Above: Graptolites: Figure from Jukes-Browne (1886, p.87).
Below: Ordovician slate, forming a slab split to reveal graptolite fossils. The fossils are identified as the species Orthograptus calcaratus priscus. Individuals are 10-22 mm long, as exposed in the plane of the slaty parting. The slate is a tabular slab, 25x11 cm by a maximum thickness of 2.7 cm. Weight is 1341.48 grams. The magnetic susceptibility averages 0.112x10-3 SI units, a typical result for metasediments. Sample 15.41.2.
"Rock of the Month # 111, posted for September 2010" ---
Graptolite fossils represent are a fascinating group of long-extinct marine life forms. Like their contemporaries the trilobites, popular interest derives at least in part from the lack of modern descendants. Orthograptus, a form of Diplograptus, may have may have been a planktonic colonial form, suspended at the water surface by a float (Ruedemann, 1895; Morley Davies, 1972, pp.179-181). A good pictorial introduction to Paleozoic fossils remains a venerable British Museum publication (Anon, 1969). Graptolites are now the focus of a detailed literature, thanks to enthusiasts like Nancy Kirk (Wyatt, 2007), Oliver Bulman (Bulman, 1970; Stubblefield, 1975) and many others.
The sample was collected on 22 August 1973 from Ordovician shale in a quarry at the south end of Abereiddy Bay, along the northern coast of the county of Pembrokeshire (now Dyfed), 7.2 km due northeast of the village of St. David's. The area of Abereiddy Bay has long been known for its locally abundant fossils of upper Llanvirn age (Wells and Kirkaldy, 1966, pp.64-65; Neville George, 1970, p.26). It lies on the north coast of Dyfed, in the area known until 1974 as the county of Pembrokeshire, 700 m northwest of the hamlet of Llanvirn (Ordnance Survey, 1965). The locality, with its beach quarry in upper Llanvirn Didymograptus murchisoni shales, is noted for "tuning fork" graptolites (Bloxam, 1971, pp.204-205). Local species include Orthograptus calcaratus priscus and Didymograptus murchisoni. These strata are approximately 463 Ma in age, and in international terms are assigned to the Darriwilian stage (Zalasiewicz et al., 2009).
Like ammonites in the Mesozoic, the importance of graptolites in Paleozoic stratigraphy is beyond question (Wells and Kirkaldy, 1966; Bennison and Wright, 1969; Zalasiewicz et al., 2009). While typical graptolites are cm- to dm- sized, examples in excess of 1 metre have been recorded (Loydell and Loveridge, 2001). No doubt more remain to be discovered, adding to our knowledge of the fossil record (Foote and Sepkoski, 1999). Comparison of graptolite faunas is critical to paleogeographic interpretations, including the history of the vanished Iapetus Ocean whose Paleozoic sediments and fossils are found from eastern North America across the British Isles to Scandinavia (Williams et al., 1992; Lenz et al., 1993; Zalasiewicz, 2001).References
Anon (1969) British Palaeozoic Fossils. British Museum (Natural History), London, 3rd edition, 208pp.
Bennison,GM and Wright,AE (1969) The Geological History of the British Isles. Edward Arnold (Publishers) Ltd, London, 406pp.
Bloxam,TW (1971) Haverfordwest, Strumble Head and Abereiddy Bay. In `Geological Excursions in South Wales & The Forest of Dean' (Bassett,DA and Bassett,MG, editors), Geologists' Association, South Wales Group, Cardiff, 267pp., 199-205.
Bulman,OMB (1970) Graptolithina: with Sections on Enteropneusta and Pterobranchia. Treatise on Invertebrate Paleontology, volume TRE-V, 195pp., 2nd edition.
Foote,M and Sepkoski,JJ (1999) Absolute measures of the completeness of the fossil record. Nature 398, 415-417.
Jukes-Browne,AJ (1886) The Student's Handbook of Historical Geology. George Bell and Sons, Covent Garden, London, 597pp.
Lenz,AC, Jin,J, MacCracken,AD, Utting,J and Westrop,SR (1993) Paleozoic biostratigraphy. Geoscience Canada 20, 41-73.
Loydell,DK and Loveridge,RF (2001) The world's longest graptolite? Geol.J. 36, 55-57.
Morley Davies,A (1961) An Introduction to Palaeontology. Thomas Murby & Company, London, 3rd edition, 322pp.
Neville George,T (1970) British Regional Geology: South Wales. IGS / HMSO, 3rd edition, 154pp.
Ordnance Survey (1965) Fishguard & Pembroke. Ordnance Survey one-inch map 138/151, 1:63,360 scale.
Ruedemann,R (1895) Synopsis of the mode of growth and development of the graptolitic genus Diplograptus. Amer.J.Sci. 149, 453-455.
Stubblefield,J (1975) Oliver Meredith Boone Bulman, 20 May 1902-18 February 1974. Biographical Memoirs of Fellows of the Royal Society 21, 175-195.
Wells,AK and Kirkaldy,JF (1966) Outline of Historical Geology. Thomas Murby & Co., London, 6th edition, 533pp.
Williams,SH, Boyce,WD and Colman-Sadd,SP (1992) A new Lower Ordovician (Arenig) faunule from the Coy Pond complex, central Newfoundland, and a refined understanding of the closure of the Iapetus Ocean. Can.J.Earth Sci. 29, 2046-2057.
Wyatt,AR (2007) Nancy Kirk: turning the world of graptolites upside down. In `The Role of Women in the History of Geology' (Burek,CV and Higgs,B editors), Geol.Soc. Spec.Publ. 281, 342pp., 325-333.
Zalasiewicz,JA (2001) Graptolites as constraints on models of sedimentation across Iapetus: a review. Proc.Geol.Assoc. 112, 237-251.
Zalasiewicz,JA, Taylor,L, Rushton,AWA, Loydell,DK, Rickards,RB and Williams,M (2009) Graptolites in British stratigraphy. Geol.Mag. 146, 785-850.
Below: Graptolites shown as impressions on either side of the plane of slaty parting in the host rock, as revealed by a careful chisel blow to the side of a thin block of slate. The fossils are identified with Orthograptus calcaratus priscus.
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