March 6. Cystoids

As I discussed yesterday, all of the modern phyla of animals except one were established during or before the Cambrian Explosion. Today let’s talk a bit about some of the variations that came about during the Ordovician diversification.

Cystoids

Echinoderms are known today in starfish and sea urchins, as well as sand dollars and sea cucumbers. They often, but not always, have a five-fold symmetry that gives them the common star shapes. There are more than 7,000 living echinoderm species. And today, at least, they only live in marine waters – there are no freshwater or land-dwelling echinoderms.

Pleurocystis filitextus,
Ordovician cystoid

The fossil record of echinoderms includes a great many varieties, some of which are extinct. One of those extinct branches of the echinoderms is the cystoids, which only lived from the Middle Ordovician, about 465 million years ago, until the extinction event at the end of the Devonian, about 360 million years ago.

Cystoids look like crinoids – which are nearly extinct but probably more familiar. Both look like plants, rooted to the sea floor, with a clear stalk and feathery arms like long petals at the top, so that crinoids especially are sometimes called sea lilies, but they are actually animals. Cystoids differ from crinoids in having triangular pores in their rigid calcareous skeletons – they are like sponges in that way – and their body forms often have a cruder expression of that five-fold symmetry than crinoids, but there are exceptions to that on both sides.

The name “cystoid” means sack-like, and that’s what their bodies typically were. Cystoids are sometimes called Lazarus animals because they seem to come and go over the time they were around. It isn’t likely that they became completely extinct and then miraculously reappeared, but it does seem that they were decimated severely by things like the end Ordovician extinction, only to reappear in good numbers in the Silurian Period. But they were definitely done for at the end of the Devonian.

—Richard I. Gibson

Left image from an old geology textbook (public domain); right image from Ernst Haeckel's Kunstformen der Natur (1904; public domain, via Wikipedia)