March 13. Bryozoans

Three or four times now, I’ve mentioned that all the modern phyla of animals were established during the Cambrian, except one. It’s time to talk about the one, the bryozoans. They began during the Ordovician, so far as we know.

Ordovician bryozoa

If you saw a bryozoan fossil, you might think it was a relatively delicate kind of coral. They have lots of diverse appearances, like corals, and their colonies are usually calcareous, made of calcium carbonate, like corals, though some bryozoans apparently had phosphatic colonies. Some bryozoans are stubby little pillars, some are lacy branches, and others are tubular, branched like staghorns, or form tiny thin hair-like crusts on other fossils such as brachiopods.

Some bryozoans that grow into flat, fan-like branches with numerous small rectangular openings in the overall structure are called fenestrate bryozoans, for the window-like openings in the colony. Fenestrate means window-like. 

Encrusting types are probably the most common, at least in living species. and there are plenty in the fossil record as well. They grow on other animals, on rocks, on modern seaweed, and one colony might have two million or more individual zooids cooperating to make a colonial structure half a meter long. These encrustations sometimes look like moss, and modern bryozoans are sometimes called “moss animals” for that reason. They were encrusters pretty much from their beginning in the Ordovician, and they are encrusters today – to the extent that they can become nuisances on ships’ hulls and dock pilings.

What are they? They do still exist – more than 4,000 modern species are known, along with 15,000 fossil species – and they are colonial animals, like corals or graptolites, meaning that the individual zooids can’t survive on their own, away from the colony. The zooids are tiny, maybe a half millimeter long, but the whole colony can be many centimeters across, even a meter in some varieties. They are filter feeders, taking nutrients out of the water the live in, and they make that happen with the help of lophophores – the tentacle-like features that make brachiopods different from clams and other mollusks. 

The oldest mineralized colonial bryozoan known is from the Lower Ordovician. Since everything else got started in the Cambrian or earlier, there’s been an intense search for Cambrian bryozoa. One candidate, described from the Upper Cambrian of Mexico in 2010, was thought to fill the bill, but it has since, just last year, 2013, been reclassified as a kind of coral. You can imagine that it’s a little challenging to be certain about these things, when the fossil remains – the structure that held the colony – might not include anything of the original animals, the zooids. It’s not as if lophophores, soft structures a tiny fraction of a millimeter long, are easy to preserve for 460 million years, but many of the modern orders of bryozoa were established by that time in the Ordovician.

It is likely, though, that bryozoans did exist during the Cambrian – but that they were late to the game of secreting calcium carbonate to make a hard skeleton, the colony. Why were they the only phylum of animals that didn’t figure out how to do that during the Cambrian explosion? There’s an enigma waiting for someone to explore.

Bryozoans contain interesting chemicals – some that cause serious skin diseases in fishermen, as well as some that show potential against Alzheimer’s disease.

—Richard I. Gibson

Photo by Mark Wilson via Wikipedia (public domain)

Further Reading
Cambrian bryozoans? Not yet.