Pliocene bryozoans in the Suffolk Coralline Crag
Paul D Taylor and Rory Milne (UK)
Britain is not richly endowed with fossiliferous Pliocene localities. However, the Red and Coralline Crags of East Anglia make up for this deficiency in the sheer abundance and quality of their fossils. Whereas the Red Crag, famous for its gastropods and bivalves, takes its name from the colour of the sediment, the Coralline Crag is named for its ‘corallines’. But what exactly are these? Despite the name, which suggests corals or perhaps coralline algae, the corallines of the Crag are actually bryozoans, popularly known as ‘moss animals’ or ‘sea-mats’ (see Issue 12 of Deposits: Bryozoans: more than meets the eye). In fact, the Coralline Crag is a bryozoan limestone and represents a rare example of a non-tropical limestone in the British geological record.
The main outcrop of the Coralline Crag runs between Gedgrave near Orford in the south, to Aldeburgh in the north, forming a low ridge almost parallel to the Suffolk coast (Fig. 1). There are also small outliers further south at Sutton and Tattingstone, but the latter is now submerged beneath a reservoir. Lateral equivalents of the Coralline Crag can be found in Belgium and Holland (for example, see Bishop & Hayward 1989).
Deposition occurred in shallow water, about 4Ma, along the margins of the ancient North Sea. Giant submarine dunes – sandwaves – swept the fragmented remains of bryozoans and other shells along the seabed, leaving behind the spectacular, cross-bedded carbonate sands (calcarenites), seen at localities such as Crag Farm, Sudbourne (Fig. 2). Detailed descriptions of this and other localities can be found in Balson (1999).
Bryozoans galore
The bryozoan fauna of the Crag is internationally famous. In the same year that Charles Darwin published the Origin of Species (1859), his friend, George Busk, brought out a monographic account of the bryozoans of the East Anglian Crags. This described 117 species of bryozoans from these deposits.
Although a full revision of Busk’s work has never been undertaken, we now know that there are at least 134 bryozoan species in the Coralline Crag. Further species doubtless await discovery. However, Busk’s Palaeontographical Society monograph still remains a key reference on the bryozoans from the Coralline Crag, despite changes to many of the generic names.
It isn’t difficult to collect bryozoans from the Coralline Crag. They are the commonest fossils in the several small pits that are occasionally worked by local farmers for gravel to surface trackways.
Bryozoans are colonial animals, each colony consisting of tens to thousands of minute individual zooids that feed on plankton. In most species, the zooids have box-like or tubular skeletons made of calcium carbonate, the source of much of the carbonate constituting the Coralline Crag. Parts of it are cemented into a hard limestone – the Bryozoan Rock Bed – used as a building stone, for example, in the tower of Chillesford Church. However, for the most part, cementation is slight and it can be crumbled in the hand.
Larger bryozoan colonies weather proud of the rock and can be carved out using a knife without the need for a hammer. Smaller colonies, and fragments of larger colonies, are better collected by taking samples of soft sediment and sieving them in the laboratory or at home. A sieve with a mesh size of about a millimetre is adequate to retain most of the decent bryozoan fragments. It is also worthwhile collecting shells of pectinid bivalves (scallops), which are very often encrusted by bryozoan colonies (Fig. 3).
One of the nice things about fossil bryozoans is that they can usually be identified from small fragments, as these preserve most of the diagnostic features of the zooidal skeleton. It is relatively easy to develop specimens carefully using a mounted needle or a vibrotool to remove sediment grains adhering to colony surfaces and filling interstices.
A binocular microscope is essential, not just for preparing specimens, but also for making accurate identifications. It would take many more pages than are available here to provide an identification guide for all of the bryozoan species recorded from the Coralline Crag. Therefore, our focus below will be on some of the commoner or more interesting species, with comments on their natural histories and significance for understanding depositional environments.
Naming the bryozoans
Coralline Crag bryozoans divide into two main types. The first group consists of those that form sheet-like or, more rarely, branching encrustations on the surfaces of shells or other bryozoans. The second comprises erect species, a variable assemblage of colonies that grew in three dimensions from a fixed base. Whereas the encrusting species are extremely diverse and can be difficult to identify, the erect bryozoans mostly consist of a few distinctive species.
Perhaps, the most characteristic bryozoans in the Coralline Crag are several globular species generally ranging from the size of a pea to a tennis ball. They include two particularly abundant genera belonging to the cyclostome (‘round-mouthed’) bryozoans, Meandropora and Blumenbachium (see Balson & Taylor 1982). In Meandropora (Fig. 4), the long tubular zooids are arranged in radial bundles with occasional lateral linkages, looking like the totally unrelated organ-pipe coral, Tubipora (see http://www.uco.es/dptos/zoologia/zoolobiolo_archivos/practicas/practica_3/C_tubipora-1.jpg).
The second genus, Blumenbachium, has more of a concentric structure when broken open, usually with a pattern of polygons on the outer colony surface resembling the corallites of a colonial coral (Fig. 5). Although the Coralline Crag is relatively young in geological terms, there are no bryozoans resembling either Meandropora or Blumenbachium in today’s oceans, and we have no idea why they became extinct.
Also present in the Crag are several species of cheilostome (‘lip-mouthed’) bryozoans growing as foliaceous (leaf-like) colonies that break down into fragments resembling potato crisps (Fig. 6).
The three commonest genera are Biflustra, Pentapora and Metrarabdotos, all having zooids opening on both surfaces of the flattened branches. A microscope or good hand lens is needed to distinguish between these genera (Fig. 7).
In Biflustra, the zooids are almost rectangular in shape with a large oval opening. Specimens in particularly good condition may preserve delicate spines projecting into the openings. Both Pentapora and Metrarabdotos have zooids with a small orifice where the tentacles emerged in the living colonies. Most of the surface of each zooid is formed by a minutely porous frontal shield. In Pentapora, a small opening may be present just beneath the orifice, while two such openings occur at either side of the orifice in Metrarabdotos. These openings are defensive zooids called avicularia. Whereas in Metrarabdotos, the older zooids near the colony base become totally obscured by calcification, those of Pentapora change little as they age.
Although a species of Pentapora lives in British waters today, where it is confusingly known as ‘Ross coral’ (see http://www.marlin.ac.uk/species/Pentaporafascialis.htm), both of the two species found in the Coralline Crag are extinct. In contrast, Metrarabdotos is absent from the modern British fauna and is regarded as a tropical or subtropical genus, one of several clues suggesting that the temperature of the North Sea during deposition of the Coralline Crag was at least as warm as the Mediterranean Sea today. Further evidence for warm water deposition comes from the presence of another bryozoan called Cupuladria. The unusual cap-shaped colonies of this bryozoan (Fig. 11) are adapted to an untethered, free-living mode of life on mobile sediments.
There are no examples of Cupuladria living around the British Isles today, and modern populations found elsewhere in the world require temperatures not to drop below 12°C. Unfortunately, Cupuladria is not easily collected from the Coralline Crag, because it has a skeleton made of aragonite rather than calcite. Most of the aragonite in the Coralline Crag was dissolved during fossilisation. This also accounts for the relative scarcity of bivalves and gastropods, which, apart from pectinids, oysters and epitomiid gastropods, had shells made of this mineral.
Living together
Aragonitic corals are also rare in the Coralline Crag, for the same reason. However, there is an unusual symbiosis between a small, cone-shaped coral called Culicia (or Cryptangia) and the bryozoan Celleporaria (Fig. 12).
The bryozoan had bushy colonies with branches that became thickened by the continuous budding of new, blister-like zooids. As they did so, corals that had settled on the surface of the bryozoan became ever more deeply embedded, while maintaining an opening on the surface by growing at the same rate as the host bryozoan. As this is an extinct symbiosis, we can only speculate about how the partners interacted during life.
However, it is likely that the tiny corals were strengthened by the enveloping bryozoan that, in return, may have received protection from the stinging cells of the corals. There are some Coralline Crag localities (for example, Ramsholt Cliff) where the aragonitic corals are preserved in situ within their calcitic bryozoan hosts, but elsewhere the corals have been dissolved to leave conical pits in the surfaces of the bryozoan colonies.
The Coralline Crag bryozoan fauna includes another example of a fossil symbiosis. The rare aragonitic bryozoan, Hippoporidra, is known today to be a symbiont of hermit crabs. Colonies develop on the outsides of gastropod shells occupied by hermit crabs, forming thick, multilayered encrustations that enlarge the size of the chamber inhabited by the crabs. Hermit crabs are rarely fossilised, but the occurrence of the distinctive colonies of Hippoporidra in the Coralline Crag (Fig. 13) proves that they colonised the Pliocene sea at the time this formation was being deposited.
Finally, when developing specimens from blocks of Coralline Crag, it is worthwhile keeping an eye out for other small fossils. Forams are quite common, though, as with many Coralline Crag fossils, they tend to be very fragile. Otoliths, tiny solitary corals, and echinoid tests and spines also occur, but again are fragile and do not commonly survive intact. Not infrequently, scallop shells will provide substrates not only for encrusting bryozoans, but also small barnacles and serpulid worms.
About the authors
Paul Taylor is a bryozoan researcher in the Palaeontology Department at the Natural History Museum, London, and past President of the International Bryozoology Association. Rory Milne is a volunteer helper in the Museum, concentrating on the bryozoan collection from the Coralline Crag.
Further reading
Bryozoans: more than meets the eye, by Paul D Taylor
References
Balson, P., 1999, The Coralline Crag. Pp. 253-288. In: Daley, B. & Balson, P. British Tertiary Stratigraphy. Joint Nature Conservation Committee, Peterborough, 388 pp.
Balson, P.S. & Taylor, P.D., 1982. Palaeobiology and systematics of large cyclostome bryozoans from the Pliocene Coralline Crag of Suffolk. Palaeontology 25, 529-554. http://www.palass.org/modules.php?name=backissues&stage=results&vol=25.
Bishop, J.D.D. & Hayward, P.J., 1989, SEM Atlas of Type and Fig. d Material from Robert Lagaaij’s ‘The Pliocene Bryozoa of the Low Countries’. Mededelingen Rijks Geologische Dienst 43, 1-64.
Busk, G., 1859, The Fossil Polyzoa of the Crag. The Palaeontographical Society, London.
Lagaaij, R., 1952, The Pliocene Bryozoa of the Low Countries. Mededelingen van de Geologische Stichting (Serie C), 5, 1-233.