Fenestella and other bryozoans in the Carboniferous rocks of the British Isles

Paul D Taylor (UK)

Ask a geologist to name a fossil bryozoan found in the rocks of the British Isles and the most likely answer will be Fenestella. The net-like fossils of Fenestella are especially abundant in the Carboniferous Limestone (Figs 1 and 2), although the genus, as used in its broadest sense, is also present in the Silurian, Devonian and Permian deposits of Britain.

While Fenestella dominates almost all bryozoan assemblages found in the British Carboniferous, a variety of other bryozoans are commonly found. Some Carboniferous bryozoans inhabited reefs or mounds, others were components of non-reef marine communities where they lived together with brachiopods, crinoids and corals at a time when the British Isles was situated close to the equator. All Carboniferous bryozoans constructed immobile colonies consisting of numerous individual zooids, with crowns of tentacles used to capture tiny planktonic algae floating in the water around.

Our knowledge of the diversity of Carboniferous bryozoans in the British Isles has increased enormously during the last 50 years through the studies of David E Owen, Ron Tavener-Smith, Adrian J Bancroft and Patrick N Wyse Jackson. Yet, and in common with bryozoans from other geological periods, Carboniferous bryozoans are too often perceived by non-specialists as a closed book. The aim of this article is to introduce some of the commoner and more interesting bryozoans that can be found in the Carboniferous rocks of the British Isles. Recognising these bryozoans and understanding how they lived adds scientific insights and interest when collecting or observing Carboniferous bryozoans.

Fenestella

Worldwide, almost 800 species have at one time or another been assigned to Fenestella, making it the bryozoan genus containing the greatest number of species. Taxonomists become very uncomfortable when a single genus harbours so many species and seek ways of splitting these ‘bloated’ genera. Such has been the case for Fenestella with the introduction of genera such as Fabifenestella, Laxifenestella and Rectifenestella.

Unfortunately, most of the new genera can only be recognised after making thin sections, as they are defined by differences in the geometrical shapes of the living chambers of the zooids not evident on the surfaces of the colonies. Where sections are unavailable and chamber shape cannot be determined, the best option is to use the genus name informally by placing it in quotes (that is, ‘Fenestella’), or with the suffix ‘s.l.’ (that is, Fenestella s.l.) to indicate sensu lato meaning in the broad sense.

The name Fenestella means ‘small window’. The windows referred to are the holes of the net (Figs 1 and 2). These are the gaps, known technically as fenestrules, between the meshwork formed by the branches and the cross-linkages (dissepiments) between them. When complete, the meshwork colonies had the form of flat, gently curved or folded fans, or cones. As the meshworks broadened during colony growth, branches occasionally bifurcated to fill the extra space available.

One surface – the obverse – of each branch bears two rows of zooidal apertures (Fig. 3), one row on each side of the branch midline, which is often raised into a keel or marked by a line of nodes. The branch-linking dissepiments are solid skeletal structures. Frustratingly, obverse surfaces of Fenestella branches showing the apertures and other features commonly used in species identification are far less often seen than branch reverse surfaces. This is because, when rocks containing Fenestella are split, the obverse surfaces usually adhere to the rock, as cemented sediment that entered into the zooidal chambers through the apertures acts to ‘lock’ the obverse surface onto the rock. In contrast, the smooth reverse surfaces form planes along which the rock splits much more readily.

The apertures on the obverse surfaces of Fenestella s.l. colonies are circular and approximately 0.1mm in diameter. During life, an inverted cone-shaped tentacle crown, with about 8 to 16 individual tentacles depending on species, would have emerged from each aperture, retracting rapidly into the safety of the box-like zooidal chamber when danger threatened. The arrangement of the apertures along the sloping flanks of the branches meant that the tentacle crowns projected into the fenestrules.

As with living bryozoans sharing the same colony-form as Fenestella s.l., seawater would have been drawn towards the zooids on the frontal surface of the colony and plankton filtered out by the tentacle crowns as it passed through the fenestrules, with depleted water exiting on the reverse surface of the meshwork (see Suárez Andrés and Wyse Jackson, 2015). In the case of colonies with conical colonies, the zooid-bearing obverse surface is normally on the outside. Thus, water was sucked through the fenestrules from the exterior of the cone, converged on the interior after filtration, and was expelled from the open top end of the cone. Fenestella s.l. colonies of all shapes functioned as highly efficient filtration fans.

Fenestella s.l., and indeed nearly all of the bryozoans you are likely to find in the British Carboniferous, belong to the subclass Palaeostomata. The calcite skeletons of palaeostomate bryozoans were enveloped during life by soft tissues, which included the epithelia responsible for secreting the mineralised skeleton. In this sense, growth of the palaeostomate skeleton was more like an echinoderm than a brachiopod, as these bryozoans were capable of adding new layers of skeleton and new skeletal structures on the exteriors of their colonies.

In Fenestella s.l., the branches often became thickened on their reverse surfaces to reinforce the meshwork against breakage. Near the bases of colonies, skeletal strengthening occurred on frontal surfaces too, with layers of calcification sealing the apertures of old zooids no longer able to feed. It is worth noting that colony bases, which would have been attached to a solid or firm substrate, are seldom found: the great majority of fossils comprise meshworks broken-off from their bases.

The ability to develop spines is another consequence of being able to calcify new skeletal structures on the outer surfaces of Fenestella s.l. colonies. Often, the spines of Fenestella s.l. are barbed, bearing small, backward-pointing spinelets (Fig. 4). The function of these barbed spines could have been in defending against predators, but it is more probable that they were used as grappling devices to hook colonies to neighbouring plants and animals for extra support.

Another kind of outgrowth found in Fenestella s.l. was initially described as a separate genus called Palaeocoryne in the mistaken belief that it was a hydroid cnidarian living symbiotically with the bryozoan (Fig. 5). The microstructure of Palaeocoryne leaves no doubt that it actually represents part of the bryozoan skeleton akin to the spines mentioned above.

‘Palaeocoryne’ outgrowths are found only on the reverse sides of a few species of Fenestella s.l. A short stem bears 8 to 12 radiating spines growing in a plane roughly parallel to the branches of the colony (Bancroft, 1988). Rarely, the spines bifurcate and anastomose. The ‘Palaeocoryne’ structures occur sporadically and are lacking in the great majority of specimens. Their growth is likely to have been triggered by an external stimulus.

Relatives of Fenestella s.l.

Fenestella belongs to the family Fenestellidae (informally ‘fenestellid’), which in turn belongs to the order Fenestrata (informally ‘fenestrate’). Having so many taxa beginning with ‘Fene-’ can be confusing, all the more so when another use of the term fenestrate for bryozoans with reticulate colonies belonging to any bryozoan group is added to make matters more complicated. For example, the fenestrate colony-form can be found also among living cheilostome and cyclostome bryozoans, providing an example of convergent evolution and reflecting the value of this particular type of colony in efficient suspension feeding.

Returning to Carboniferous bryozoans, other genera belonging to the order Fenestrata are quite common in the British Isles. Hemitrypa is closely similar to Fenestella s.l. in having two rows of apertures along branches linked by dissepiments, but differs in possessing a curious ‘superstructure’ positioned slightly above the obverse surface of the colony (Bancroft, 1986). The superstructure of Hemitrypa takes the form of a hexagonal grid (Fig. 6), each hole in the grid positioned above an aperture on the meshwork of branches below.

There can be little doubt that the superstructure would have strengthened the colony, but it may also have performed a protective function in shielding the lophophores that opened into the space between the superstructure and the meshwork. Colonies of Hemitrypa from Early Carboniferous reefs are sometimes acutely conical, resembling some sponges in overall shape (Fig. 7). Like these sponges, they would have fed on suspended food particles in seawater sucked from the exterior to the interior of the cone and expelled through the open top.

The meshwork of Polypora (Fig. 8) resembles Fenestella s.l. but close inspection of the branches shows them to have more than two rows of apertures on their obverse surface and be correspondingly broader and without a central keel. The two genera are frequently found together (Fig. 9), evidently prospering in the same habitats.

Another genus placed in the order Fenestrata, Ptylopora, is much less common in the British Carboniferous. Ptylopora colonies have a central stem from which a Fenestella-like meshwork diverges on either side (Fig. 10). Fragments of the meshwork detached from the central stem of Ptylopora could easily be mistaken for Fenestella s.l., but generally lack or have few branch bifurcations.

The central stem is thickly calcified and robust, possibly allowing Ptylopora to live in higher energy environments with stronger currents, although this was not always the case to judge from a study of the environments where this genus and other fenestrates lived during the Carboniferous in the south-eastern USA (McKinney and Gault, 1980). Recalling modern sea-fans (gorgonian cnidarians), colonies of Ptylopora were likely oriented at right angles to the prevailing direction of environmental current flow, as this would facilitate the passage of plankton-laden water through the fenestrules.

Fragments of delicate branches of the fenestrate genus Penniretepora can be quite common in the British Carboniferous. Unlike Fenestella s.l. and the other genera mentioned above, Penniretepora lacks dissepiments and does not form a meshwork but is instead ‘pinnate’, with a series of parallel secondary branches diverging from opposite sides of a slightly thicker primary branch (Fig. 11), recalling the geometry of veins on a leaf.

Both the primary and secondary branches have two rows of apertures along each side of the obverse surface, as in Fenestella s.l., whereas the reverse surface is often striated. The fragility of Penniretepora compared with other genera suggests that colonies were better adapted to quieter water environments and sheltered habitats.

Other bryozoans

While fenestrates are the most common bryozoans to be found in the Carboniferous of the British Isles, genera belonging to some other orders of palaeostomates are also present. Ramose colonies with apertures opening all around the circumferences of the branches can be found in some trepostomes as well as rhabdomesine cryptostomes. Trepostomes generally have thicker branches than rhabdomesines, with diameters exceeding 2mm (Fig. 12), and the arrangement of the zooidal apertures is less orderly (Fig. 13). Several genera of ramose trepostomes can be found in the British Carboniferous, notably Stenopora, Tabulipora, Stenophragmidium, Leioclema and Stenodiscus, but thin sections revealing internal features must be prepared to distinguish between them, as they can look very similar on the surface.

Short spines or tubercles often surround the zooidal apertures of both trepostomes and rhabdomesines (Fig. 14). These are known in bryozoan parlance as ‘styles’ and are the surface expressions of cone-in-cone laminations within the skeletal walls of the zooids. They probably acted like miniature rivets in strengthening the skeleton, as well as supporting the soft tissues investing the colony.

The two commonest rhabdomesine genera found in the British Carboniferous are Rhombopora and Rhabdomeson. Both genera frequently have straggly, sparsely branched colonies (Fig. 15). New branches may be formed at the growing tips of the colonies by the equal division of a parent branch, or less often arise in more basal parts of colonies as lateral branches oriented at right angles to the parent branch. Some lateral branches would have formed when colonies were toppled over to re-establish upward growth away from the sediment surface.

Unlike Rhombopora, Rhabdomeson is unusual in having a narrow axial cylinder running along the centre of the branches (Fig. 16). An early interpretation of the axial cylinder considered it to represent the location of a thin substrate around which the Rhabdomeson branches grew, for example, an algal thread. However, the finding of occasional cross-partitions dividing the axial cylinder makes this idea untenable (Wyse Jackson and Bancroft, 1995), and the axial canal seems instead to have developed as an integral part of branch growth, the outer surface functioning as the site for the budding of new zooids.

Bryozoans with erect colonies dominate British Carboniferous fossil assemblages. However, encrusting bryozoans can sometimes be found growing on brachiopods, corals, crinoids and erect bryozoans. Some of these encrusters are trepostomes, but these are generally outnumbered by Fistulipora (Fig. 17) and Eridopora, both of which belong to another bryozoan order called the cystoporates. This order takes its name from the presence of vesicle-like cystopores between the zooids, best seen in thin sections. Whereas trepostomes have polygonal apertures, cystoporates tend to have more rounded apertures, sometimes with a cowl-like structure called a lunarium partly overlapping one side.

Some notable localities

Bryozoans can be found widely in marine rocks of the British Lower Carboniferous, from the Mendips into South Wales, North Wales and the Pennines, the Midland Valley of Scotland and across broad swathes of Ireland. They occur in limestones, as well as in associated shales. The collections of the Natural History Museum, London contain particularly notable bryozoans from Halkyn in Flintshire, Ravenstonedale in Cumbria, and Carrick Lough in County Fermanagh. Bryozoan faunas from these localities differ in the species present, their preservation and palaeoecology.

Bryozoans from the Brigantian Halkyn Shale are mostly fenestellids (Figs 1 and 18). Although colonies are somewhat crushed, their white colour stands out against the black shale matrix making them particularly photogenic. No palaeoecological research has been undertaken on these bryozoans, which were first described by George W Shrubsole in the nineteenth century. However, as the colonies are relatively intact, they are unlikely to have been transported, even though the black shale suggests low oxygen conditions on the seafloor unfavourable for bryozoans. The solution to this dilemma may be that the bryozoan colonies lived attached to arborescent animals and plants that elevated them into faster flowing and better oxygenated water significantly above the seafloor. Such is the case for some bryozoans living today.

Treak Cliff near Castleton in Derbyshire is famed for its geology (see Ford, 1996). Among the features to be seen here is an Early Carboniferous (Asbian) apron reef complex containing bryozoans (Owen, 1966). As with so many reefs in the Carboniferous, including the Waulsortian Reefs of Belgium and Ireland, this reef lacks obvious reef-builders and has been classified as a mud mound. Fenestellid and other bryozoans are common components. The fenestellids may have played some part in mud mound growth, for example, acting as current baffles that promoted the deposition of fine particles of sediment.

The Lower Carboniferous of the region between Ravenstonedale and Shap in Cumbria consists of a sequence of sandstones, mudstones and limestones deposited in shallow water close to the shoreline. Well-preserved fenestellid, trepostome and other bryozoans can be found in some of the mud-rich levels, such as those developed in the Ashfell Sandstone (Arundian) and overlying Ashfell Limestone (Holkerian). The striking bedding plane assemblage illustrated in Fig. 19 is strewn with fragments of Fenestella s.l. belonging to several species distinguished by their different meshwork sizes, together with long branches of trepostomes and shells of spiriferid brachiopods.

A rich fauna of silicified bryozoans is revealed by acid dissolution of the Glencar Limestone from Carrick Lough in Northern Ireland (Fig. 20). Almost 70 species have been recorded from this Asbian deposit, including 26 species of Fenestella s.l. (Tavener-Smith, 1973; Wyse Jackson, 1996). While the preservation of these delicate silicified fossils can be exquisite (Figs 3, 4 and 6), the fragmentary condition of the colonies has led to the suggestion that the bryozoans were transported downslope to their site of burial from a shallower water environment.

These are just a few of many places where Fenestella s.l. and other Carboniferous bryozoans can be found in the British Isles. Wherever marine Lower Carboniferous rocks outcrop, there is a good chance of finding and collecting bryozoans. In contrast, the British Upper Carboniferous, including the Coal Measures, contains very few bryozoans, which is not surprising given the predominantly non-marine origin of these deposits.

Unless you have the opportunity of making thin sections, it is not always possible to identify the species or genera. But you can try to imagine how these colonial animals lived and their roles in the ecology of seabed communities, capturing phytoplankton, forming habitats for other animals and doubtless the foodstuff of others.

References

Bancroft, A.J. 1986. The Carboniferous fenestrate bryozoan Hemitrypa hibernica M’Coy. Irish Journal of Earth Sciences 7: 111–124.

Bancroft, A.J. 1988. Palaeocorynid-type appendages in Upper Palaeozoic fenestellid Bryozoa. Palaeontology 31: 665–675.

Ford, T. D. 1996. The Castleton Area, Derbyshire. Geologists’ Association Guide No. 56, 93 pp.

Owen, D. E. 1966. New Carboniferous Polyzoa from Derbyshire. Geological Journal 5: 135–148.

Suárez Andrés, J.L. & Wyse Jackson, P.N. 2015. Feeding currents: a limiting factor for disparity of Palaeozoic fenestrate bryozoans. Palaeogeography, Palaeoclimatology, Palaeoecology 433: 219–232.

Tavener-Smith, R. 1973. Fenestrate Bryozoa from the Viséan of County Fermanagh, Ireland. Bulletin of the British Museum (Natural History) (Geology Series) 23: 391–493.

Wyse Jackson, P.N. 1996. Bryozoa from the Lower Carboniferous (Visean) of County Fermanagh, Ireland. Bulletin of the Natural History Museum (Geology Series) 52: 119–171.

Wyse Jackson, P.N. & Bancroft, A.J. 1995.Generic revision of the cryptostome bryozoan Rhabdomeson Young & Young, 1874, with descriptions of two species from the Carboniferous of the British Isles. Journal of Paleontology 69: 28–45.