WERE BLASTOIDS (ECHINODERMATA) DISASTER TAXA OR JUST REALLY WELL ADAPTED?

Disaster taxa, such as stromatolites, are opportunistic species that increased in dominance after ecological perturbations including mass extinctions. This presentation hypothesizes that blastoid abundance and diversity patterns are consistent with that of disaster taxa. Although the typical blastoid genus is monospecific, geographically and temporally restricted, a few genera are long lived, wide ranging and diverse. Most blastoids occur as relatively minor members of Paleozoic shallow marine communities, but occur in the tens of thousands and locally dominate communities in the Lower Devonian of Spain, the Mississippian of North America and Europe and the Permian of Indonesia. The Mississippian is hailed as the Age of Crinoids and the acme of crinoid (and blastoid) abundance and diversity. Although crinoids and blastoids were part of Lower and Middle Devonian reef communities, their abundance and diversity increased significantly after the catastrophic demise of metazoan reef communities at the Frasnian – Famennian boundary. Recent work in the Central Asian Orogenic Belt has demonstrated a dramatic increase in the echinoderm component of carbonate production within ~100000 years of this event in the Hongguleleng Formation. Echinoderms are a significant, but not dominant, component of this community. The collapse of the global reef ecosystem in the Late Devonian changed the structure of the shallow seas from platforms to ramps allowing echinoderms to dominant carbonate production in the Early and Middle Mississippian. The resulting regional encrinites often produced an unstable substrate which inhibited colonization of brachiopod and bryozoan larvae further enhancing echinoderm dominance. The acme of crinoid and blastoid dominance ended at the beginning of the Serpukhovian as did the regional encrinites. This also marks the re-emergence of metazoan reef communities in the Akiyoshi Limestone in Japan. Within the Mississippian echinoderm communities, blastoid dominance fluctuated, but was most prominent during the transition between the Middle and Upper Paleozoic Crinoid Macroevolutionary Faunas. Blastoids flourished during times of ecological and evolutionary stress within the crinoid communities suggesting they functioned as disaster taxa.