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 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)