred functions of colonies of the extinct trepostomes with post-Triassic fossil and living stenolaemates suggests that walls of trepostome autozooids grew continuously outward so that living chambers starting from their basal diaphragms ranged from shallow to full-sized on colony surfaces. Under-sized polypides apparently grew with their under-sized living chambers and fed as they regenerated to full size, as in living stenolaemates. Actively feeding colony surfaces included autozooids either having polypides at similar or different stages of polypide regeneration, or fully regenerated. Nonfeeding colony surfaces included autozooids either having degenerated polypides, autozooids with diaphragms too closely spaced to skeletal apertures to have housed polypides, or possibly, autozooids that stopped skeletal growth in proximal regions of some large colonies.

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