Britain isn’t richly endowed with fossiliferous Pliocene localities. However, the Red and Coralline Crags of East Anglia make up for this deficiency within the sheer abundance and high quality of their fossils. Whereas the Red Crag, well-known for its gastropods and bivalves, takes its identify from the color of the sediment, the Coralline Crag is known as for its ‘corallines’. But what precisely are these? Despite the identify, which suggests corals or maybe coralline algae, the corallines of the Crag are literally bryozoans, popularly generally known as ‘moss animals’ or ‘sea-mats’ (see Issue 12 of Deposits: Bryozoans: greater than meets the attention). In reality, the Coralline Crag is a bryozoan limestone and represents a uncommon instance of a non-tropical limestone within the British geological document.

The most important outcrop of the Coralline Crag runs between Gedgrave close to Orford within the south, to Aldeburgh within the north, forming a low ridge virtually parallel to the Suffolk coast (Fig. S). There are additionally small outliers additional south at Sutton and Tattingstone, however the latter is now submerged beneath a reservoir. Lateral equivalents of the Coralline Crag could be present in Belgium and Holland (for instance, see Bishop & Hayward 1989).

Deposition occurred in shallow water, about 4mya, alongside the margins of the traditional North Sea. Giant submarine dunes – sandwaves – swept the fragmented stays of bryozoans and different shells alongside the seabed, abandoning the spectacular, cross-bedded carbonate sands (calcarenites), seen at localities comparable to Crag Farm, Sudbourne (Fig. S). Detailed descriptions of this and different localities might be present in Balson (1999).

Bryozoans galore

The bryozoan fauna of the Crag is internationally well-known. In the identical yr that Charles Darwin revealed the Origin of Species (1859), his good friend, George Busk, introduced 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 by no means been undertaken, we now know that there are at the least 134 bryozoan species within the Coralline Crag. Further species probably await discovery. However, Busk’s Palaeontographical Society monograph nonetheless stays a key reference on the bryozoans from the Coralline Crag, regardless of modifications to most of the generic names.

It isn’t troublesome to gather bryozoans from the Coralline Crag. They are the most typical fossils within the a number of small pits which are sometimes labored by native farmers for gravel to floor trackways.

Bryozoans are colonial animals, every colony consisting of tens to hundreds of minute particular person zooids that feed on plankton. In most species, the zooids have field-like or tubular skeletons manufactured from calcium carbonate, the supply of a lot of the carbonate constituting the Coralline Crag. Parts of it are cemented into a tough limestone – the Bryozoan Rock Bed – used as a constructing stone, for instance, within the tower of Chillesford Church. However, for probably the most half, cementation is slight and it may be crumbled within the hand. Larger bryozoan colonies climate pleased with the rock and could be carved out utilizing a knife with out the necessity for a hammer. Smaller colonies, and fragments of bigger colonies, are higher collected by taking samples of sentimental sediment and sieving them within the laboratory or at house. A sieve with a mesh measurement of a few millimetre is satisfactory to retain a lot of the respectable bryozoan fragments. It can also be worthwhile amassing shells of pectinid bivalves (scallops), that are fairly often encrusted by bryozoan colonies (Fig. B).

One of the great issues about fossil bryozoans is that they will often be recognized from small fragments, as these protect a lot of the diagnostic options of the zooidal skeleton. It is comparatively straightforward to develop specimens rigorously utilizing a mounted needle or a vibrotool to take away sediment grains adhering to colony surfaces and filling interstices.

A binocular microscope is important, not only for getting ready specimens, but in addition for making correct identifications. It would take many extra pages than can be found right here to offer an identification information for all the bryozoan species recorded from the Coralline Crag. Therefore, our focus under shall be on a few of the commoner or extra fascinating species, with feedback on their pure histories and significance for understanding depositional environments.

Naming the bryozoans

Coralline Crag bryozoans divide into two essential varieties. The first group consists of people who type sheet-like or, extra not often, branching encrustations on the surfaces of shells or different bryozoans. The second includes erect species, a variable assemblage of colonies that grew in three dimensions from a hard and fast base. Whereas the encrusting species are extraordinarily numerous and might be troublesome to determine, the erect bryozoans principally consist of some distinctive species.

Perhaps, probably the most attribute bryozoans within the Coralline Crag are a number of globular species usually starting from the dimensions of a pea to a tennis ball. They embrace two notably plentiful genera belonging to the cyclostome (‘spherical-mouthed’) bryozoans, Meandropora and Blumenbachium (see Balson & Taylor 1982). In Meandropora (Fig. P), the lengthy tubular zooids are organized in radial bundles with occasional lateral linkages, wanting just like the completely 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 extra of a concentric construction when damaged open, often with a sample of polygons on the outer colony floor resembling the corallites of a colonial coral (Fig. O). Although the Coralline Crag is comparatively younger in geological phrases, there are not any bryozoans resembling both Meandropora or Blumenbachium in in the present day’s oceans, and we do not know why they turned extinct.

Also current within the Crag are a number of species of cheilostome (‘lip-mouthed’) bryozoans rising as foliaceous (leaf-like) colonies that break down into fragments resembling potato crisps (Fig. A). The three commonest genera are Biflustra, Pentapora and Metrarabdotos, all having zooids opening on each surfaces of the flattened branches. A microscope or good hand lens is required to differentiate between these genera (Fig. eleven). In Biflustra, the zooids are virtually rectangular in form with a big oval opening. Specimens in notably good situation might protect delicate spines projecting into the openings. Both Pentapora and Metrarabdotos have zooids with a small orifice the place the tentacles emerged within the dwelling colonies. Most of the floor of every zooid is shaped by a minutely porous frontal defend. In Pentapora, a small opening could also be current simply beneath the orifice, whereas two such openings happen at both aspect of the orifice in Metrarabdotos. These openings are defensive zooids referred to as avicularia. Whereas in Metrarabdotos, the older zooids close to the colony base turn into completely obscured by calcification, these of Pentapora change little as they age.

Although a species of Pentapora lives in British waters at present, the place it’s confusingly often known as ‘Ross coral’ (see http://www.marlin.ac.uk/species/Pentaporafascialis.htm), each of the 2 species discovered within the Coralline Crag are extinct. In distinction, Metrarabdotos is absent from the fashionable British fauna and is considered a tropical or subtropical genus, certainly one of a number of clues suggesting that the temperature of the North Sea throughout deposition of the Coralline Crag was no less than as heat because the Mediterranean Sea at the moment. Further proof for heat water deposition comes from the presence of one other bryozoan referred to as Cupuladria. The uncommon cap-formed colonies of this bryozoan (Fig. 12) are tailored to an untethered, free-dwelling mode of life on cellular sediments. There are not any examples of Cupuladria dwelling across the British Isles immediately, and trendy populations discovered elsewhere on the earth require temperatures to not drop under 12°H. Unfortunately, Cupuladria just isn’t simply collected from the Coralline Crag, as a result of it has a skeleton made from aragonite quite than calcite. Most of the aragonite within the Coralline Crag was dissolved throughout fossilisation. This additionally accounts for the relative shortage of bivalves and gastropods, which, aside from pectinids, oysters and epitomiid gastropods, had shells manufactured from this mineral.

Living collectively

Aragonitic corals are additionally uncommon within the Coralline Crag, for a similar cause. However, there’s an uncommon symbiosis between a small, cone-formed coral referred to as Culicia (or Cryptangia) and the bryozoan Celleporaria (Fig. H). The bryozoan had bushy colonies with branches that turned thickened by the continual budding of latest, blister-like zooids. As they did so, corals that had settled on the floor of the bryozoan turned ever extra deeply embedded, whereas sustaining a gap on the floor by rising on the similar price because the host bryozoan. As that is an extinct symbiosis, we will solely speculate about how the companions interacted throughout life. However, it’s probably that the tiny corals have been strengthened by the enveloping bryozoan that, in return, might have acquired safety from the stinging cells of the corals. There are some Coralline Crag localities (for instance, Ramsholt Cliff) the place the aragonitic corals are preserved in situ inside their calcitic bryozoan hosts, however elsewhere the corals have been dissolved to go away conical pits within the surfaces of the bryozoan colonies.

The Coralline Crag bryozoan fauna consists of one other instance of a fossil symbiosis. The uncommon aragonitic bryozoan, Hippoporidra, is understood at this time 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 dimensions of the chamber inhabited by the crabs. Hermit crabs are not often fossilised, however the prevalence of the distinctive colonies of Hippoporidra within the Coralline Crag (Fig. thirteen) proves that they colonised the Pliocene sea on the time this formation was being deposited.

Finally, when creating specimens from blocks of Coralline Crag, it’s worthwhile holding an eye fixed out for different small fossils. Forams are fairly widespread, although, as with many Coralline Crag fossils, they are typically very fragile. Otoliths, tiny solitary corals, and echinoid checks and spines additionally happen, however once more are fragile and don’t generally survive intact. Not occasionally, scallop shells will present substrates not just for encrusting bryozoans, but in addition small barnacles and serpulid worms.

References

Balson, K., 1999, The Coralline Crag. Pp. 253-288. In: Daley, S. & Balson, G. British Tertiary Stratigraphy. Joint Nature Conservation Committee, Peterborough, 388 pp.

Balson, G.R. & Taylor, K.A., 1982. Palaeobiology and systematics of huge 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, C.A.A. & Hayward, G.M., 1989, SEM Atlas of Type and Fig. d Material from Robert Lagaaij’s ‘The Pliocene Bryozoa of the Low Countries’. Mededelingen Rijks Geologische Dienst forty three, B-sixty four.

Busk, M., 1859, The Fossil Polyzoa of the Crag. The Palaeontographical Society, London.

Lagaaij, J., 1952, The Pliocene Bryozoa of the Low Countries. Mededelingen van de Geologische Stichting (Serie M), H, B-233.

Paul A Taylor and Rory Milne

Paul Taylor is a bryozoan researcher within the Palaeontology Department on the Natural History Museum, London, and previous President of the International Bryozoology Association. Rory Milne is a volunteer helper within the Museum, concentrating on the bryozoan assortment from the Coralline Crag.

Filed underneath: fossils Tagged: Biflustra, Blumenbachium, Bryozoan, Bryozoans, Coralline Crag, Crag Farm, Culicia, fossils, Meandropora., Pectinid, Pentapora, Ramsholt, Sudbourne., Suffolk, Suffolk Knoll
Deposits Magazine