CHAETETID GROWTH FORM AND ITS CONTROLLING FACTORS

Chaetetid sponge morphology is examined to provide details on growth styles and their controlling factors. Chaetetid growth forms range from laminar to domical, bulbous, columnar and complex branching in a variety of sizes. The chaetetid skeleton began as a laminar unit comprising growth of many calicles across the substrate at the same time. Several styles of early growth, involving differential calicle growth rates and varying directions of adjacent calicle growth, are recognized, and result in complex arrangements of calicles in the skeleton. Despite this, the cross-sectional profile of the gross morphology at any stage of growth is usually a simple outline, implying that internal complexities of calicle development are modulated to produce an optimum cross-sectional outline for the individual chaetetid. The morphological range of chaetetids is similar to stromatoporoids, some tabulate, heliolitid and colonial rugose corals, some bryozoans, stromatolites, encrusting foraminifera and calcareous algae; the common environmental controlling factors of sedimentation and turbulence profoundly influenced growth form in all these organisms by virtue of their common sessile shallow marine habit. Chaetetid growth forms show a general relationship to the environment; columnar and branching forms grew in quiet water, while laminar and domical were better adapted to environments of higher energies. The environmental adaptations of laminar forms, however, remain problematic, because they are found in both high and low energy facies, and interpretation depends strongly on facies study. Also, interpretation of all growth forms is suspected to relate to taxonomic aspects, as has been recognized for other groups. Unfortunately, chaetetid taxonomy is in need of revision, and at present no certain relationship has been demonstrated between taxonomy and growth form. Some modern calcareous sponges with a chaetetiform architecture also show similarities in growth form to fossil types, and may be subject to similar controls.