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Graptolites in British Stratigraphy

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Graptolites in British Stratigraphy Geol. Mag. 146 (6), 2009, pp. 785–850. c Cambridge University Press 2009 785 doi:10.1017/S0016756809990434 Graptolites in British stratigraphy J. A. ZALASIEWICZ*†, L. TAYLOR*, A. W. A. RUSHTON‡,D.K.LOYDELL§, R. B. RICKARDS¶ & M. WILLIAMS* ∗ Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK ‡Department of Paleontology, Natural History Museum, Cromwell Road, London, SW7 5BD, UK §School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK ¶Department of Earth Sciences, University of Cambridge, Sedgwick Museum, Downing Street, Cambridge CB2 3EQ, UK (Received 2 January 2008; accepted 25 September 2008; First published online 9 September 2009) Abstract – 697 taxa of planktonic graptolites are recorded, and their stratigraphical ranges are given, through 60 biozones and subzones in the Ordovician and Silurian strata of England, Wales and Scotland, in the first such stratigraphical compilation for Great Britain since the synthesis of Elles & Wood (1901–1918). Keywords: graptolites, graptolite zones, biozones, biostratigraphy, Britain. 1. Introduction to the monograptids, which diversified to dominate the Silurian and Devonian seas worldwide (although Graptolites are extinct colonial hemichordates, gener- in Great Britain, graptolites disappeared from the ally considered to be closely related to the present-day shallowing marine basins late in the Ludlow). Normal pterobranchs. They range in age from the middle of diplograptids persisted for a short while into the the Cambrian to the Carboniferous. The graptolites Silurian, while retiolitid (‘meshwork’) graptolites were include the exclusively planktonic graptoloids, the locally common and survived into the Ludlow. The largely benthic dendroids and also the benthic crust- morphologically diverse and rapidly evolving mono- oids, tuboids, cameroids and dithecoids (Rickards & graptids provide a fine resolution for the Silurian, Durman, 2006). The graptoloids are the focus of this with graptolite zones lasting, on average, well under account. They provide the primary means of correlation a million years (Rickards, 1976, 1989; Zalasiewicz, of Ordovician and Silurian strata in the UK, and 1990; Hughes, 1995; Melchin, Cooper & Sadler, 2004); are fundamental to resolving the stratigraphical and by contrast, the duration of graptolite zones in the structural architecture of these rocks (e.g. Zalasiewicz, British Ordovician averages c. 2 Ma (Rushton, 1990, 2001), which were laid down in sedimentary basins on cf. Cooper & Sadler, 2004). the margins of the Palaeozoic Iapetus Ocean. Major outcrops are in the Southern Uplands of Scotland, the Lake District and the Howgill Fells of northern England, and Wales and the Welsh Borderland. Outside 2. Palaeoecology, provincialism and distribution Great Britain, graptolites are important also in early Graptolites are commonly held to be ‘ideal’ zone Devonian successions. fossils, because they were widely distributed in marine The British graptolite biozonal and subzonal waters and so not bound by facies. The situation, schemes are shown in Figures 1 and 2. In the though, is not as simple as this. The graptoloids were early Ordovician, planktonic dendroids, and then the probably holoplanktonic, although there is still much dichograptid graptoloids, were the most important debate about whether they floated more or less passively groups. These were succeeded by the diplograptids, (e.g. Bulman, 1964; see also discussion in Rigby & which were dominant throughout the rest of the Rickards, 1990 and Palmer & Rickards, 1991) or Ordovician. Dicranograptids and nemagraptids were actively propelled themselves through the water (e.g. also important elements at various times in the mid- to Kirk, 1978; Bates & Kirk, 1984, 1985; Rickards et al. late Ordovician. (Such terms are used here in a general 1998; Melchin & DeMont, 1995). sense; higher-level graptolite taxonomy is discussed Planktonic graptolites have long been interpreted in more detail in Mitchell, 1987 and Mitchell et al. as largely ‘open ocean’ dwellers, common in off- 2007). shore pelagic and hemipelagic sequences (‘graptolite Following near-extinction during the latest Or- facies’), and rare or absent in shallow water deposits dovician glaciation, a few species of diplograptids (‘shelly facies’). Subsequent elaborations of this gen- survived into the earliest Silurian. These gave rise eral observation included suggestions that graptolites were subject to depth control, with near-surface and †Author for correspondence: [email protected] deep-living taxa (e.g. Berry & Boucot, 1972; Bates & Downloaded from https:/www.cambridge.org/core. Open University Library, on 18 Jan 2017 at 09:15:53, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756809990434 786 J. A. ZALASIEWICZ AND OTHERS Figure 1. British Ordovician graptolite biozones and subzones. Zonal scheme based on that proposed by Fortey et al. (1995), and adopted in the Ordovician Correlation Report of Fortey et al. (2000), with subsequent modification to the Arenig to early Llanvirn by Cooper et al. (2004), refinement of the Caradoc by Bettley, Fortey & Siveter (2001), while the Caradoc/Ashgill section of England and Wales shows the modified correlations suggested by Rickards (2002). Chronostratigraphy and radiometric dates after Ogg, Ogg & Gradstein (2008). Downloaded from https:/www.cambridge.org/core. Open University Library, on 18 Jan 2017 at 09:15:53, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756809990434 Graptolites in British stratigraphy 787 Figure 2. British Silurian graptolite biozones and subzones. Zonal scheme follows Rickards (1976, depicted) with modifications from Loydell (1992–1993a), Zalasiewicz (1994), Loydell & Cave (1993, 1996), Zalasiewicz & Williams (1999) and those proposed herein. Chronostratigraphy and radiometric dates after Ogg, Ogg & Gradstein (2008). Kirk, 1984; Erdtmann, 1976) or that they were competition from other (soft-bodied) macrozoo- controlled by ‘water mass specificity’ with particular plankton may also have restricted their occurrence assemblages of taxa adapted to particular conditions (Zalasiewicz, 2001). Finney & Berry (1997) disputed of temperature and chemistry (Finney, 1986), while the notion of graptolites as truly ocean-going, noting Downloaded from https:/www.cambridge.org/core. Open University Library, on 18 Jan 2017 at 09:15:53, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756809990434 788 J. A. ZALASIEWICZ AND OTHERS their absence from many deep-water, anoxic deposits, ‘balloon’ taxa. There are many instances of the type ma- and suggested that they were largely confined to the terial of a ‘classical’ species being too poorly preserved, region of the outer shelf and continental slope. or inadequately described, to serve as a reference Provincialism in the graptolites was particularly specimen by modern standards. It has been common, marked in the Ordovician, with well-defined ‘Atlantic’ also, for several distinct taxa to be described, at various and ‘Pacific’ provinces between which it is often times, as the same species. Conversely, Loydell (1993b, difficult to correlate. This may have been due in pp. 330–1) has shown how Stomatograptus longus part to pronounced climatic gradients (Skevington, Obut, 1949 has, over time, acquired five additional 1974), the Atlantic province representing temperate, species names (junior synonyms). There remains and the Pacific representing equatorial, waters. More much taxonomic ‘housekeeping’ to do, much of it local provincialism is also common, however, with straightforward but time-consuming. Better definition apparently coeval faunas of markedly different com- of taxa, for instance by the wholesale refiguring of position being reported from different regions of the type material (Zalasiewicz et al. 2000; Zalasiewicz & USA interior (Finney, 1986), and from the western Rushton, 2008) should lead to greater refinement in and eastern parts of the Welsh Basin (e.g. compare the graptolite biostratigraphy. faunal successions in Hughes, 1989 and Zalasiewicz, 1992a). Provincialism is less pronounced in the Silurian 4. Preservation (Melchin, 1989), and the British biozonal system can be applied for much of the Silurian throughout much of the The widespread occurrence of graptolites in British world, with relatively minor modifications (see Koren’ early Palaeozoic successions is due to a phenomenon et al. 1996 and Melchin, Cooper & Sadler, 2004). which is absent from recent oceans: that of prolonged, widespread periods of sea-floor anoxia (Page et al. 2007 and references therein). Thus, graptolites are typ- 3. Taxonomy ically found in finely laminated hemipelagic deposits One of the fundamental constraints on graptolite (‘graptolite shales’) laid down in anoxic conditions biostratigraphy is the ability to discriminate consist- that excluded benthos. In Britain, graptolites are ently between taxa. Many graptolites have a complex generally rare or absent in rocks that were laid down morphology, with many identifiable features that can under oxygenated sea-floor conditions and that were be measured and tabulated. Nevertheless, the identific- colonized, and bioturbated, by a benthic fauna (e.g. ation of graptolites is not by any means universally Davies et al. 1997). straightforward,
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