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7. Recommended Methods

Calcified bryozoan colonies may be preserved in similar situations to molluscs, ostracods and calcareous foraminifera. Chitinous statoblasts may be preserved in similar situations to insects. The most likely scenario in which bryozoans may be encountered archaeologically is as epibiont organisms on marine shells. Encrustations may also occur on artefacts that have been submerged, for example at Walpole in Somerset, where bryozoans were recorded on pieces of fired clay (Figure 4). Shells and artefacts bearing bryozoan encrustations should not be subjected to normal finds washing procedure as this is likely to cause damage to, or even completely deface, diagnostic features. Should any cleaning be required, gentle brushing with a fine sable paintbrush and a pipette of deionised water should be used.

Figure 4

Figure 4: Colony of Conopeum seurati (Canu 1928) on archaeological fired clay, Walpole, Somerset, UK

Loose strands of zooids may also be recovered within sediment samples. This was the case at Weston College, Weston-super-Mare, where strands of Flustra foliacea (Linnaeus 1758) were washed into a tidal creek or ditch and recovered by the author in samples for molluscan analysis (Figure 5). Techniques for the extraction of bryozoans are essentially the same as for non-marine Mollusca (Evans 1972; Davies 2008). A standard weight of air-dried sediment should be selected (1.5kg was used at Weston College and at Walpole where samples were also processed for Mollusca; much less will suffice for recovery of statoblasts from freshwater deposits). Some sediments may require disaggregation in dilute sodium pyrophosphate. Residues should be washed through a 250 µm mesh sieve and oven dried. Statoblasts of the freshwater Phylactolaemata may be found in freshwater-lain sediments. Use of a 125 µm mesh sieve is recommended where their recovery is sought. Extraction should take place using a fine sable paintbrush or fine forceps under a low-power stereo microscope. Identification of bryozoan colonies relies on the characteristics of the frontal surface, so the basal surface may be mounted with a water-soluble glue or gum tragacanth on black card. Statoblasts should be mounted onto glass slides.

Figure 5

Figure 5: Strands of Flustra foliacea (Linnaeus 1758) recovered from samples of a tidal creek or ditch, Weston-super-Mare, UK

Identification should be carried out using a low-powered microscope at 50x magnification with reference to modern specimens (Figures 6 and 7). Francis (2001, 109) notes that some statoblasts of Plumatella spp. can only be differentiated using scanning electron microscopy. Zooid and statoblast size may be measured using a micrometer eyepiece (O'Dea and Okamura 2000a, 323).

The best general text on bryozoans is still that of Ryland (1970). A key to British and European freshwater taxa, including statoblasts, has been published by Mundy (1980), and a thorough review of methods by Francis (2001). Statoblasts of Cristatella spp. decay in a distinctive fashion, and Hall et al. (2003, fig. 3) present a decay sequence allowing identification of even poorly preserved specimens. A good starting point for marine taxa is Ryland (1995), although a number of more detailed texts exist in the Synopses of the British Fauna series (Ryland and Hayward 1977; Hayward and Ryland 1979; Hayward 1985; Hayward and Ryland 1985; 1996; 1999), as well as two keys in the journal Field Studies (Ryland 1962; Ryland 1974).

Figure 6

Figure 6: Colony of Conopeum reticulum (Linnaeus 1767) on a modern mussel (Mytilus edulis Linnaeus 1758) shell.

Figure 7

Figure 7: Colonies of Escharella variolosa (Johnston 1838) on a modern common whelk (Buccinum undatum (Linnaeus 1758)) shell.


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