Bryozoans in the British Jurassic

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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).

Fig. 1. Jurassic bryozoans with encrusting colony-forms: Sheets (A, B), runners (C) and a ribbon (D). (A) Several disc-shaped colonies of ‘Berenicea’ growing on a cup of the crinoid Apiocrinus elegans (Bathonian, Bradford Clay; Wiltshire); (B) Kololophos sp with apertures arranged along radial ridges, growing on the ventral valve of a brachiopod (Aalenian or Bajocoan, Inferior Oolite; probably Gloucestershire); (C) Colonies belonging to two species of Stomatopora ramifying over the internal surface of an oyster (Bathonian, Gloucestershire); and (D) Idmonea triquetra growing over the dorsal valve of a brachiopod (Bathonian, Bradford Clay; Wiltshire). Scale bars are 1cm.

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.

Fig. 2. Small colonies of two sheet-like colonies (A and B) contrast with a runner-like colony (C) from the Jurassic: (A) Young, fan-shaped colony of Reptomultisparsa (Aalenian, Lower Inferior Oolite, Painswick, Gloucestershire); (B) Circular colony of Mesonopora with a circumferential growing edge from which new zooids were budded (Bathonian, Bradford Clay; Wiltshire); and (C) Early growth stages of a branching colony of Stomatopora (Bathonian, White Limestone Formation; Foss Cross Quarry, Gloucestershire). Scale bars are 500µm.

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).

Fig. 3. Details of the surface of two sheet-like colonies of the ‘Berenicea’ type: (A) In Hyporosopora dilatata, the colony surface is relatively flat, save for the slightly convex zooids and faint growth lines (Upper Callovian or Lower Oxfordian, Oxford Clay; Stanton Harcourt, Oxfordshire); and (B) Well-defined transverse ridges cross the colony surface in Rugosopora enstonensis (Bathonian, Hampen Marly Beds; Enstone, Oxfordshire). Scale bars are 500µm.

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.

Fig. 4. Gonozooids of contrasting shape in two Jurassic cyclostomes, formerly placed in the form-genus Berenicea. Arrows point to the openings of the gonozooids where the swimming larvae would have exited from their incubation chamber: (A) Reptomultisparsa has an elongate gonozooid about twice the width of an autozooid (Bajocian, Microzoa Beds; Shipton Gorge, Dorset); and (B) The gonozooid of Hyporosopora is broad, in this example shaped like a boomerang with lobes extending beyond the position of the opening (Bathonian, Fullers Earth Rock; East Cranmore, Somerset). Scale bars are 200µm.

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.

Fig. 5. Bushy colony of Collapora straminea weathering out from a block of limestone (Bajocian, Middle Inferior Oolite; Leckhampton, Gloucestershire). Scale bar is 1cm.
Fig. 6. Jurassic cyclostome bryozoans with bush-like erect colonies: (A) Part of a branch of Collapora straminea, the species which gave its name to the Millepore Bed (Bajocian, Cloughton Formation, Lebberston Member; Yorkshire coast); and (B) Holotype colony of the rare bryozoan Fasciculipora waltoni, in which the feeding zooids open at the branch tips instead of along the sides as in Collapora straminea and most other bush-like colonies (Bathonian, Great Oolite; Bathampton, Somerset). Scale bars: A = 500µm; B = 1mm.
Fig. 7. Fragment of the foliaceous erect bryozoan Mesenteripora michelini. Almost all of the apertures are closed by terminal diaphragms, showing that this is an older part of the colony where feeding had ceased. Scale bar is 1mm.

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’.

Fig. 8. Low-angled cross-bedding in the Millepore Bed at Yons Nab in Cayton Bay, North Yorkshire.

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.

Fig. 9. Branches of the bryozoan Collapora straminea in a shale layer of the Millepore Bed (Bajocian, Cloughton Formation, Lebberston Member; Yons Nab). The colony from which these branches are broken would have looked originally like the colony depicted in Fig. 5.

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.

Fig. 10. Pyriporopsis portlandensis from the Portland Stone is one of the oldest species belonging to the Cheilostomata, which are the most diverse order of bryozoans living in modern seas. The zooids of this encrusting colony, probably from Whitchurch in Buckinghamshire, appear chaotically arranged. Scale bar is 500µm.

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).

Fig. 11. Vertically sectioned bryozoan reef from the Portland Limestone of the Isle of Portland, Dorset. The cream-coloured ‘matrix’ consists of multiple layers of encrusting cyclostomes within which articulated oysters that fouled the surface of the reef have become embedded during upward reef growth.


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.

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