Sprinkle, James Department of Geological Sciences, University of Texas, Austin, Texas.
- Links to Primary Literature
- Additional Readings
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
The content above is only an excerpt.
for your institution. Subscribe
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
to your librarian. Recommend
Let your librarian know about the award-winning gateway to the most trustworthy and accurate scientific information.
AccessScience provides the most accurate and trustworthy scientific information available.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Its dedicated editorial team is led by Sagan Award winner John Rennie. Contributors include more than 9000 highly qualified scientists and 42 Nobel Prize winners.
MORE THAN 8500 articles and Research Reviews covering all major scientific disciplines and encompassing the McGraw-Hill Encyclopedia of Science & Technology and McGraw-Hill Yearbook of Science & Technology
115,000-PLUS definitions from the McGraw-Hill Dictionary of Scientific and Technical Terms
3000 biographies of notable scientific figures
MORE THAN 17,000 downloadable images and animations illustrating key topics
ENGAGING VIDEOS highlighting the life and work of award-winning scientists
SUGGESTIONS FOR FURTHER STUDY and additional readings to guide students to deeper understanding and research
LINKS TO CITABLE LITERATURE help students expand their knowledge using primary sources of information