June 5. Blastoids

First, an update. Some new work seems to have pinned down the cause of one of the multiple mass extinctions during the middle and late Cambrian period. The extinction at 510 to 511 million years ago correlates well with voluminous volcanism in Australia. I’ve put a link on the blog episode for February 28 to an article about this.

Now, back to the Mississippian. It’s June 5, and today’s topic is blastoids. Blastoids are a class of echinoderms, quite similar to crinoids. Crinoids have decreased tremendously from their peak during the Paleozoic, but they’re still with us today. Blastoids however are extinct – they didn’t make it past the Great Dying at the end of the Permian Period. But since they began during the Ordovician, or possibly in the late Cambrian, blastoids had a run of more than 230 million years.

Photo by DanielCD via Wikimedia Commons under GFDL.

Like crinoids, blastoids were animals that had a plant-like stalk and a system of holdfasts, root-like structures that held them to the sea floor. Unlike crinoids, blastoids’ skeletons were held together by solid interlocking calcareous plates. In most crinoids, their bodies were held together by muscular tissues, so after death, crinoids tended to fall apart. Blastoids are often found intact. It also seems that blastoids and crinoids may have had different systems for moving water through their bodies to provide oxygen to the animal.

The pentagonal symmetry typical of echinoderms is usually well displayed by blastoids, and the body fossils often look like a flower bud or some kind of nut. There’s a wide range in size, of course, but some of the most common genus, Pentremites, are on the order of a half inch to an inch long.

Blastoids reached their peak of diversity and numbers during the Mississippian, and in some places, such as the fossil locality known as Pentremites Hollow, near Bloomington, Indiana, they are exceedingly abundant.

—Richard I. Gibson

Photo by DanielCD via Wikimedia Commons under GFDL.