Glycimeris, a fossil bivalve from the Red Crag

--- late Pliocene strata on the coast of Essex, southeast England

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Figure 1. Eight shells of the bivalve genus Glycimeris, from a set of 32 such shells, which vary in size from 52x50x10 mm to 27x25x4 mm. Most bivalves are broadly symmetrical across the hinge line, so, were these shells all matched, the 32 valves would have "clothed" sixteen of the molluscs that secreted them. Glycimeris is a widespread marine bivalve ranging in time from the early Cretaceous to the Recent, and is found across North America, in western Europe (including the British Eocene: Wells and Kirkaldy, 1966), the Bay of Bengal and elsewhere. Perforated Glycimeris shells have been found in Prehistoric necklaces in Spain.

"Rock of the Month #183, posted for September 2016" ---

The bivalve Glycimeris

is the most abundant fossil, to the naked eye, in the orange shelly sands that constitute the crumbling cliffs found at Walton-on-the-Naze, just northeast of the English seaside town of Clacton-on-Sea. The cliffs, along the southern coast of East Anglia, are geologically youthful, the rock ascribed to the upper Pliocene or early Pleistocene period, roughly 3.0 to 2.5 million years old (cf. Greensmith et al., 1973; Head, 1998). They suffer seasonal erosion from the storms of the North Sea. The cliffed promontory ("Naze"), backed by salt marshes around a marine inlet known as Hamford Water, is 220 km west of Rotterdam in the Netherlands, and 120 km northwest of Dunkirk, France.

More on Bivalves such as Glycimeris

Bivalves (often called pelecypods in North America, and referred to in older textbooks as lamellibranchs), together with cephalopods and gastropods, are the most common classes of mollusc. Many books refer to the genus Glycimeris (e.g., Morley Davies, 1961, pp.35-42; Anon, 1971; Black, 1972, pp.52-53; Turek et al., 1988; Chaumeton and Magnan, 2005). The shells shown here display typical morphology of bivalves, including:

  • A hinge near the umbo (the "beak" at the top of each shell, above the hinge line), about which the two valves are joined in life by a ligament.
  • The hinge is decorated by teeth which interlocked the two shells. There are many small teeth and sockets along the hinge plate ("taxodont" dentition).
  • A pair of adductor muscle scars inside each shell, to which were attached muscles controlling opening and closing of the two parts of the shell, attached to each other by the ligament.
  • Concentric growth bands on the outsides of each shell, and
  • Crenulated interior margins.

The Red Crag

The shells are shown in Figs. 1 and 2 against a background of the host rock, a shelly sand composed largely of carbonate shell fragments (bivalves ["cockles"], also gastropods ["whelks"] and coralline algae). The sand and the shells are darkened by rufous ferric iron oxyhydroxide (goethite or limonite). Water, carrying iron and other elements in solution, can easily percolate through the poorly consolidated young sediment. The Crag also contains cylindrical, cm-scale tubes with hollow cores, which probably represent better-cemented escape tubes by which water may have moved upwards under pressure. If this is the case, then these tubes might be viewed as pseudofossils - they are not the remains of organisms, true fossils, such as the bivalve shells. An alternative origin would be as burrows of organisms (such as crustaceans or worms) which lived under the sea floor. In this case, the tube-burrows are known as trace fossils (ichnofossils), evidence (such as footprints or vertebrate animals) which tell us about the mode of life of the organism that produced them.

Outliers of Red Crag are the oldest (late Pliocene to) Pleistocene deposits in the London Basin (Blezard et al., 1967). The Tertiary strata of the region contain occurrences of phosphate (coprolites, or fossil excrement, in the lower Red Crag), sand and gravel, brick earth and boulder clay (IGS, 1963). The Red Crag is noted for fossils such as Neptunea and Nucella, Hinea and Scaphella (gastropods), Pholas (Barnea), Cardium and (of course) Glycimeris (Chatwin, 1961).

The base of the Red Crag is undulating, with sediments infilling deep depressions in the underlying Eocene London Clay (Hunt, 1989). The Red Crag is also described by Hamblin et al., 1997) and in the regional synthesis by Gerald Lucy (1999, pp.53-60). The latter notes that, in addition to molluscs and other small fry, the Crag contains remains of larger creatures. Most notable are the jagged teeth of Carcharodon megalodon, the largest known shark, with teeth up to 10 cm long, and a likely body length that approached 12 metres.

The molluscan fauna of the Red Crag has been described on numerous occasions, early examples being Wood (1868), Kendall (1931) and Jukes-Browne (1886). The latter grouped the Miocene, Pliocene and Pleistocene strata as the "Icenian system" (ibid., pp.486-488). The shells of the Red Crag even merit a mention in books devoted more to rocks and minerals (e.g., Ellis, 1957). In the instance of the estate of a storied collector and dealer, John (Frederick) Calvert (1825-1897), an expert palaeontologist saw the collection in 1900 and opined that there were "several tons of rubbish, but he pulled out a few good Red Crag fossils" (Cooper, 2007, pp.85-105 and especially p.101).

As a secondary point of interest, amber (fossil tree resin) samples are known along the east coast of England, including sites such as Southwold in Suffolk and Walton-on-the-Naze. It has been demonstrated (Beck and Shennan, 1991) that the east coast finds are Baltic amber, not native resins.

Pliocene to Pleistocene records of fauna and flora can help to constrain the climate of the time, using the likes of marine gastropods (Coles and Higgs, 1969) and pollen (Hunt, 1989; Head, 1998). There is evidence of a warm spell in the late Pliocene, and cooler temperatures in the succeeding Pleistocene time (Hunt, 1989; Head, 1998). There was cooling near the start of the Pleistocene, at 2.55 Ma, with reduced species diversity in the Red Crag compared to the older Coralline Crag (Head, 1998).

The samples discussed here are F24 (the "iron pipes"), F31 (a large jar of fossiliferous Red Crag sand), and 7.153.1, Glycimeris glycimeris. The UTM grid reference for the locality is circa TM266235. Collection date(s) uncertain, certainly pre-1975, and probably in the period 1970-1973. NOTE that today the cliffs are a Site of Special Scientific Interest (SSSI), so direct collection from the cliff faces is prohibited. However, landslips ensure that there is plenty of loose material to examine below the cliffs, down to the beach.

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Figure 2. View of five shells. Note the small cylindrical holes in two shells, probably due to the rasping tongue of a predatory gastropod. Other shells may be invested by the outlines of coralline algae, which invested the shells after the death and decay of a shell's owner.

References, in Chronological Order

Wood,SV (1868) On the structure of the Red Crag. Quart.J.Geol.Soc.London 22, 538-552.

Jukes-Browne,AJ (1886) The Student's Handbook of Historical Geology. George Bell and Sons, Covent Garden, London, 597pp.

Kendall,PF (1931) The Red Crag of Walton-on-the-Naze. Geol.Mag. 68, 405-420.

Ellis,C (1957) The Pebbles on the Beach. Faber and Faber Limited, London, 2nd edition, 163pp.

Chatwin,CP (1961) British Regional Geology. East Anglia. Institute of Geological Sciences / HMSO, London, 4th edition, 102pp.

Morley Davies,A (1961) An Introduction to Palaeontology. Thomas Murby & Company, London, 3rd edition, 322pp.

IGS (1963) Woodbridge & Felixstowe. Institute of Geological Sciences map, England and Wales, sheet 208 & 225, 1:63,360 scale, solid and drift edition.

Wells,AK and Kirkaldy,JF (1966) Outline of Historical Geology. Thomas Murby & Co., London, 6th edition, 533pp.

Blezard,RG, Bromley,RG, Hancock,JM, Hester,SW, Hey,RW and Kirkaldy,JF (1967) The London Region (North of the Thames). Geologists' Association Guide 30A, 34pp.

Coles,JM and Higgs,ES (1969) The Archaeology of Early Man. Penguin Books, 454pp.

Anon (1971) British Caenozoic Fossils. British Museum (Natural History), London, 4th edition, 132pp.

Black,RM (1972) The Elements of Palaeontology. Cambridge University Press, 339pp.

Greensmith,JT, Blezard,RG, Bristow,CR, Markham,R and Tucker,EV (1973) The Estuarine Region of Suffolk and Essex. Geologists' Association Guide 12, 41pp.

Turek,V, Marek,J and Benes,J (1988) Fossils of the World: a Comprehensive Practical Guide to Collecting and Studying Fossils. Arch Cape Press, New York, 1990 edition, 495pp.

Hunt,CO (1989) The palynology and correlation of the Walton Crag (Red Crag Formation, Pliocene). J.Geol.Soc.London 146, 743-745.

Beck,C and Shennan,S (1991) Amber in Prehistoric Britain. Oxbow Books, Oxford, Oxbow Monograph 8, 232pp.

Hamblin,RJO, Moorlock,BSP, Booth,SJ, Jeffery,DH and Morigi,AN (1997) The Red Crag and Norwich Crag formations in eastern Suffolk. Proc.Geol.Assoc. 108, 11-23.

Head,MJ (1998) Pollen and dinoflagellates from the Red Crag at Walton-on-the-Naze, Essex: evidence for a mild climatic phase during the early late Pliocene of eastern England. Geol.Mag. 135, 803-817.

Lucy,G (1999) Essex Rock, a Look Beneath the Essex Landscape. Essex Rock and Mineral Society, 128pp.

Chaumeton,H and Magnan,D (2005) Fossielen. Konemann, Tandem Verlag GmbH, 431pp. (in Dutch).

Cooper,MP (2007) Robbing the Sparry Garniture: A 200-Year History of British Mineral Dealers. Mineralogical Record, 1st edition, 358pp., page 101.

Graham Wilson, 05 September 2016, last modified 08 January 2017

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