Bryozoans

Bryozoans:

These small colonial creatures are familiar to all who have looked closely at a rock pool. Commonly known as moss animals or sea-mats, bryozoans consist of mats, threads or branches of half-millimetre-long tubes, joined together into a fantastic array of colony shapes. Although the particular groups of bryozoans living today were probably absent in the Ordovician, the same basic structure was already there. I say “already” because bryozoans were relative late-comers to the biosphere. They are the only phylum with a mineralised skeleton that did not appear in the Cambrian, although there is a strong suspicion that their early evolution was hidden by the absence of a skeleton at that stage. (There are several groups of bryozoans that appear rather later, but that’s hardly surprising, since they don’t have a mineral skeleton, and only turn up as moulds underneath encrusting oysters and the like.) Whatever their origins, by the Middle Ordovician, they were diversifying rapidly, and appearing in many types of ecosystem around the globe.

Because of the nature of their skeleton, bryozoans appreciate a solid base from which to grow. Once this is found, the colony can grow as a series of threads, an encrusting mat, a fan-shaped mesh of reticulating strands, an undulating curtain or a bush of solid branches. In shallow-water sandstones, large branching colonies were thriving in the area, along with blobs and spheres. The largest colonies found so far from Builth are about 10 cm tall, but there are probably still larger ones out there; however, a more typical size is just a few centimetres, with branches often only a millimetre across. In contrast, in the mudstones of the teretiusculus Zone and Llanfawr, with their soft, muddy sea floors, the majority of species were directly encrusting onto the shells of other creatures. Nautiloids and occasionally trilobite fragments were particularly prone to their attention, and in some cases were completely covered. These types are quite difficult to work with, because their growth was so unconstrained; but several species were probably present at any one time, and their total diversity from the area is probably greatly underestimated.

The composition of the skeleton was probably calcite with a high proportion of magnesium, a substance fairly prone to dissolution. Specimens are correspondingly preserved as moulds, which can give a very misleading appearance to those unfamiliar with them. Distinguishing species is done primarily on the basis of the internal skeletal structure, which is simply not preserved in most cases; you need limestones, and in the Builth Inlier, limestones are rare. Fortunately, at this time bryozoans are rare enough that some attempt at identification can be made, although the material could not really be used to define new species. Although a relatively minor component of the fauna, their specialised lifestyle is worthy of note, and adds an extra dimension to our picture of the local ecosystems.

Many bryozoans in the Ordovician grew as mats over the sediment surface – in Builth, some are found in coarse sandstones and some in teretiusculus Zone mudstones. This life habit is extremely rare today; species that use this strategy are only found in the absence of bioturbating (burrowing or sediment grubbing) organisms. One study found that one particular species was opportunistic, growing very quickly in areas where bioturbators are temporarily absent. As soon as the bioturbators re-occupy the area, of course, the bryozoan colonies are destroyed. In the Ordovician this was clearly not such a problem, showing one way in which Ordovician ecosystems were slightly different from modern ones. Certainly Builth Inlier rocks show little evidence of bioturbation. The near absence of structures that can definitely be assigned to bioturbation does not in itself prove that none was present: the disturbance may have been so complete that the evidence for disturbance was itself destroyed. There are organisms known from the inlier that would probably have burrowed (bivalves, palaeoscolecids) or dug around in the surface of the sea floor (trilobites), but they haven’t left many traces of their activities. The presence of these mat-like bryozoans suggests that, at least in some cases, the apparent absence of bioturbation is genuine.

[2]Batostoma sp. Colony up to 90 mm tall.

[5]Bryozoan? (possibly ptilodictyine cryptostome, or fenestellid), at least 10 mm.

[2]Bryozoan indet. A. Branch diameter ~ 0.8-1.0 mm.

[2]Bryozoan indet. B. Branch diameter ~ 1.5 mm.

[2]Bryozoan indet. C. Branch diameter ~ 0.4 mm.

[5]Bryozoan indet. D. Colony up to ~ 40 mm diameter.

[2]Bryozoan indet. E. Frondose; colony height up to ~ 25 mm.

[2]Bryozoan indet. F. Branch diameter ~ 8 mm.

[4]Bryozoan indet. G. Only known specimen encrusts Selkirkia? tube; colony diameter ~15 mm.

[2]Bryozoan indet. H. Planar, frondose form known only from few small fragments; colony diameter ~2 mm.

[2]Bryozoan indet. I. Single fragment with stellate apertures and small spines between pores; branch diameter approx. 0.8 mm.

[2]Bryozoan indet. J. Possibly domal form with relatively large pores, and distinct spines (acanthostyles?) over surface. Colony diameter 1.5 mm.

[2]Bryozoan indet. K. Dendroid trepostome with orthogonal pores at surface, suddenly deflected along axis in the interior. Branch diameter 1.5 mm.

[2]Bryozoan indet. L. Dendroid form (cystoporayte?) with spaced pores in semi-regular array. Branch diameter 2-3 mm.

[1]Bryozoan indet. M. Dendroid, irregular form with sub-polygonal pores and gradual, irregular change in pore angle internally; branch diameter 1.5-2 mm.

[1]Bryozoan indet. N. Sheet-like, probably erect colony with irregular arrangement of slightly elongated pores. Colony diameter at least 20 mm.

[2]Ctenostome? borings on brachiopod (Macrocoelia), with detail of clearly-preserved zoecia chain.

[2]Cyclostome? indet. sp. A. Branch diameter ~ 0.5 mm.

[2]Cyclostome? indet. sp. B. Known only from one fragment, and internal structure unknown. Branch diameter ~ 0.7 mm.

[2]Fenestellid indet. Colony up to 30 mm tall.

[2,?5]"Monticulipora turnbulli" Name assigned to a specimen on display in the Sedgwick Museum, Cambridge, but so far we haven't been able to find where it came from! Sheet-like, probably erect foliose colony with prominent monticules (mounds), separated by 1-2 mm. In at least some specimens, it is bilaminate, although a similar bryozoan has been found encrusting a hard substrate in the gracilis Biozone. Colony diameter at least 40 mm.