FUNCTIONAL-SIGNIFICANCE OF REGULAR ARCHAEOCYATHAN CENTRAL CAVITY DIAMETER - A BIOMECHANICAL AND PALEOECOLOGICAL TEST

Theoretical and experimental biomechanical approaches are used to test the effect regular archaeocyathan central cavity diameter has on the generation of passive now through the skeleton. These results are then used to predict a correspondence between gross morphology and paleoen vironmental occurrence. Previous work has demonstrated that regular ar chaeocyathan morphology generates passive flow, via Bernoulli and visc ous entrainment effects, through its porous walls for suspension feedi ng, a phenomenon that occurs in modern sponges. Efficacy of entrainmen t depends upon the area of the excurrent pore (i.e., central cavity) o ver which the ambient flow is moving. Consequently, archaeocyaths shou ld have maximized their central cavity diameters. Five-centimeter-long . conical and cylindrical acrylic pipes with varying end diameters wer e tested in a flume to document the relative effects of Bernoulli and viscous entrainment. Each pipe was oriented perpendicular to the now d irection in a uniform now held, and fluorescein dye was injected at th e pipe's mid-length for flow visualization. Models with different-size d apertures consistently exhibit dye movement to the larger opening an d greater dye entrainment speeds than models with identically sized ap ertures, thereby suggesting that viscous entrainment effects are signi ficant and operating in concert with Bernoulli effects. To test for si milar effects in archaeocyaths, four brass models were constructed wit h varying central cavity diameters. Both volume flux and excurrent flo w speed of the exiting water increased as the central cavity diameter increased. An analysis of the morphologies that occur in nature confir m these results. Regular archaeocyaths most commonly have central cavi ty diameters close to their outer wall diameter, thereby maximizing th e excurrent pore area.These results have implications for archaeocyath an paleoecology. Environments with low-magnitude currents should suppo rt individuals with larger central cavity diameters than higher energy settings. Data on the occurrence of morphotypes within bioherms of va rying flow energies from South Australia support this prediction.