Bryozoans: more than meets the eye

By Paul D Taylor

A few years ago a survey was undertaken of the changing proportion of bryozoans relative to other fossils at an Ordovician locality near Cincinnati popular with fossil collectors. The site was revisited annually over a ten-year period, random collections of fossils were made and the numbers of crinoids, trilobites, brachiopods, bryozoans and other fossils were counted. At the beginning of the study, about 25% of the fossils consisted of bryozoans, by the end the proportion had gone up to 75%. The author of the study referred to this phenomenon as “bryo-enhancement”. Of course, the bryozoans had not been increasing in an absolute sense, rather visitors to the site had preferentially collected other fossil groups, causing the proportion of bryozoans left behind to rise.

The Cincinnati study is a fair reflection of the unpopularity of bryozoans among fossil collectors. Fossil bryozoans generally languish unloved among the sponges and trace fossils in the bottom drawers of collectors’ cabinets and are seldom seen for sale in fossil shops or on the Internet. Why should this be? Part of the reason is that few bryozoans have the aesthetic appeal of such fossils as ammonites or trilobites – well at least to the naked eye.

They are also difficult to identify and all too often mistaken as corals, sponges or algae. But another reason is that bryozoans have a low public profile. Unlike molluscs and crustaceans, you won’t find bryozoans on the menu at fish restaurants, even in the Far East. However, I did once hear of a bryozoan that had been eaten by some Japanese students and found to be tasteless but acceptable when accompanied by soy sauce! To quote from a sentence about bryozoans written by one of my colleagues for a booklet on sorting microfossils:

Their complicated structure and bewildering terminology is such that an awareness of their presence in samples is usually enough”.

This short article aims to take some of the mystery out of bryozoans.

Invertebrate phylum

But what exactly are bryozoans? Bryozoans are a major group – a phylum – of animals. Despite appearances, they are most closely related to brachiopods and molluscs, and distant from corals. Bryozoans are always colonial, each colony comprising anywhere between a few and hundreds of thousands of genetically identical individuals known as zooids. While individual zooids seldom measure more than a millimetre in length, colonies occasionally grow up to a metre in diameter. However, they are more usually between about one and ten centimetres in size.

There are at least 10,000 species of bryozoans living at the present day. The majority inhabit the sea but a few live in freshwater lakes and rivers. Over 400 species have been recorded from British coastal waters. Some of these can be found cast ashore along our beaches, although they are seldom recognized for what they are. Perhaps the commonest bryozoan found in British beach drift is Flustra foliacea, a foliaceous species (that is, it resembles a leaf or leaves) that is often mistaken for seaweed, as reflected in its common name the Horny Wrack. (Wracks are seaweeds.) True seaweeds may be encrusted by the delicate bryozoans Membranipora membranacea and Electra pilosa that are known, respectively, as the Common Seamat and the Hairy Seamat. These species are very unusual in having vernacular names – almost all bryozoans are known only by their scientific names.

Fossil record

Fortunately for palaeontologists, the majority of bryozoans secrete hard, calcareous skeletons that fossilize very well, especially as they mostly consist of calcite rather than the less stable mineral aragonite. The fossil record of bryozoans stretches back to the Early Ordovician. In some strata, bryozoans are so common as to form limestones. A good example is the Pliocene Coralline Crag of Suffolk.

The term ‘Coralline’ is something of a misnomer as corals are rare in the Coralline Crag – the corallines are in fact bryozoans. There are many other stratigraphical units in Britain containing diverse and abundant bryozoan fossils. These include the Silurian Wenlock Limestone of the Welsh Borderlands (however, many apparent bryozoans in the Wenlock Limestone are actually tabulate corals), the Permian Magnesian Limestone of County Durham, the Jurassic Inferior Oolite of Gloucestershire and Dorset and the Cretaceous Chalk, especially in Norfolk and southern England. The Carboniferous Limestone is also an excellent source of bryozoan fossils.

Recognition

How can fossil bryozoans be recognized?

There is no straightforward answer to this question because of the enormous variety of colony shapes found among bryozoans. They range from simple, sheet-like encrustations on shells or rocks, to net-like fronds or small bushy colonies that often break up into their constituent stick-like branches after death. Dome-shaped colonies and branching, runner-like encrusting colonies can also be found. The calcareous skeletons of bryozoan zooids may be box-shaped or tubular.

In most cases, when seen on the colony surface, they have rectangular, rhombic or hexagonal outlines. An aperture is present through which the tentacle crown of the living zooid was protruded during life to feed on plankton. This opening may occupy the entire frontal surface or it may be more restricted, set within a calcareous frontal wall and circular, oval, semi-elliptical in shape.

Other pores and spines are often present on the frontal surface of the zooids and these are important in species identification. Compared with other groups that can be mistaken for bryozoans, the well-defined modular zooids generally permit distinction from sponges that tend to have less organized structures. The millimetric scale of the zooids and lack of septa enable bryozoans to be separated from corals that have larger zooids (corallites) with septa, while the calcareous bryozoan skeleton provides a clear distinction from graptolites whose organic skeletons are usually preserved as black carbonaceous streaks.

Living in a colony

The shape of bryozoan colonies can be a poor indicator of their identity because of convergent evolution – time and time again different groups of bryozoans have evolved almost identical colony shapes reflecting adaptations to particular environments and modes of life. For example, net-like fronds occur in Carboniferous fenestellid bryozoans, abundant in the Carboniferous Limestone, but also in some Cenozoic cyclostome and cheilostome bryozoans found in the Coralline Crag. During life, the nets would have formed plankton filtration structures, each zooid helping to pump water through the holes and capturing food particles as they passed.

Perhaps the most distinctive of all bryozoans is Archimedes, named for its screw-shaped axis looking similar to a water pump invented by the Greek philosopher Archimedes. Complete Archimedes colonies have a continuous net attached to the flanges of the screw but this often breaks off during fossilization leaving the robust calcite screw.

Lunulite bryozoans are unusual in being free-living. Instead of anchoring to a heavy substrate, lunulite colonies develop on tiny shell fragments or sand grains, rapidly outgrowing them and coming to rest freely on the seabed. Colonies are generally less than 1cm in diameter and shaped like a Chinese peasant’s hat. Observations of living lunulites have shown that they are able to dig themselves out of the sand or silt if buried. Some species can even walk around using coordinated movements of hair-like appendages.

When fragile lunulite colonies are broken, the fragments can regenerate and grow into complete colonies. This form of asexual propagation predominates in some lunulite populations and can be recognized by the wedge-shaped remains of the parent colony visible on the underside of the new colony. In other cases, lunulite colonies are the products of sexual reproduction and begin life from a larva like other bryozoans.

The common Chalk echinoid Echinocorys is a feature of many fossil collections. Look at almost any specimen of Echinocorys that hasn’t been cleaned too vigorously and you will find encrusting bryozoan colonies on the surface. Sometimes, two such colonies will have grown next to one another. When this happens, they compete for living space, the dominant competitor overgrowing the weaker one. Such observations can provide rare instances of competition frozen in the fossil record. Palaeontologists are able to study overgrowths to work out how different bryozoans have competed over millions of years of geological time and gain insight into the evolution of competition.

Microscopic structure

To appreciate the fine structure of bryozoans, it is necessary to study them under a microscope. Binocular microscopes are generally adequate for species identification, but a scanning electron microscope is needed to see bryozoans in their full glory. The delicate micro-engineering of their calcareous skeletons, especially the tessellated patterns of their interlocked zooids, is astonishing.

Bryozoans are remarkable in having zooids specialized to perform different functions, leading to polymorphism. Colonies may contain feeding, defensive and reproductive zooids that look totally different from one another even though they are genetically identical. The hair-like appendages of lunulites are borne by polymorphic zooids called “vibracula”. Polymorphic zooids that brooded embryos are also often present. These can be easily recognized in fossils using a microscope, allowing us to diagnose ancient pregnancies.

Sometimes brooding zooids are large, bulbous structures in which tens of embryos were housed prior to being released as larvae to swim away and found new colonies. In other cases, the embryos are brooded one at a time in small, hood-shaped structures. Another type of zooid called an “avicularium” is equipped with a beak-like mandible used to restrain would-be predators.

Next time you’re out in the field collecting fossils, don’t ignore the bryozoans. You may discover branches of erect colonies or entire encrusting colonies attached to the surface of oysters and other fossils. Take them home, carefully clean them (a soft toothbrush is ideal), and get hold of a binocular microscope to observe their fine structure. You may be surprised with what you can observe.

Further reading

Pliocene bryozoans in the Suffolk Coralline Crag, by Paul D Taylor and Rory Milne

References

Bock, P. E. The Bryozoa Home Page: http://www.bryozoa.net/

Taylor, P.D. & Lewis, D.N., 2005. Fossil Invertebrates. The Natural History Museum, London, 208 pp.