FOOD PARTICLE SIZE LIMITATION IN BLASTOIDS: A KEY TO MISSISSIPPIAN DIVERSIFICATION

Blastoids are an extinct class of stalked echinoderms that originated in the Late Ordovician and became extinct during the end-Permian mass extinction. Blastoid diversity remained low until the Devonian Frasnian-Famennian mass extinction. Following this close brush with extinction, blastoid diversity resurged to attain an all-time high (> 40 genera) during the Early Mississippian (Viséan) coinciding with peak crinoid diversity. Thereafter blastoid diversity declined sharply before a final surge in the Permian prior to ultimate extinction. Blastoids utilized numerous bristle-like brachioles bearing presumably ciliated food grooves to collect food particles and convey them to the mouth through increasingly wider but usually covered grooves traversing the five ambulacral tracts of the plated theca. A primary limiting factor on the food particle size utilized by blastoids was the narrow width of the brachiolar food groove (BFG). On the basis of exquisitely preserved specimens, previously published, and our new measurements, the BFG ranged from <100 - <300 µm in width, imposing a maximal width of particles that could be entrained and passed through the brachiolar pore before entering the wider food grooves on the theca. Lacking information on the length of brachioles and stalk, some of the niche partitioning parameters applicable to crinoids cannot be evaluated for blastoids, but available data suggest that blastoids were restricted to very small particles collected by a “bottle-brush” filtration fan that was conserved throughout their history. Although crinoids occupied a broader trophic ecospace, blastoids were restricted to collection of very small particles. The significance of this particle-size limitation may have been realized during the Viséan when a “bloom” of microplankton or bacterioplankton resulting from a global drop in CO₂ provided a rich source of very fine particles that helped fuel this all-time peak of pelmatozoan diversity as suggested by Riding (2009). Blastoids at least as old as the Devonian were well equipped to exploit extremely fine particles, a source of nutrition that received a significant Viséan increase and may have marked an innovation in the spectrum of suspended particles available to suspension feeding marine invertebrates.