CONSTRUCTIONAL MORPHOLOGY OF BLASTOID FEEDING

The architect of constructional morphology, Adolph Seilacher, wrote frequently on the historically constrained design of the blastoid theca and feeding structures compared to crinoids. Although feeding posture for rheophillic blastoids is modeled on modern stemmed crinoids, the two groups have fundamental differences. Blastoids elevated their theca using an articulated stem as do crinoids. Blastoids were not able to produce a large, branching planar filtration fan as do crinoids because they had incorporated the ambulacral system into the theca. The brachioles, containing the podia, were limited in length and were unable to branch. Seilacher viewed the brachiolar feeding system of blastoids as an evolutionary handicap resulting from the ambulacra’s integral role in respiration through internal hydrospires. Seilacher reached a number of conclusions about the historical contraints and mistakes of blastoid theca design and the contingent nature of evolution. For the past three years, our group has been modeling blastoid feeding and thecal design using computational fluid dynamics (CFD) allowing us to assess some of Seilacher’s hypotheses. The blastoid feeding structure is a three dimensional array of brachioles that are approximately equal to thecal length. The thickness of the filtration system is a function of ambulacral length, which varies greatly with thecal morphology. Some spherical blastoids more closely resembled dandelions than crinoids. Although different in structure, blastoid and crinoid feeding structures produce similar adoral vortices aiding food capture. Blastoid thecal profiles vary from circular to ellipsoidal to cone shaped. These theca produce different feeding structures, and have a range of hydrodynamic efficiencies suggesting that different thecal designs were optimized for specific ranges of current velocities. Subtle changes in thecal construction produced significant changes in hydrodynamics similar to the benefit of winglets added to aircraft wings. Although historically constrained in feeding by its ambulacral system, blastoids were able to adapt to disruptive Paleozoic patterns of planktonic food supply, ecosystem collapse, and climate change dominating echinoderm communities during times of crinoid community reorganization.