Bryozoans in the British Jurassic

Dr Paul D Taylor (UK)

Fossil collectors often overlook, or worse discard, bryozoans. There are several reasons: some bryozoans are small and not easily spotted in the field, others are mistaken for non-descript sponges or algae, while bryozoans cemented to the surfaces of other fossils can be cursed for detracting from the value of the main fossil. But bryozoans are fascinating fossils in their own right and ought not to be ignored.

Bryozoans are a morphologically varied phylum of colonial invertebrates. The myriad of colony-forms they exhibit reflect adaptations that evolved to allow them to prosper as immobile colonial animals living on the seabed and feeding on passing plankton (Taylor, 2020). The majority of the more than 6,000 bryozoan species living today possess resistant skeletons of calcium carbonate, and the calcareous skeletons of fossil bryozoans are abundant globally in rocks ranging back to the Early Ordovician, some 480 million years.

Fossil bryozoans in Britain occur in marine sedimentary rocks from every post-Cambrian geological period except the Triassic. Ordovician bryozoans can be found in the Welsh Borderlands and in southern Scotland, Silurian bryozoans in the West Midlands and Shropshire, Devonian bryozoans in Devon, Carboniferous bryozoans in the Pennines and other places where the Carboniferous Limestone outcrops, and Permian bryozoans in the Magnesian Limestone of northeast England. Previous contributions to Deposits have described bryozoans from the Chalk of Late Cretaceous age (Taylor, 2018) and the Pliocene Coralline Crag of Suffolk (Taylor and Milne, 2009). Here, I focus on British Jurassic bryozoans.

Jurassic bryozoans

Britain is second only to France in the diversity of its Jurassic bryozoan fauna. Globally, approximately 150 species of Jurassic bryozoans have been described, more than a third of which can be found in Britain. A striking characteristic of Jurassic bryozoans is that they are considerably more abundant and diverse in northern and western Europe than they are anywhere else in the world (Taylor and Ernst, 2008).

Nevertheless, there is no monographic publication describing British Jurassic bryozoans, making it difficult for vocational palaeontologists and non-vocational collectors alike to identify them. The only substantial works date from the nineteenth century, when scientists did not have the benefit of scanning electron microscopes to study fine details of these fossils and to make accurate illustrations. Nor did they have sufficient understanding of which features in fossil bryozoan skeletons are important in distinguishing between different genera and species. The most comprehensive reference to aid in the identification of Jurassic bryozoans dates back 50 years, is inadequately illustrated by modern standards, is written in the French language and mainly concerns French bryozoans (Walter, 1970).

Few bryozoans have been found in the Early Jurassic rocks of Britain, or indeed anywhere else in the world: bryozoans took a long time to recover after their collapse during the Late Permian mass extinction. In the Middle Jurassic, however, bryozoans once again became abundant and diverse prior to a global decline in the Late Jurassic. Nearly all Jurassic bryozoans belong to the Order Cyclostomata. In fact, the Jurassic is the only geological period in which cyclostomes are the dominant bryozoan order. The name cyclostome means ‘round mouth’ and the majority of Jurassic cyclostomes do indeed have circular or oval openings in the skeleton through which the tentacle crowns of the individual zooids were protruded during life for capturing planktonic food.

The epicentre for British Jurassic bryozoans is undoubtedly the Cotswolds. Here, bryozoans can be found in limestones of the Inferior Oolite (Aalenian and Bajocian) forming the escarpment above Cheltenham and the Great Oolite (Bathonian) around Bath and across Gloucestershire as far east as Oxford. Further south, bryozoans are locally abundant in the Inferior Oolite of Somerset and Dorset, as well as in parts of the Bathonian, such as the Boueti Bed of Herbury on the Fleet lagoon. The Cornbrash also contains bryozoans, but these are nearly all encrusting species, as are those found in the younger Oxford and Kimmeridge clays. Bryozoans are rare in the limestones of the Corallian, but are moderately common in the Portland Stone deposited at the end of the Jurassic period.

Staying low

The bryozoans most often found in the British Jurassic are species with encrusting colonies (Fig. 1).

Erect species are less common overall but, when present, are more conspicuous as they are generally larger and therefore more noticeable. Encrusting bryozoans commonly occur on the surfaces of brachiopod (Fig. 1B and D) and bivalve shells, especially oysters (Fig. 1C). They are occasionally found growing on irregular echinoids, crinoids (Fig. 1A), corals and ammonites, while sedimentary hardgrounds, limestone clasts and ‘algal’ oncoids may also be covered by bryozoans. Because the small size of encrusting colonies makes them difficult to see in the field, bulk sampling of shells and shell fragments for later washing and microscopic inspection is often the best collecting strategy.

There are three basic types of encrusting colonies found among Jurassic bryozoans: sheets, runners and ribbons. Sheets have tightly packed zooids and no gaps are left in the colonies as they spread across the substrate. Colonies are initially fan-shaped (Fig. 2A), but become almost circular (Fig. 2B) as they continue to grow by adding new zooids around the circumference. The largest colonies sometimes develop overgrowths: older zooids that no longer feed become covered by newly budded, younger zooids.

Most of the sheets in the Jurassic have historically been identified as ‘Berenicea’. This is now regarded as a form-genus, which still has utility when names are needed for specimens that cannot be more precisely identified. The apertures of the zooids through which the tentacle crowns protruded during life are typically 0.10 to 0.15mm in diameter and are arranged evenly across the colony surface.

Two other genera – Theonoa and Kololophos – both less common in the Jurassic than ‘Berenicea’, are distinguished by having the apertures aligned in raised radial rows on the colony surface (Fig. 1B), superficially resembling solitary corals with septa. Colonies having this morphology are believed to have created a specific pattern of water flow for feeding, with water being pulled towards the aperture rows by the tentacle crowns of the zooids, filtered of plankton, and then passed along the valleys towards the centre of the colony for expulsion.

Discriminating between different species of ‘Berenicea’ has been a major challenge. Sometimes, there are clear differences in the sizes of the zooids or the flatness of the colony surface (Fig. 3). A new genus – Rugosopora – was proposed recently for species with the colony surface crossed by transverse ridges (Fig. 3B).

Another key character is the shape of the gonozooids (Fig. 4). These enlarged zooids are used for brooding embryos before their release as larvae that could swim off to found a new colony. Unfortunately, only a minority of Jurassic ‘Berenicea’ colonies developed gonozooids – it may be necessary to collect tens or even hundreds of colonies before an all-important gonozooid is found. Several Jurassic genera belonging to the form-genus ‘Berenicea’ have been proposed based on differently shaped gonozooids, such as Microeciella, Reptomultisparsa, Hyporosopora and Mesonopora.

Runners in the Jurassic are represented by the genus Stomatopora (not to be confused with the sponge Stromatopora, or the microbial structures called stromatolites). Stomatopora has colonies with zooids arranged in single lines, one after the other, to form thin branches that bifurcate periodically (Figs. 1C and 2C). These colonies were like weeds: difficult to destroy completely, fast growing, and adept at populating new surfaces when they became available (for example, the interiors of shells following death of the shell maker). There are no gonozooids in Stomatopora, but differences in size can be used to discriminate between species. The two commonest species in the British Jurassic are Stomatopora bajocensis and Stomatopora dichotomoides, in which the branches are less and more than 0.2mm wide, respectively.

Like runners, ribbons have branching colonies, but here the branches are several zooids wide. This colony-form is best seen in Idmonea triquetra (Fig. 1D), an attractive species found in the Bradford Clay, Boueti Bed and other Bathonian deposits in southern England. The branches of Idmonea triquetra are triangular in cross-section, their basal underside cemented to the substrate and the two upward-facing sides bearing the apertures of the feeding zooids arranged in transverse rows.

Growing erect

Erect bryozoans that grew up from the substrate into bush-like (Figs. 5 and 6B) or frondose (Fig. 7) colonies are found at a smaller number of British Jurassic localities, almost all in the Middle Jurassic.

Although originally reaching 10cm or more in height, these colonies are most often broken into branch fragments a few millimetres long. Two contrasting localities illustrate the occurrence of erect bryozoans – Cleeve Hill in Gloucestershire and Yons Nab in Yorkshire – both notable for exposing beds specifically named for their bryozoan content.

Cleeve Hill is the highest point of the Cotswolds. Close to the summit are some old quarry faces that now form cliffs overlooking the Vale of Severn. These expose the Lower Freestone of the Inferior Oolite (now the Birdlip Limestone Formation), a cross-bedded oolitic limestone. At its base is the Polyzoa Bed, Polyzoa being an old name for Bryozoa.

About 16 species of bryozoans are known from the Polyzoa Bed, which has been interpreted as a stabilised ooid shoal. Half of these bryozoans have erect colonies; some are bushy with cylindrical branches, but most are foliaceous and consist of flattened fronds. Many of the erect colonies have hollow tubular bases indicating growth around the cylindrical stems of unknown animals or plants no longer preserved. Whatever these stems were, they would have held the bryozoan colonies aloft above the seabed into waters where currents flowed more quickly, thus supplying a plentiful supply of planktonic food.

Yons Nab is a narrow rocky promontory exposed during low tide at the northern end of Cayton Bay, between Scarborough and Filey (Fig. 8). The Middle Jurassic succession along the North Yorkshire coast consists mainly of non-marine, fluviatile and deltaic deposits, famous for fossil plants and dinosaur footprints, hence the name ‘Dinosaur Coast’.

However, there are several thin intercalations of marine sediments. Among these is the ‘Millepore Bed’ (Lebberston Member of the Cloughton Formation), a cross-bedded oolitic limestone of Bajocian age best seen at Yons Nab beneath a covering of seaweeds and barnacles. This bed takes its name from a fossil originally described as Millepora straminea by John Phillips in 1829.

As Millepora is the name of a living hydrozoan coral, the bryozoan is now placed in a different genus, Collapora, as Collapora straminea, although, in many older references, it is referred to as Haplooecia straminea. Branch fragments of Collapora straminea can be found throughout the Millepore Bed, but are best preserved in shale partings (Figs. 6A and 9). This is the only erect bryozoan found in the Millepore Bed at Yons Nab, although tiny encrusting colonies of ‘Berenicea’ and Stomatopora occasionally grow on the branches of Collapora straminea.

Portland’s special bryozoans

All of the bryozoans mentioned so far are cyclostomes. However, two other orders of bryozoans are known in the British Jurassic: ctenostomes and cheilostomes. Ctenostomes have entirely soft-bodied colonies, which may be preserved as borings in shells, or as natural casts on the undersides of shells that overgrew them during life, a process known as bioimmuration.

Cheilostomes are by far the dominant bryozoan order living today and include genera such as Membranipora, Electra and Flustra that are abundant along the British coast. These calcified bryozoans did not appear until the Late Jurassic. One of the earliest is Pyriporopsis portlandensis. As the name suggests, this occurs on the Isle of Portland, but can also be found elsewhere in southern England where the Tithonian-aged Portland Stone outcrops. The encrusting colonies of Pyriporopsis portlandensis consist of strings of oval zooids with large apertures (Fig. 10), quite different from those of cyclostomes.

It is remarkable that this common bryozoan, of huge importance in understanding the evolutionary origin of cheilostomes, was not discovered until the early 1970s. However, colonies at first glance can be mistaken for ooids adhering to the surfaces of shells. A block of Portland Stone that had been on display for many years in the then Geological Museum on Exhibition Road in London, was found to include examples of Pyriporopsis portlandensis that had gone totally unnoticed.

Finally, the Portland Stone is also notable for containing small patch reefs constructed by cyclostome bryozoans (Fürsich et al, 1994). Although less common as reef-builders in the Portland Stone than the alga Neosolenopora jurassica, the bryozoan reefs are exceptional and have no close equivalents in the Mesozoic. They consist of multiple layers of encrusting bryozoans of the ‘Berenicea’ type, possibly belonging to Rugosopora portlandica, which is the commonest bryozoan species in the Portland Limestone. Oysters periodically grew on the reef surface, before being enveloped by more layers of bryozoans and incorporated into the reef structure (Fig. 11).

References

Fürsich, F.T., Palmer, T.J. & Goodyear, K.L. 1994. Growth and disintegration of bivalve-dominated patch reefs in the Upper Jurassic of southern England. Palaeontology 37: 131–171.

Taylor, P.D. 2018. Bryozoans in the English Chalk. Deposits Magazine 55: 33–37.

Taylor, P.D. 2020. Bryozoan Paleobiology. Wiley Blackwell, Oxford, 320 pp.

Taylor, P.D. & Ernst, A. 2008. Bryozoans in transition: the depauperate and patchy Jurassic biota. Palaeogeography, Palaeoclimatology, Palaeoecology 263: 9–23.

Taylor, P.D. & Milne, R. 2009. The ‘corallines’ of the Coralline Crag: bryozoans in a Suffolk Pliocene bryozoan limestone. Deposits 19: 22-25.

Walter, B. 1970. Les bryozoaires jurassiques en France. Documents des Laboratoires de Géologie de la Faculté des Sciences de Lyon 35 [for 1969]: 1–328.