Sprinkle, James Department of Geological Sciences, University of Texas, Austin, Texas.
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A small extinct class of relatively advanced blastozoan echinoderms containing five genera (three named), ranging from the middle Early Ordovician to the early Late Ordovician in eastern Canada; northeastern, eastern, south-central, and western United States; south and north Wales; and near St. Petersburg, western Russia. Parablastoids have a bud-shaped theca or body with well-developed pentameral symmetry. Thecal plates include basals, small or large radials, and sometimes other small plates in the lower theca; large distinctive triangular-to-parabolic deltoids between the ambulacra in the upper theca (see illustration); and small oral plates and an oral crest surrounding and covering the mouth on the summit. Parablastoids have single or multiple slits through the lower deltoids that are connected by internal folds (cataspires) to pores that open between the single ambulacral plates at the edge of each ambulacrum; presumably they were respiratory organs. Short-to-long biserial brachioles were attached to the edges of the five ambulacra and served as the main food-gathering structures; when not feeding, the brachioles folded in to lie against a high T-shaped ambulacral crest in the center of each ambulacrum in the best-known genus. A stem with one-piece columnals attached the theca to the sea floor, suggesting that parablastoids were attached, medium- to high-level, suspension feeders. Some parablastoids are found in bank or reef deposits, suggesting they may have been adapted for rough-water conditions. Although they converged on blastoids in thecal design and way of life, parablastoids had differences in their plating, ambulacra, and respiratory structures that indicate a separate origin and evolutionary history. This justifies assigning parablastoids and blastoids to different blastozoan classes. See also: Blastoidea; Echinodermata; Ordovician
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