Wednesday, September 1, 2010
Fossil of the Week
9/1 – Blastoids
Blastoids are extinct echinoderms (related to living seastars, sea urchins, sea cucumbers, and crinoids) that are quite common fossils. Although not as well known as crinoids*, they share many similarities in life style and history. Blastoids appeared during the Ordovician Period and disappeared during the end-Permian extinction, having reached their peak during the Lower Carboniferous (approx. 350 million years ago). Like other echinoderms, they have radial symmetry, which is particularly obvious in the specimen pictured here on the right in which five ridges radiate away from the mouth.
The part of the organism that is pictured here is called the “theca” and is made up of a series of tightly interlocking calcium carbonate plates. This makes the theca quite robust, so blastoids have a good fossil record and are common as fossils in rocks of the right age and environment. Blastoids are often described as looking like fossilized hickory nuts, but the thecae vary in shape and size from fairly globular, like these specimens – Pentremites godani from the Mississippian of the Paint Creek Formation, St. Clair County, Illinois, PRI Acc. #1493 – to more elongated or much more angular.
Like crinoids, living blastoids had a stalk that was attached to the ocean floor by a holdfast. The stalk held the theca above the sediment, allowing the organism to feed. The stalks were not as tough as the thecae, and they fell apart more easily after death, so fossilized thecae are usually found without stalks. You can see in the specimen on the left where the stalk would have been attached to the theca at the bottom. Blastoids fed on plankton that they filtered from the water using delicate structures called “brachioles” that are rarely preserved. The brachioles radiated from the five “ambulacra” that are clearly visible on these specimens – they form the ridges that radiate away from the top of the specimen on the right and can be seen to truncate before reaching the bottom end of the theca on the left.
Text by Ursula Smith (reprinted from “Fossil Focus” in American Paleontologist, Spring 2010)
*See Fossil of the Week 4/29/09 - Crinoids.