BOTTOM DWELLING BLASTOIDS

Blastoids are an extinct class of blastozoan echinoderms ranging in age from the Upper Ordovician to the Upper Permian. Most blastoids are pelmatozoans and were mid-tier filter feeders attached by a stem of approximately 10 cm. At various times during their geologic history,however, blastoids moved into the lower tier near or at the sediment-water interface by developing a bottom-dwelling asymmetrical stemless thecal morphotype or a diminutive stemmed morphology. Eleutherocrinu, a hyperoblastoid, is oldest such example and is known from the Eifelian of Bolivia and the Givetian of eastern North America. Tripoblastus and Astrocrinus are both stemless fissiculates known respectively from the Late Famennian of China and the Late Visean of the British Isles. Both time intervals contain additional genera of diminutive stemmed blastoids in addition to typically-sized taxa. The occurrences of these bottom-dwelling morphologies are associated with changes in blastoid diversity and the evolution of crinoid communities in the Late Devonian, Lower Carboniferous, and Upper Carboniferous and may represent paedomorphic expansion into new niches in the presence of increasing echinoderm diversity.