Palaeoecology of a solitary coral, Farley, Wenlock Edge, Shropshire (Silurian)

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1 ISSN X (Print) ISSN (Online) Palaeoecology of a solitary coral, Farley, Wenlock Edge, Shropshire (Silurian) Stephen K. Donovan 1 DONOVAN, S.K. (2012). Palaeoecology of a solitary coral, Farley, Wenlock Edge, Shropshire (Silurian). Proceedings of the Shropshire Geological Society, 17, The Much Wenlock Limestone Formation continues to provide new data and insights to geologists and palaeontologists even after being studied for over 200 years. A specimen of the solitary rugose coral Dokophyllum sp. from the old road cutting at Farley, near Much Wenlock, Shropshire, formed part of the fossiliferous debris on a largely overgrown and degraded site. This specimen preserves stark evidence of how it lived and died. After toppling over while still immature, the coral was growing around a tight corner, seeking to elevate the calice above the sediment surface, in which it was partly buried, when it is presumed to have died by burial due to another influx of sediment. Evidence for this scenario includes preservation of the sediment surface (now lithified) in which it was embedded, angling of the new growth away from the old calice so as to leave a wedge-shaped gap between them and growth of root-like rhizoids on the base of the new growth. 1 Department of Geology, NCB Naturalis, Postbus 9517, 2300 RA Leiden, The Netherlands. Steve.Donovan@ncbnaturalis.nl INTRODUCTION The diversity of fossil invertebrates of the Much Wenlock Limestone Formation, which crops out widely in the west of central England, including Shropshire, is exceptional and is not surpassed by that of any other sedimentary unit in the Lower Palaeozoic of the British Isles. Its fauna has been recognised for over 200 years, although the modern monographic treatment of its treasures perhaps dates from as recently as Murchison (1839). That the fauna is particularly well known is undoubted, but that is not to imply that our knowledge of these fossils is complete. Systematic revisions and descriptions of new species are continuing (e.g., starfishes, Herringshaw et al., 2007; brachiopods, Bassett, , in progress), and the study of stratigraphy (e.g., Ray et al., 2010), palaeoecology, taphonomy and related subjects continues. Such studies must rely, in part, on existing collections in museums, but the importance of new collecting and observations should never be underestimated. Herein, the palaeoautecology (sensu Ager, 1963, p. 19) is discussed of a specimen of the solitary rugose coral Dokophyllum sp. that the author collected from the Much Wenlock area in December The fossil first came to his attention as a large specimen in the float, but close examination suggested that it was worthy of closer examination as it told the story of the animal s life and death in unusual detail. For explanation of the terminology of the skeleton of rugose corals, see Hill (1981, pp. F6-F36) or Clarkson (1998, pp ). The specimen is now part of the collection of NCB Naturalis, Leiden (RGM ). LOCALITY The old road cutting at Farley, Shropshire, is on west side of the A4169 road (formerly B4378) at Farley, 2 km north of Much Wenlock and about 200 m north of the former Rock House public house, now a private dwelling [NGR SJ ] (Figure 1). Note that this is not the same locality discussed by Derek Siveter (2000, pp ). Although rock exposure is limited, fossiliferous debris is moderately common in the more or less overgrown scree, including tabulate corals, solitary rugose corals, brachiopods, bryozoans, crinoid debris, and rarer gastropods and other invertebrates. This site is best known for fossil crinoids, such as Myelodactylus ammonis (Bather), Gissocrinus sp. and calceocrinids (Donovan & Sevastopulo, 1989; Donovan et al., 2008; Donovan & Lewis, 2010). Much Wenlock Limestone Formation; Silurian, Wenlock, Homerian. Proceedings of the Shropshire Geological Society, 17, Shropshire Geological Society

2 S.K. DONOVAN distinct changes in growth directions between regions 1, 2 and 3 (Fig. 2C), but the conical morphology is continuous. In contrast, regions 3 and 4 are separated by a wedge-shaped break in the cone (Fig. 2B, D), with bases of rhizoid processes developed on base of 4. Part of the calice at the apex of 3 is exposed, although septa are only weakly apparent. Region 4 is distinctly wedge-shaped and strongly angled to the long axis of region 3 (Fig. 2D). Calice (region 4) is elliptical in outline, deep and conical, but much of the centre is obscured by limestone. Septa numerous, radial, sloping towards centre. No dissepiments apparent. Figure 1. Outline map of the area northeast of Much Wenlock, Shropshire, showing the position of the fossil locality discussed herein (*), adjacent to the A4169 road (after Donovan et al., 2008, fig. 1). NGR co-ordinates indicated on margins. Key: B = Buildwas; F = Farley Quarry; MW = Much Wenlock; RS = River Severn; S = Shadwell Quarry. Only main roads are marked. For a geological map of this area, see David Siveter et al. (1989, fig. 15). DESCRIPTION RGM is a moderately large solitary coral (maximum dimension mm, maximum width 46.8 mm), horn-shaped (Fig. 2A, B, D), almost circular in section at midheight, curved at extremities. Corallum divided into four regions, informally labelled 1 to 4 herein and each having grown in a different orientation (Fig. 2C, D). Epitheca with sculpture of both low, longitudinal ridges corresponding to positions of septa, and concentric growth lines, more or less raised, and more prominent towards calice. Outgrowths of the epitheca that appear as wart-like structures (Fig. 2A, B) are bases for root-like rhizoid processes (=rhizosessile; Scrutton, 1998, p. 6, figs 3G, 4; Eliason, 2000, p. 65), some showing distinct orientation towards base. Corallum shows DISCUSSION The above description agrees well with that of Eliason (2000, p. 65) of Dokophyllum; Solitary, large coral with an almost straight conical skeleton bearing root-like extensions. The corallite has a flat, wide bottom [Hill, 1981, fig a]. It can attain a height of 20 cm. Internal features, such as the flat base to the corallite and the presence of a dissepimentarium (Hill, 1981, p. F219) cannot be confirmed without sectioning the specimen, which has not been done to preserve the external features discussed herein. However, Hill (1981, p. F219) implied that the development of dissepiments was a late growth feature; this specimen, being only half the height determined by Eliason, may be immature and, hence, dissepiments are absent. The particular interest of this specimen is in its early and late growth stages. The four regions, 1 to 4 (Fig. 2C, D), are each interpreted as a different stage in the continuous growth of the corallum, each separated from the next by a change in the direction of growth. Such changes are widely known from solitary rugose corals (disturbance variation; Scrutton, 1998, p. 16, fig. 11) and it is usual for the early growth stages to result in a more or less geniculate geometry due to the uncemented individual being more or less toppled in unlithified sediment (see, for example, Hubbard, 1970; Clarkson, 1998, p. 120). So, the transitions from regions 1 to 2 to 3 are interpreted as due to changes in orientation following reorientation of the corallum due to sediment settling, erosion or (less likely) deposition, due to the influence of Proceedings of the Shropshire Geological Society, 17, Shropshire Geological Society

3 PALAEOECOLOGY OF A SOLITARY CORAL Figure 2. Dokophyllum sp., RGM , Much Wenlock Limestone Formation, Farley, Shropshire. (A, B) Two views of complete corallum. (A) The view into the calice at the top of this view demonstrates the curvature of the specimen. (B) The change of growth direction indicated by the gap in the epitheca at about two thirds the height of the specimen. (C) Detail of the base of the corallum, showing the three earliest growth directions, numbered 1, 2 and 3. (D) Life position of the colony at time of death as indicated by the prominent bedded limestone layer in the central half of the corallum. Growth directions 1-3 as (C); the extreme change in direction between 3 and 4 is indicated by the wedge-shaped gap in the epitheca. Scale bars represent 10 mm. Proceedings of the Shropshire Geological Society, 17, Shropshire Geological Society

4 S.K. DONOVAN changing bottom currents, or all of these. Each of these regions would have been perpendicular to the sediment surface at the time of growth. The transition from regions 3 to 4 is somewhat different and is the most unusual feature of this coral, which has been interpreted using multiple lines of evidence. I construe this transition as the result of the corallum toppling over to lie horizontally and partly embedded in the unlithified sediment. Although collected from float, this horizontal attitude is indicated by the (now lithified) remnant of a bed of limestone that is preserved as a rib along one side of the corallum (Fig. 2D, just above midline in this view and parallel to the bottom of the page). This is indicative of the coral s attitude after final burial, but other lines of evidence strongly suggest that the corallum was already recumbent before then. This evidence is provided by the distinctive features of the transition from regions 3 to 4 (Fig. 2B, D). Unlike other changes of growth direction that were minor and merely engendered alterations of orientation of the conical corallum, region 3 had its growth partly terminated (lower half of Fig. 2D) where the calice was buried and preserved in sediment. Region 4 grew in contact with only the upper half of region 3, that is, the part of the coral above the sediment surface, and they show a strong angular separation. The calice of region 4 is consequently more elliptical than that of region 3 as it sought to grow around the corner. The exposed base of the corallum of region 4 also grew new root-like rhizoids into the sediment for stabilisation (Fig. 2B, D). The coral, after toppling over, was fighting to survive and was growing around a tight corner, seeking to elevate the calice above the sediment surface, when it is presumed to have died by burial due to another influx of sediment. CONCLUSIONS Take nothing for granted. The Much Wenlock Limestone Formation has yielded many, but certainly not all, of its secrets to geologists over some hundreds of years. The specimen of Dokophyllum sp. discussed herein represents a well-known Wenlock coral, yet this unique individual has told a tale of life and death as detailed and riveting as many a novel. ACKNOWLEDGEMENTS Many thanks to Jill Darrell and Dr. Brian Rosen (both the Natural History Museum, London (BMNH)), who identified this specimen, provided much useful comment regarding its morphology and pointed me to the relevant literature. My research at the BMNH was supported by NCB Naturalis, Leiden. REFERENCES Ager, D.V. (1963). Principles of Paleoecology: An introduction to the Study of how and where Animals and Plants lived in the past. McGraw-Hill, New York, xi+371 pp. Bassett, M.G. ( ). The articulate brachiopods from the Wenlock Series of the Welsh Borderland and South Wales. Monograph of the Palaeontographical Society, London, 123 (525), 1-26 [1970]; 126 (532), [1972]; 128 (541), [1974]; 130 (547), [1977]; [in progress]. Clarkson, E.N.K. (1998). Invertebrate Palaeontology and Evolution. Fourth edition. Blackwell Science, Oxford, xvi+452 pp. Donovan, S.K. & Lewis, D.N. (2010). Aspects of crinoid palaeontology, Much Wenlock Limestone Formation, Wenlock Edge, Shropshire (Silurian). Proceedings of the Yorkshire Geological Society, 58, Donovan, S.K., Lewis, D.N. & Veltkamp, C.J. (2008). The cladid crinoid Gissocrinus Angelin from Wenlock Edge, Shropshire (Silurian). Proceedings of the Yorkshire Geological Society, 57, Donovan, S.K. & Sevastopulo, G.D. (1989). Myelodactylid crinoids from the Silurian of the British Isles. Palaeontology, 32, Eliason, S. (2000). Sunstones and Catskulls: Guide to the Fossils and Geology of Gotland. Gotlands Fornsal, Visby, Gotland, 166 pp. Herringshaw, L.G., Thomas, A.T. & Smith, M.P. (2007). Starfish diversity in the Wenlock of England. Palaeontology, 50, Hill, D. (1981). Treatise on Invertebrate Paleontology. Part F. Coelenterata. Supplement 1. Rugosa and Tabulata. In two volumes. Geological Society of America and University of Kansas Press, Boulder, Colorado and Lawrence, Kansas, xl + F1- F762. Proceedings of the Shropshire Geological Society, 17, Shropshire Geological Society

5 Hubbard, J.A.E.B. (1970). Sedimentological factors affecting the distribution and growth of Viséan caninioid corals in north-west Ireland. Palaeontology, 13, Murchison, R.I. (1839). The Silurian System. Founded on geological researches in the counties of Salop, Hereford, Radnor, Montgomery, Carmarthen, Brecon, Pembroke, Monmouth, Gloucester, Worcester, and Stafford; with descriptions of the coal-fields and overlying formations. In two parts. John Murray, London, xxxii+768 pp. Ray, D.C., Brett, C.E., Thomas, A.T. & Collings, A.V.J. (2010). Late Wenlock sequence stratigraphy in central England. Geological Magazine, 147, Scrutton, C.T. (1998). The Palaeozoic corals, II: structure, variation and palaeoecology. Proceedings of the Yorkshire Geological Society, 52, Siveter, David J., Owens, R.M. & Thomas, A.T. (1989). Silurian Field Excursions: A Geotraverse across Wales and the Welsh Borderland. National Museum of Wales, Cardiff, 133 pp. Siveter, Derek J. (2000). The Wenlock Series. In: (Aldridge, R. J., Siveter, David J., Siveter, Derek J., Lane, P. D., Palmer, D. & Woodcock, N. H.) British Silurian Stratigraphy. Geological Conservation Review Series. Joint Nature Conservation Committee, Peterborough, pp PALAEOECOLOGY OF A SOLITARY CORAL Copyright Shropshire Geological Society ISSN x Proceedings of the Shropshire Geological Society, 17, Shropshire Geological Society