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The Herefordshire Lagerstätte: Fleshing out Silurian Marine Life

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The Herefordshire Lagerstätte: Fleshing out Silurian Marine Life Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021 Review Focus Journal of the Geological Society Published online October 10, 2019 https://doi.org/10.1144/jgs2019-110 | Vol. 177 | 2020 | pp. 1–13 The Herefordshire Lagerstätte: fleshing out Silurian marine life Derek J. Siveter1,2*, Derek E. G. Briggs3, David J. Siveter4 & Mark D. Sutton5 1 Earth Collections, University Museum of Natural History, Oxford OX1 3PW, UK 2 Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK 3 Department of Geology and Geophysics and Yale Peabody Museum of Natural History, Yale University, PO Box 208109, New Haven, CT 06520-8109, USA 4 School of Geography, Geology and the Environment, University of Leicester, Leicester LE1 7RH, UK 5 Department of Earth Sciences and Engineering, Imperial College London, London SW7 2BP, UK DeJS, 0000-0002-5305-2192; DEGB, 0000-0003-0649-6417; DaJS, 0000-0002-1716-5819; MDS, 0000-0002-7137-7572 * Correspondence: [email protected] Abstract: The Herefordshire Lagerstätte (c. 430 Ma) from the UK is a rare example of soft-tissue preservation in the Silurian. It yields a wide range of invertebrates in unparalleled three-dimensional detail, dominated by arthropods and sponges. The fossils are exceptionally preserved as calcitic void infills in early diagenetic carbonate concretions within a volcaniclastic (bentonite) horizon. The Lagerstätte occurs in an outer shelf/upper slope setting in the Welsh Basin, which was located on Avalonia in the southern subtropics. The specimens are investigated by serial grinding, digital photography and rendering in the round as ‘virtual fossils’ by computer. The fossils contribute much to our understanding of the palaeobiology and early history of the groups represented. They are important in demonstrating unusual character combinations that illuminate relationships; in calibrating molecular clocks; in variously linking with taxa in both earlier and later Paleozoic Lagerstätten; and in providing evidence of the early evolution of crown-group representatives of several groups. Received 9 July 2019; revised 19 August 2019; accepted 21 August 2019 The Herefordshire Lagerstätte is unique in preserving representa- bentonite is unique in the British stratigraphic record, at least, in tives of many major invertebrate groups in high-fidelity 3-D, as reaching a thickness of more than 1 m in places. The bentonite was calcite in-fills within concretions in a marine volcaniclastic deposit. deposited within mudstones of the Wenlock Series Coalbrookdale The Herefordshire fossils are unimpressive at first sight, yet they Formation (Fig. 1d), which rests on the slightly older Dolyhir and yield unrivalled evidence about the palaeobiology and evolutionary Nash Scar Limestone Formation. In the Dolyhir region these coral significance of Silurian animals. They provide, for example, the and algal-rich limestones lie with angular unconformity on a earliest evidence for the time of origin of certain major crown- Precambrian sedimentary basement (Woodcock 1993). At nearby groups (i.e. the living biota), for instance sea spiders (pycnogonids). Nash Scar, laterally equivalent carbonates sit, possibly disconform- The locality lies in the Welsh Borderland, a historic region central ably (Woodcock 1993), on the shallow-marine, upper Llandovery to R. I. Murchison’s pioneering research which established the Series Folly Sandstone Formation and a hardground is developed Silurian System in the 1830s. In 1990 Bob King, mineralogist and locally on the limestones (Hurst et al. 1978). retired Curator in the Department of Geology at the University of The host bentonite lies approximately at the boundary between Leicester, collected concretions in float from the locality and the Sheinwoodian and Homerian stages (Fig. 1d), which is presented nine of them to the Leicester collections. Curator Roy radiometrically constrained to 430.5 ± 0.7 Ma (Cohen et al. 2013, Clements asked David Siveter to examine one of the concretions updated 2018). Various evidence from brachiopods, graptolites and when he was accessioning the material in 1994. What David conodonts indicates a Wenlock age for the Dolyhir and Nash Scar recognized under the microscope, an arthropod with preserved Limestone Formation in addition to the overlying Coalbrookdale limbs (later named Offacolus kingi), was confirmed by Derek Formation (Bassett 1974; Cocks et al. 1992). The Sheinwoodian Siveter and, together with Bob King, they identified the conodont Ozarkodina sagitta rhenana has been recovered from the stratigraphic horizon and collected more concretions. In 1995 limestones at Dolyhir and Nash Scar (Bassett 1974; Aldridge et al. Derek Briggs joined the team and, following more visits to the 1981). The presence of Homerian Cyrtograptus lundgreni Biozone locality, we published an initial paper flagging the significance of graptolites in the Coalbrookdale Formation mudstones immediately the discovery (Briggs et al. 1996). NERC and Leverhulme Trust above the limestones at Nash Scar (Bassett 1974) suggests that funding enabled Patrick Orr and later Mark Sutton to work on the deposition of these mudstones did not commence locally until Lagerstätte, initially as postdoctoral researchers. The support and C. lundgreni times (Hurst et al. 1978). This is consistent with the co-operation of the site owners and help from English Nature have presence of the limestone hardground that presumably formed been crucial in realizing its scientific potential. during a hiatus in deposition in the late Sheinwoodian. The recovery of the chitinozoans Margachitina margaritana, Locality, age and palaeogeographical setting Ancyrochitina plurispinosa and Cingulochitina cingulata from 20 to 50 cm below the bentonite also indicates an upper Sheinwoodian The Herefordshire site lies in the Old Radnor to Presteigne area of to Homerian age for the Coalbrookdale Formation (G. Mullins, the Welsh Borderland (Fig. 1a) in the vicinity of the Church Stretton pers. comm. 2000; see also Steeman et al. 2015). That age is Fault Zone. The fossil-bearing concretions (Fig. 1b and c) occur in a unchallenged by ostracods (David Siveter, unpublished informa- soft, fine-grained, cream-coloured, weathered and largely uncon- tion) and radiolarians (Siveter et al. 2007a) recovered from solidated bentonite (Fig. 1b) that crops out over about 30 m. The concretions in the bentonite. © 2019 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021 2 D. J. Siveter et al. Fig. 1. Provenance and palaeogeography. (a) Welsh Borderland location of the Herefordshire Lagerstätte with regional geology. (b) The volcanic ash (bentonite), with concretions in situ, in contact with Coalbrookdale Formation shales above. (c) Typical concretion, 6 × 4 cm, concentrically weathered, lacking the blue-grey-hearted centre of calcium carbonate that is present in partially weathered examples, and incorporating an eccentric specimen of the aplacophoran Acaenoplax.(d) Local stratigraphy of the Dolyhir–Presteigne area. Radiometric dates are those given in Cohen et al. (2013, updated 2018), the International Commission on Stratigraphy (ICS) Chronostratigraphic Chart. Cramer et al. (2012) give marginally different radiometric dates for some of the boundaries based on sampling from Gotland and the West Midlands, UK, for example 429.5 Ma for the Sheinwoodian–Homerian boundary. However, the majority margin of error on these dates (±0.7 Ma) provides overlap with the dates given in the ICS Chronostratigraphic Chart. (e) Welsh Basin palaeogeography at approximately the Sheinwoodian–Homerian boundary (modified from Cherns et al. 2006, fig. 4.13a). (f) Eastern Laurussian palaeogeography during the Wenlock (modified from Cocks & Torsvik 2005, fig. 9). Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021 The Herefordshire Lagerstätte 3 The volcanic ash in which the Herefordshire Lagerstätte is constituent of the void fill. Clay minerals precipitated on and preserved accumulated in the Welsh Basin (Fig. 1e). This lay on the underneath the exoskeleton, and in some examples the gut trace was microcontinent of Avalonia, which included southern Britain replicated in calcium phosphate. Ferroan dolomite (ankerite) formed (Fig. 1f; Cocks & Torsvik 2011). Avalonia and adjacent Baltica at a later stage perhaps coincident with the transformation on burial were in southern tropical/subtropical palaeolatitudes in the mid- of smectite to illite. Abundant radiolarians are evident on the surface Silurian, separated from Laurentia by a remnant Iapetus Ocean. of split concretions (Siveter et al. 2007a; Fig. 5x). Their Shallow-water muds characterized much of the Welsh Borderland preservation shows parallels to that of Offacolus (Orr et al. 2002). and central England east of the Church Stretton Fault Zone (Bassett Calcite and pyrite precipitated in the void vacated by the cytoplasm et al. 1992; Cherns et al. 2006). Coeval fine clastics and turbidites within the radiolarian tests; this space was not invaded by matrix. accumulated to the west in the deepest parts of the Welsh Basin. The precipitation of calcite retained the shape of the test when the Palaeogeographical and sedimentological evidence indicates that opaline silica that formed it was replaced during diagenesis
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