Archive for January 16th, 2011

Wooster’s Fossil of the Week: A cystoid (Middle Ordovician of northeastern Estonia)

January 16th, 2011

Fossils don’t get much more spherical than Echinosphaerites aurantium, an extinct creature common in the Early and Middle Ordovician of North America and Europe. These are cystoids, a somewhat informal category of filter-feeding, stalked echinoderms that are relatives of the better known crinoids. My students and I found bucketloads of them in the oil shales of the Baltic country Estonia three years ago. They are like stony golf balls.

A typical cystoid has a sac-like theca forming the bulk of the body. This theca is made of dozens to hundreds of plates of the mineral calcite fitted together like tiles. On one end of the theca is a small stem to attach it to the substrate; the other end has short brachioles, which are filter-feeding arms surrounding a tiny mouth at their base. An anus is present on the side, distinguished by a circlet of special plates.

If you look carefully at the specimen on the left in the above illustration, you’ll see at least two sclerobionts (hard-substrate dwellers) attached to the theca.  The black branching form is a graptolite (like our last Fossil of the Week) called Thallograptus sphaericola (the species name means “sphere dweller”) and the raised disk is a bryozoan.

Every once in awhile the cystoids in Estonia were buried quickly and did not fill with sediment. The hollow space within became a kind of geode with crystals of calcite growing from the thecal plates inward. Each plate is a single crystal of calcite, so the crystals grew syntaxially (maintaining crystallographic continuity). These specimens are spectacular if broken open carefully so they don’t shatter into a thousand sparkles.