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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 marine

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 in unparalleled three-dimensional detail, dominated by and . The 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 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 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 . 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- 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 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 , 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 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 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 -bearing concretions (Fig. 1b and c) occur in a unchallenged by (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

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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 above. (c) Typical concretion, 6 × 4 cm, concentrically weathered, lacking the blue-grey-hearted centre of 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 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 by a the Coalbrookdale Formation in the Dolyhir–Nash Scar area of the mixture of ankerite and diagenetic clay minerals; replacement of Church Stretton Fault Zone accumulated in a moderately deep, outer silica by ankerite has also been observed in siliceous - shelf/shelf slope environment (Briggs et al. 1996); the presence of spicules (Nadhira et al. 2019). Although there are similarities with the Visbyella community suggests water depths of the diagenetic sequence displayed by radiolarians in concretions 100–200 m (Hurst et al. 1978; Brett et al. 1993). Geochemical elsewhere (Holdsworth 1967; Orr et al. 2002), the nature of the analyses suggest a destructive plate margin as the source of the preservation of soft tissue morphology at the Herefordshire site volcanic ash (Riley 2012). The closest known volcanic centres of remains unique. Wenlock age (Cocks & Torsvik 2005) are the Mendip Hills in SW England (Avalonia) and the Dingle peninsula in SW Ireland Releasing the information from the fossils (Avalonia). The Herefordshire bentonite is geochemically similar to volcanic rocks in both areas (Riley 2012). A more distant candidate The Herefordshire concretions are collected in bulk using earth is the Czech Republic, then on Perunica, a microcontinent in the moving equipment. Each scoop is tipped slowly on to a waste-pile; centre of the Rheic Ocean (Fig. 1f). concretions roll out and are collected manually. The concretions are split repeatedly in the laboratory with a manual rock-splitter; the Taphonomy process is halted if a fossil is found (Fig. 3; c. 50% of concretions). Specimens are identified provisionally, photographed with a Leica The volcanic ash that hosts the Herefordshire biota was deposited on DFC420 digital camera mounted on a Leica MZ8 binocular an erosion surface on massive limestone and, in places, muds; the microscope with a thin layer of water over the specimen to enhance uneven topography likely reflects the original Wenlock seafloor. contrast, and catalogued in a custom online database. The ash layer varies from a few tens of centimetres to over 1 m thick The fossils record copious anatomical information but present and is highly weathered at least in outcrop: no evidence for primary challenges for its extraction. Manual or chemical preparation of sedimentary structures is preserved. Thus, it is unclear whether the specimens (see e.g. Sutton 2008) is impractical. Study of the ash represents in situ settling or was reworked, or whether there was Herefordshire fauna is unique among invertebrate Lagerstätten in one or several ashfalls. Occasional examples of unsuccessful escape relying almost exclusively on virtual reconstructions. Indeed, traces (Orr et al. 2000) indicate that at least some of the animals were research on the fossils has played an important role in the buried alive, which is consistent with the remarkable preservation. development of such techniques and their wider application The bentonite has yielded at least five thousand randomly (Sutton et al. 2001b, 2014). Non-destructive approaches to data- distributed calcite-cemented (Fig. 3) concretions 2–25 cm in capture, such as X-ray computed tomography, are clearly preferable diameter (Orr et al. 2000; Fig. 1b and c). Unusually for fossils as a basis for virtual reconstructions but are largely ineffective for preserved in this way, the organisms do not appear to have Herefordshire fossils: X-ray absorption contrast between fossil and influenced the size or shape of the concretions, nor are they usually matrix is very low. Limited success has been achieved with some in the centre (Fig. 1c). Fossils are only preserved where they were specimens by using phase-contrast synchrotron tomography (see incorporated into concretions but it is not clear what determined the Sutton et al. 2014, p. 78), but a destructive yet highly effective locus of concretion formation. The timing and nature of the growth physical-optical tomography ‘serial grinding’ technique is, of of the Herefordshire concretions merits further investigation. necessity, our method of choice. This takes advantage of the The fossils show high-fidelity three-dimensional preservation – strong optical contrast between specimen and matrix and has been the threads that attach the juveniles to the arthropod Aquilonifer used to reconstruct over 100 specimens. (Briggs et al. 2016), for example, are less than 50 µm in diameter Specimens for reconstruction are trimmed with a fine rock-saw to (Fig. 4h). About half the taxa lacked biomineralized hard parts and c. 10 mm cuboids and mounted on a Buehler ‘slide holder’ which the biomineralized taxa preserve remarkable details in addition to consists of an inner metal cylinder which can be adjusted to elevate features preserved in the normal fossil record. The soft tissues show specimens a precise distance above a hard outer ceramic disc. negligible collapse (Fig. 3a and d) indicating that the ash stiffened Specimens are ground to remove a consistent thickness (20–50 µm), and recorded the outer morphology of the animals before significant washed, and photographed. This process is repeated until the entire decay took place. The spherical morphology of the concretions specimen has been digitized, typically in 200–400 increments. Part suggests that they formed rapidly, before compaction. The and counterpart are ground separately, and large specimens are fossils are preserved as voids that were infilled with calcite (Fig. 3) processed in several pieces which are re-united digitally, following which also dominates the concretion cement (Orr et al. 2000). The reconstruction, using the custom SPIERS software suite (Sutton biomineralized skeletons of brachiopods, gastropods, and et al. 2012b). Reconstruction begins with manual or semi-automatic ostracods are preserved, but organic remains, even non-biominer- registering (aligning) of photographs using the cut edges of cuboids alized arthropod , are lost – the periderm of the rare as a guide. Data are then prepared for reconstruction by carefully graptolites is an exception. Preservation was investigated in detail in distinguishing pixels representing the fossil from those correspond- the arthropod Offacolus (Orr et al. 2000). Sparry calcite infilled the ing to matrix. Software exists to automate this process but an external mould of Offacolus, sometimes with an initial phase of understanding of the taphonomy and likely of the finer fibrous calcite around the periphery (Figs 3a, 4c and f). Pyrite specimen, and of artefacts generated by the process (e.g. ‘out-of- precipitated on the boundary between calcite crystals but as a minor plane’ structures visible through bubbles in the encasing resin), is Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

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Box 1. The faunal composition of the Herefordshire Lagerstätte Thirty-two , including two under open nomenclature, have been described from the Herefordshire Lagerstätte, and at least 29 await description (Table 1). Some 75% of Herefordshire species (excluding radiolarians) are non-biomineralized but this reflects bias in initial taxa selected for study and may fall to c. 50% when other biomineralized taxa, including trilobites and the extensive sponge fauna, are described. Arthropods are diverse (Fig. 2b): 19 species have been recorded, 17 of which have been fully described. They include representatives of most major euarthropod groups: megacheirans, pycnogonids, chelicerates, trilobites, , stem mandibulates and pancrustaceans (this last by far the most species-rich, with 9 described species) as well as one lobopodian. Sponges are the most abundant and diverse of the other major groups, probably comprising at least 20 species, although all but one await detailed investigation. Molluscs are represented by two species of aplacophorans and two gastropods (one uninvestigated), one bivalve and an orthoconic nautiloid (uninvestigated). There are at least four species, an asteroid, edrioasteroid, ophiocistioid and a (the last uninvestigated), at least three brachiopods (one an uninvestigated lingulid), an , possibly two cnidarians (a coral and a possible hydrozoan, both uninvestigated), and two (a graptoloid and a possible dendroid, both uninvestigated). The large number of unidentified specimens (1506; 41.0%) reflects poorly preserved examples and also the difficulty of identifying specimens on split concretions prior to reconstruction. Radiolarians are not included in the faunal composition assessment (Fig. 2a) because of the difficulty in estimating their specimen numbers (which are in the hundreds or possibly thousands). Additionally, a few thousand concretions remain unsplit.

The chelicerate Offacolus kingi is the most abundant species at more than 800 specimens (Fig. 2b), an order of magnitude more than the mandibulate Tanazios dokeron and the malacostracan Cinerocaris magnifica. Most of the arthropods are known from just a few specimens (e.g. the megacheiran Enalikter aphson), or single individuals, such as the durgae. Of non-arthropod taxa the annelid Kenostrychus clementsi is the most abundant at over 250 specimens. Many of the non-arthropods, however, such as the edrioasteroid Heropyrgus disterminus, are based on a few tens of specimens and some, such as the bivalve Praectenodonta ludensis, on one specimen only.

Table 1. Faunal composition of the Herefordshire Lagerstätte Tanazios dokeron (Siveter et al. 2007d; Fig. 5p and t) Sponges Carduispongia pedicula (Nadhira et al. 2019; Fig. 5y) Malacostracans Some 20 other species (uninvestigated) Cinerocaris magnifica (Briggs et al. 2003; Fig. 5c and d) Cnidarians Cascolus ravitis (Siveter et al. 2017; Fig. 5l and m) ? Hydroid (uninvestigated colonial organism) Cirripedes Coral (uninvestigated) Rhamphoverritor reduncus (Briggs et al. 2005; Brachiopods Fig. 4n and o) Bethia serraticulma (Sutton et al. 2005a; Fig. 4k) Ostracods At least two other brachiopod species (both uninvestigated), one of - Myodocopes them a lingulid Colymbosathon ecplecticos (Siveter et al. 2003; Fig. 4a) Lophophorate indet. Nymphatelina gravida (Siveter et al. 2007c; Drakozoon kalumon (Sutton et al. 2010; Fig. 4j) Fig. 4b, q and r) Nasunaris flata (Siveter et al. 2010; Fig. 5z) Kenostrychus clementsi (Sutton et al. 2001c; Fig. 5s) Pauline avibella (Siveter et al. 2013; Fig. 5zz) Molluscs Spiricopia aurita (Siveter et al. 2018a; Fig. 5u) Aplacophorans - Podocopes and palaeocopes Fragments of unidentified species belonging to these groups have been Acaenoplax hayae (Sutton et al. 2001a; Fig. 4s and t) recovered from acid residues Kulindroplax perissokomos (Sutton et al. 2012a; Fig. 4l and m) Pentastomids Invavita piratica (Siveter et al. 2015b, Fig. 4p, q and r) Praectenodonta ludensis Reed, 1931 (Fu 2016) Gastropods Asteroids Platyceras? sp. (of Sutton et al. 2006; Fig. 5v) Bdellacoma sp. (of Sutton et al. 2005b; Fig. 5f–i) A high spired species (uninvestigated) Edrioasteroids Heropyrgus disterminus (Briggs et al. 2017; Fig. 5j and k) Nautiloids (uninvestigated) (uninvestigated sp.) Panarthropods Ophiocistioids Lobopodians Sollasina cthulhu (Rahman et al. 2019; Fig. 5q) Thanahita distos (Siveter et al. 2018b; Fig. 5a and b) Hemichordates Megacheirans Pterobranchs Enalikter aphson (Siveter et al. 2014a, 2015a; Fig. 5e) Graptoloids (uninvestigated) Pycnogonids Dendroids? (uninvestigated) Haliestes dasos (Siveter et al. 2004; Fig. 5n and o) Radiolarians Chelicerates Inanihella sagena (Siveter et al. 2007a; Fig. 5x) Offacolus kingi (Orr et al. 2000; Fig. 4c and f) Inanihella sp. (of Siveter et al. 2007a) Dibasterium durgae (Briggs et al. 2012; Fig. 4d, e, g and i) Haplentactinia armista (Siveter et al. 2007a) Trilobites Parasecuicollacta hexactinia (Won et al. 2002) Dalmanites sp. (uninvestigated) Mazuelloids Tapinocalymene sp. (uninvestigated) Fragments of unidentified mazuelloid species have been Marrellomorphs recovered from acid residues Xylokorys chledophilia (Siveter et al. 2007b; Fig. 5r and w) Mandibulates The relevant primary reference is given after the species name. Other relevant Aquilonifer spinosus (Briggs et al. 2016; Fig. 4h) references are given throughout the text. Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

The Herefordshire Lagerstätte 5

Fig. 2. (a) Faunal composition of the major groups of the Herefordshire Lagerstätte. Percentage abundance is based on number of specimens (n = 3670); some specimens as yet undescribed and unassigned at species level are included, and radiolarians are excluded. (b) Panarthropod faunal composition of the Herefordshire Lagerstätte. Percentage abundance is based on number of specimens (n = 982). The chelicerate Offacolus is by far the best represented species (833 specimens = 84.8% of the total number); all other species are represented by less than 10 specimens each (= less than 1%), except for the mandibulate Tanazios (88 specimens = 9.0%) and the malacostracan Cinerocaris (18 = 1.8%).

essential to extract maximum morphological information. Data are Deep molluscan phylogeny hinges on the position of the ‘marked up’ to identify separate structures for visualization, e.g. a which comprise two -like groups, particular arthropod appendage. Prepared datasets are reconstructed Chaetodermomorpha and Neomeniomorpha, united in lacking a to triangle-mesh isosurfaces using the Marching Cubes algorithm shell and fully developed molluscan foot. Aplacophorans were (Lorensen & Cline 1987). These are visualized and studied using the traditionally interpreted (e.g. by Salvini-Plawen 1991) as the earliest SPIERSview component of the software suite, which can combine branch of the mollusc lineage (Fig. 4u), but an alternative model places multi-part models (e.g. part and counterpart) and enables them as sister to the multivalved Polyplacophora () in a stereoscopic viewing, arbitrary rotation, scaling, dissection and termed Aculifera, which is in turn the to all other molluscs sectioning. Models are exported into VAXML/STL format (Sutton (Conchifera). This ‘Aculifera hypothesis’ is supported by both et al. 2012b) for publication, or to the open-source rendering molecular and morphological data (Sigwart & Sutton 2007; package Blender (Blender Foundation 2019) to produce maximal- Wanninger & Wollesen 2018;butseeSalvini-Plawen & Steiner quality ray-traced images and animations (see Sutton et al. 2014, 2014). Resolution of the debate is critical for reconstructing molluscan pp. 27–31, 155–158 for details of the process). The quality of evolution (did the ancestor of crown-group molluscs have a shell and/ preservation is such that a single specimen can provide abundant or foot?) and for determining their position in the . data for detailed analysis. Aplacophorans were unknown as fossils before they were discovered in the Herefordshire biota. Multivalved molluscs had Unexpected character combinations and a possible role been described from the Paleozoic (e.g. Cherns 1998a, b), but were treated as early polyplacophorans (chitons). The discovery of for evolutionary development Acaenoplax hayae from Herefordshire (Sutton et al. 2001a, 2004; The reconstruction of phylogenies and the ordering and timing of Dean et al. 2015) revealed an aplacophoran-like form bearing character-acquisition has been an overarching project in evolution- spicules, lacking a true foot and possessing a posterior respiratory ary and palaeontology for over 150 years. Reconstructing cavity (Fig. 4s and t). Nonetheless Acaenoplax shows serialization the based on the living biota alone is attractive as vast and multiple dorsal valves reminiscent of Polyplacophora. The quantities of data are obtainable, including the genetic sequences subsequent discovery of Kulindroplax perissokomos (Sutton et al. that have revolutionized phylogenetic practice in recent years. 2012a) documented an even more aplacophoran-like body with no However, such data are restricted to crown-groups; no amount of trace of a foot but bearing a set of typical Paleozoic ‘polyplaco- avian sequences would reveal the origin of birds within Dinosauria, phoran’ valves (Fig. 4l and m). This association appeared chimaeric for example. Fossils document otherwise unknown character but only because it represents an extinct character combination. combinations that did not survive to the present. These combina- Acaenoplax and Kulindroplax provide support for Aculifera (e.g. tions can illuminate relationships and evolutionary history (e.g. in Vinther 2015), indicating that the ‘simple’ morphology of arthropods: Legg et al. 2013), a reminder that the significance of aplacophorans is derived rather than primitive for the . fossils may be out of proportion to their morphological information Furthermore, these Herefordshire fossils reveal a sequence of content. It is no surprise therefore that the high-fidelity soft-tissue evolutionary events that cannot be deduced from extant forms: a anatomy preserved in the Herefordshire fossils provides important body predated the loss of valves, and a diversity of new data for determining the course of evolution in many groups, as ‘plated aplacophorans’, previously misinterpreted as polyplaco- illustrated by molluscs. phorans, was present during the early Paleozoic. Modern Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

6 D. J. Siveter et al.

Fig. 3. Examples of Herefordshire animals on the split surface of concretions. (a) Longitudinal horizontal section of the chelicerate Offacolus kingi.(b) Longitudinal subcentral section of the edrioasteroid Heropyrgus disterminus.(c) Longitudinal subcentral section of the sponge Carduispongia pedicula.(d) Longitudinal subcentral section of the aculiferan Acaenoplax hayae. (a) and (c) show grey- to light blue-coloured carbonate-rich matrix of the concretion, within and around the fossil; (b) and (d) show orange - to yellow-coloured matrix of the concretion, indicating weathered (leached) examples. Scale bars are all 1 mm. aplacophorans evolved through a secondary loss of valves and are a Dibasterium. This weaker expression disappears in later embryonic remnant of a once larger group (Fig. 4v). stages echoing the loss of the outer ramus during the evolution of the Research on evolutionary development has demonstrated how group (Briggs et al. 2012). The chelicera of Dibasterium (limb 1) is control genes can initiate major changes in morphology. Modern also unusual in being an elongate flexible antenna-like appendage, polyplacophorans (chitons) have a series of eight dorsal valves in contrast to the familiar claw of Limulus. Here, too, knockdown of associated with the expression of the engrailed gene (Jacobs et al. genes such as Dll and dachsund may have led to the loss of distal 2000), whereas Acaenoplax and Kulindroplax (Fig. 4l, m, s and t) podomeres and the evolution of the shorter chelicera in modern have only seven. Such seven-fold seriality is expressed during the horseshoe crabs, echoing experimental results on the harvestman ontogeny of living aculiferans (Scherholz et al. 2013) and may Phalangium (Sharma et al. 2013). reflect an ancestral morphology evident in Kulindroplax (Wanninger & Wollesen 2018). The addition of an eighth shell, Extended stratigraphic ranges and the calibration of which occurs after in Polyplacophora, appears to be molecular clocks an advanced condition (Wanninger & Wollesen 2018). Acaenoplax is unusual in having an obvious gap between valves 6 and 7 where The Herefordshire Lagerstätte is one of very few windows on soft- an additional valve might have been placed (Sutton et al. 2001a, bodied organisms during the Silurian. It is not surprising, therefore, 2004). The ‘missing’ valve may reflect the ancestral 7-valve that the fossils provide important examples of stratigraphic range morphology or, given that the space for an additional valve is extensions. Thanahita (Fig. 5a and b), for example, falls in a clade present, may have been lost secondarily from an 8-valved form. with the three known species of the iconic lower to mid- The remarkable details preserved in some of the Herefordshire lobopodian Hallucigenia (Siveter et al. 2018b). The long slender arthropods provide evidence of ancestral morphologies and prompt trunk appendages of Thanahita contrast with those of short-legged questions as to how they might have been transformed into modern lobopodians, such as the Cambrian Antennacanthopodia (Ou et al. forms. Martin et al. (2016) used CRISPR/Cas9 mutagenesis and 2011). Long-legged lobopodians are also represented by RNAi knockdown to show how shifting domains in a Carbotubulus from the late Mazon Creek living amphipod crustacean could contribute to macroevolutionary Lagerstätte of Illinois (Haug et al. 2012) which is some 135 Ma changes in the . The limbs of the Herefordshire younger than Thanahita. chelicerates Dibasterium (Briggs et al. 2012; Fig. 4d, e, g and i) The discovery of the arthropod Enalikter (Siveter et al. 2014a, and Offacolus (Sutton et al. 2002; Fig. 4c and f) can be 2015a; Fig. 5e) revealed that megacheirans (short-great-appendage homologized readily with those of the living horseshoe crab arthropods) were present in the Silurian. Phylogenetic analysis Limulus but they differ in fundamental ways. Limbs 2 to 5 in the showed that Bundenbachiellus from the Hunsrück Slate, anterior division of the body (prosoma) of the Herefordshire not previously interpreted as a megacheiran, is sister to Enalikter, Silurian forms have two branches, an endopod and exopod, both of extending the range of megacheirans nearly 100 Ma beyond the which are robust and segmented, whereas only one branch is present previously youngest known short-great-appendage arthropod, in Limulus. Unlike the branches of limbs in other arthropods, ? sp. from the mid-Cambrian of Utah (Briggs et al. however, the two branches are not connected to a common 2008). segment: they insert in different places on the body wall. This Xylokorys is the only known Silurian euarthropod arrangement can be interpreted as a stage in the loss of the exopod to (Siveter et al. 2007b; Fig. 5r and w). It belongs to the Acercostraca yield a limb with a single ramus, as in Limulus. The expression of (see Legg 2016) which, in contrast to the familiar Cambrian Distal-less (Dll) in larval Limulus (Mittmann & Scholtz 2001) Marrella, is characterized by a large shield-like carapace which suggests that the loss of the exopod may be developmental. Limulus covers the head and trunk. Xylokorys extends the Cambrian shows a strong expression of Dll associated with the origin of the (Primicaris and ) and (Enosiaspis) acercostracan endopod and a weaker expression, in the early embryo, in an record and heralds the youngest known example, Vachonisia, in the adjacent position corresponding to the insertion of the exopods in Devonian Hunsrück Slate. Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

The Herefordshire Lagerstätte 7

Fig. 4. Herefordshire Lagerstätte species. (a) Colymbosathon ecplecticos, valves removed, lateral view. (b) Nymphatelina gravida, left valve removed, posterolateral view. (c, f) Offacolus kingi, dorsal, ventrolateral views (see Fig. 3a). (d, e, g, i) Dibasterium durgae, ventrolateral stereo-pair, dorsal view; chelicerae (first appendages), lateral view; prosomal area and anterior part of opisthosoma, ventral view. (h) Aquilonifer spinosus, dorsal view. ( j) Drakozoon kalumon, ventral view. (k) Bethia serraticulma, with attached Drakozoon kalumon and ?atrypide brachiopod, lateral view. (l, m) Kulindroplax perissokomos, lateral, dorsal views. (n, o) Rhamphoverritor reduncus, attached juvenile, free swimming stage, lateral views. (p) Invavita piratica, lateral view. (q, r) Nymphatelina gravida, with valves, and valves removed, with an attached and an internal specimen of Invavita piratica, lateral views. (s, t) Acaenoplax hayae, anterior part of body, dorsolateral view; main and posterior parts of body, dorsolateral view (see Fig. 3d). (u, v) Cladograms depicting competing molluscan phylogenies (simplified from Wanninger & Wollesen 2018): (u) ‘Testaria’ hypothesis, one of several phylogenetic models in which aplacophorans are primitive with respect to all other molluscs; (v) ‘Aculifera’ hypothesis, in which aplacophorans and polyplacophorans form a sister clade (Aculifera) to all other molluscs (Conchifera). Scale bars are all 500 µm. Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

8 D. J. Siveter et al.

Fig. 5. Herefordshire Lagerstätte species. (a, b) Thanahita distos, dorsal, right lateral views. (c, d) Cinerocaris magnifica, right lateral, posterior ventrolateral views. (e) Enalikter aphson, dorsal view. (f–i) Bdellacoma sp., isolated example of pedicellaria with one valve removed, internal, lateral views; pedicellariae attached to arm; portion of arm with spines and podia, sub-adoral view. ( j, k) Heropyrgus disterminus, dorsal, lateral views (see Fig. 3b). (l, m) Cascolus ravitis, anterolateral, dorsal views. (n, o) Haliestes dasos, dorsal, anterolateral view. (p, t) Tanazios dokeron, dorsal, anterolateral views. (q) Sollasina cthulhu, aboral view. (r, w) Xylokorys chledophilia, ventrolateral, dorsal views. (s) Kenostrychus clementsi, anterolateral view. (u) Spirocopia aurita, valves removed, right lateral view. (v) Platyceras sp., anterior part of shell removed, view from position normal to viscera. (x) Inanihella sagena. (y) Carduispongia pedicula, lateral view (see Fig. 3c). (z) Nasunaris flata, right valve removed, lateral view. (zz) Pauline avibella, right lateral view. Scale bars are all 500 µm.

Haliestes (Siveter et al. 2004; Fig. 5n and o) is arguably the best pycnogonid, Cambropycnogon klausmuelleri, from the upper preserved and most complete of just 13 known species (Sabroux Cambrian Orsten of Scandinavia (Waloszek & Dunlop 2002), and et al. 2019) of fossil pycnogonid. These include a putative larval a basal stem group pycnogonid, Palaeomarachne granulata, from Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

The Herefordshire Lagerstätte 9

Box 2. Comparison of Silurian Lagerstätten The early Ordovician Fezouata biota of Morocco (Van Roy et al. 2015) is the most diverse open-marine Lagerstätte in the interval between the late Cambrian Weeks Formation of Utah (Lerosey-Aubril et al. 2018) and the mid-Silurian Herefordshire Lagerstätte (other Ordovician Lagerstätten are briefly reviewed in Van Roy et al. 2015). The Fezouata biota is much more diverse than Herefordshire, comprising over 160 genera, of which shelly taxa account for about 50%. As in Herefordshire, sponges are diverse and panarthropods dominate (over 60 taxa). Fezouata yields a greater diversity of shelly taxa including conulariids, bryozoans, annelids, rostroconch and helcionelloid bivalves, hyolithoids and echinoderms.

Apart from Herefordshire, open-marine Lagerstätten of Silurian age are rare (Fig. 6). Other exceptionally preserved faunas are almost exclusively preserved via Burgess -type pathways as degraded organic (carbonaceous) compression fossils and almost all represent marginal-marine or ‘stressed’ environments (Orr 2014). The Ludlow age Eramosa Lagerstätte of Ontario includes three biotas representing different environments. Soft-bodied fossils are characteristic of Biota 3, which contains a diverse marine fauna of conulariids, a lobopodian, trilobites, , xiphosurans, diverse crustaceans, brachiopods, polychaete annelids, echinoderms, conodonts and possible fish. The fauna of the late Llandovery Waukesha Lagerstätte from Wisconsin (Mikulic et al. 1985a, b) shows some similarity to Eramosa but is peritidal. It is dominated by arthropods, especially trilobites (13 genera), and includes a xiphosuran and phyllocarid, ostracod and thylacocephalan crustaceans. Conulariids and graptolites are common, but brachiopods, bryozoans, corals, molluscs and echinoderms are rare or absent. The Prídolí̌ Bertie Group of Ontario and New York State is dominated by eurypterids, xiphosurans, scorpions and phyllocarids preserved in fine carbonate muds (Clarke & Ruedemann 1912; Copeland & Bolton 1985). The presence of salt hoppers was long regarded as evidence of hypersaline conditions, but it is likely that they were precipitated in the sediment after deposition and the Bertie Group represents a shallow subtidal setting (Vrazo et al. 2016). The deeper-marine Herefordshire fauna includes numerous sponges, two genera, and a greater representation of major arthropod groups, as well as four major (fully stenohaline) echinoderm taxa (Box 1, Table 1).

The other major Silurian Lagerstätten show a greater terrestrial influence. Late Llandovery to early Wenlock Lagerstätten in the Midland Valley of Scotland from the Priesthill and Waterhead groups of Lesmahagow, and correlatives in the Hagshaw and Pentland Hills, represent restricted quasi-marine to lacustrine settings and are typically dominated by eurypterids, phyllocarids and fish (Ritchie 1968, 1985; Rolfe 1973; Allison & Briggs 1991; Siveter 2010a). The Stonehaven Lagerstätte (Siveter & Palmer 2010) in Scotland, which yields (Wilson & Anderson 2004), was considered to be of late Wenlock/Ludlow age on the basis of palynomorphs (Marshall 1991; Wellman 1993) but has recently been radiometrically dated as lowermost Devonian, Lochkovian (Suarez et al. 2017; see also Shillito & Davies 2017 for ichnological evidence). The basal Prídolí̌ Ludlow Bone Bed Lagerstätte at Ludford Lane and Corner in the Welsh Borderland yields – in addition to fish bone fragments and thelodonts – , arthropleurid and kampecarid myriapods, eurypterids and scorpions (Jeram et al. 1990; Siveter 2000; Siveter 2010b).

Fig. 6. Stratigraphic occurrence of Silurian Lagerstätten. The radiometric dates are those given in Cohen et al. (2013, updated 2018), the International Commission on Stratigraphy (ICS) Chronostratigraphic Chart.

the Ordovician William Lake Lagerstätte of Canada (Rudkin et al. discovery of Luprisca incubi from the late Ordovician Beecher’s 2013). The Herefordshire pycnogonid Haliestes shows reduction of Trilobite Bed of New York State (Siveter et al. 2014b; Wolfe et al. the body to a small trunk projection beyond the posteriormost limbs. 2016). Invavita piratica from the Herefordshire Lagerstätte (Siveter This is a feature of Pantopoda, indicating that crown-group et al. 2015b; Fig. 4p–r), the earliest adult pentastomid pancrusta- pycnogonids extended back to the Silurian (Siveter et al. 2004; cean, is arguably a more secure calibration than Boeckelericambria Arango & Wheeler 2007; Dunlop 2010; but see Charbonier et al. pelturae from the Cambrian Orsten (Walossek & Müller 1994), 2007). Five pycnogonid species are known from the Devonian which is based on a (Wolfe et al. 2016). Hunsrück Slate, of which Palaeopantopus maucheri and The described Herefordshire biota includes five species of Palaeothea devonica show trunk end reduction. Three species ostracods with preserved appendages, all of which are myodocopes: from the of La Voulte-sur-Rhȏne, France, may also belong Colymbosathon ecplecticos, Nymphatelina gravida, Nasunaris to extant pantopod families (Charbonier et al. 2007). flata, Pauline avibella and Spiricopia aurita (Siveter et al. 2003, Where Herefordshire taxa extend the range of particular 2007c, 2010, 2013, 2015b, 2018a). All of these, except N. gravida, back in time, they provide fossil calibrations for the tree of life, represent the earliest representatives of crown-group cylindroleber- notably in the case of arthropods. Haliestes provides one example idids, which are characterized by the presence of . The only (Wolfe et al. 2016), the Rhamphoverritor reduncus (Briggs other fossil myodocopes with possible preservation are et al. 2003; Fig. 4n and o) provides a calibration for crown-group Triadocypris from the lower of Spitzbergen (Weitschat Thecostraca, and the phyllocarid Cinerocaris magnifica (Briggs 1983) and Juraleberis from the upper Jurassic of Russia (Vannier & et al. 2005, Fig. 5c and d) for crown (Wolfe et al. Siveter 1996). 2016). Herefordshire ostracods (Figs 4a, b, q, r; 5u, z, zz) provided The Herefordshire molluscs Acaenoplax and Kulindroplax the calibration for crown Ostracoda (Oakley et al. 2013) prior to the (Sutton et al. 2001a, 2004, 2012a; Sigwart & Sutton 2007; Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

10 D. J. Siveter et al.

2016; Fig. 4h). Such data suggest that complexity of brooding strategies probably evolved early in the history of the Arthropoda. Specimens representing a free-living ‘cyprid’ larval stage and an attached juvenile of the barnacle Rhampoverritor reduncus demonstrate that the lifecycle of cirripedes has remained essentially the same over 430 myr (Briggs et al. 2005; Fig. 4n and o). A boring through a valve of Rhampoverritor (Fig. 4n) also provides evidence of predation in the biota. The discovery of a pentastomid pancrustacean, Invavita piratica, parasitic on the ostracod Nymphatelina gravida is the only known fossil pentastomid preserved with its host and suggests that the group may have originated as ectoparasites on (Siveter et al. 2015b; Fig. 4p–r). Bethia serraticulma is the only described fossil rhynchonelliformean brachiopod with preserved soft parts (Fig. 4k). Evidence of its mode of life, shell size and configuration indicates that the only known specimen is an immature example. Its unusual pedicle morphology differs from that in living species, urging caution in inferring stem-group anatomy based on crown- group species (Sutton et al. 2005a). The specimen of B. serraticulma bears an in vivo epifauna including a tiny unmineralized lophophorate, Drakozoon kalumon (Fig. 4j), which may allude to a widespread occurrence of similar but unknown lophophorates in the Paleozoic (Sutton et al. 2010). Fig. 7. Ecological types and percentage abundance based on number of The majority of the 63 known Herefordshire species lived on the species (n = 63). It includes the four radiolarian species, and some species seafloor (Table 1; Fig. 7), either as sessile or vagile epibenthos. belonging to other major groups that are as yet undescribed and Unless the Herefordshire animals were transported from signifi- unassigned at species level. cantly shallower waters, it appears that the benthic components of the biota lived in dim light conditions at best. Several of the benthic Fig. 4l, m, s and t) also provide molecular clock calibrations. Both animals lack and there is an absence of photic-zone indicators represent total group aplacophorans and crown-group Aculifera such as algae. Given the evidence for rapid burial it is likely that the (Vinther et al. 2017; Wanninger & Wollesen 2018). The water column biota is under-represented. Sponges are the dominant Herefordshire polychaete worm Kenostrychus represents crown sessile element, with brachiopods, echinoderms (the edrioasteroid Aciculata even though its original placement as a stem-group Heropyrgus and a crinoid), a cirripede, coral, and a possible phyllodocid (Sutton et al. 2001c; Fig. 5s) has been revised to stem dendroid graptolite and gastropod (Platyceras?) as minor compo- amphinomid (Parry et al. 2016). nents. Most of the vagile epibenthos are arthropods, but this group also includes two echinoderms (the asteroid Bdellacoma and ophiocistioid Sollasina), the aplacophoran mollusc Acaenoplax,a Mode of life and palaeoecology gastropod and a bivalve mollusc. The nektobenthos is made up Herefordshire taxa reveal aspects of gender, larval development and entirely of arthropods, with ostracods the most species abundant. A brood care in fossil arthropods. The recognition of hemipenes in the nautiloid is the only nektonic element. A graptolite and radiolarians myodocope ostracod Colymbosathon (Siveter et al. 2003; Fig. 4a) comprise the pelagic . A possible infaunal/semi-infaunal and and a juvenile, in situ, within the carapace of the mode of life is interpreted for just three taxa, a lingulid brachiopod, myodocope Nymphatelina (Siveter et al. 2007c, 2015b; Fig. 4b) the aplacophoran Kulindroplax and the annelid Kenostrychus. The allow male and female to be distinguished in early Paleozoic numerical dominance of Offaculus within the biota is probably real animals. The brood care strategy evident in Nymphatelina remains rather than representing an artefact of sampling, as concretions are essentially unchanged in myodocopes today. A more complex and selected and split randomly and, as one of the smaller taxa, its high unique brooding behaviour is preserved in the stem mandibulate yield is unlikely to result from being selectively preserved. The biota Aquilonifer; the presumed female carries ten tiny juveniles each includes representatives of many major invertebrate groups but there within a cuticular capsule tethered by a delicate thread (Briggs et al. is no obvious reason why some groups known from the Silurian of

Box 3. Outstanding questions

(1) Precisely when and how were the Herefordshire concretions formed?

(2) Will more sensitive scanning techniques become available in the future which can facilitate imaging the fossils, even if only for initial identifications?

(3) What will be revealed when additional fossils are reconstructed? A diversity of sponges, as well as cnidarians, brachiopods, molluscs, trilobites and other arthropods, crinoids and other echinoderms, graptolites and various remain to be investigated.

(4) How does the Lagerstätte fauna compare with that of the interbedded Coalbrookdale Formation mudstones?

(5) What further insights on the palaeoecology will emerge when more taxa have been described?

(6) Lagerstätten rarely prove unique. Where are there other examples in the stratigraphic record preserved in a similar fashion to the Herefordshire example? No other Silurian locality has yet been identified, even within the Welsh Basin. Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

The Herefordshire Lagerstätte 11 the Welsh Basin elsewhere, such as and bryozoans, have Acknowledgements Carolyn Lewis (Oxford University Museum of not been found in the concretions. Natural History) is thanked for technical assistance. We are grateful for insightful comments from Patrick Orr, Robert Gaines and a third anonymous reviewer. Why is the Herefordshire Lagerstätte apparently Funding Funding is gratefully acknowledged from the Leverhulme Trust unique? (grant no. EM-2014-068), the Natural Environment Research Council (grant no. NF/F0108037/1), the Yale Peabody Museum of Natural History Invertebrate The Herefordshire biota is one of a number of examples of soft Division, and English Nature. tissue preservation in concretions. Concretion formation requires carbonate (or more rarely silica) supersaturation and the presence of Author contributions DerekJS led funding acquisition and specimen a nucleus. In some cases, a buried carcass can provide both. Soft curation and coordinated this review; all authors contributed equally to research tissues can be preserved within concretions in a number of ways and to the preparation of the paper. (McCoy et al. 2015): as carbonaceous remains, as a result of Scientific editing by Philip Donoghue replication in a variety of authigenic minerals, or by void fill, as in the case of the Herefordshire biota. Exceptional preservation is References promoted where concretion growth is rapid, and void fills are more likely to occur where cementation occurs from the inside out, Aldridge, R.J., Dorning, K.J. & Siveter, D.J. 1981. Distribution of groups across the Wenlock Shelf of the Welsh Basin. In: Neale, J.W. & slowing diffusion at an early stage and isolating the organism from Brasier, M.D. (eds) Microfossils from Recent and fossil Shelf Seas. British its environment (McCoy et al. 2015). An analysis of factors Micropalaeontological Society, 18–30. associated with exceptional preservation in concretions (based on Allison, P.A. & Briggs, D.E.G. 1991. Taphonomy of nonmineralized tissues. In: Allison, P.A. & Briggs, D.E.G. (eds) Taphonomy: Releasing the Data Locked 88 concretion sites, of which 20% preserve soft-tissues) showed that in the Fossil Record. Plenum Press, New York, 25–70. concretion formation enhances the chances of exceptional preser- Arango, C.P. & Wheeler, W.C. 2007. Phylogeny of the sea spiders (Arthropoda, vation only where other conditions are favourable, such as burial in Pycnogonida) based on direct optimization of six loci and morphology. , 23, 255–293, https://doi.org/10.1111/j.1096-0031.2007.00143.x fine-grained sediment (McCoy et al. 2015). The volcanic ash that Bassett, M.G. 1974. Review of the stratigraphy of the Wenlock Series in the buried the Herefordshire organisms may have played a role in their Welsh Borderland and South Wales. Palaeontology, 17, 745–777. exceptional preservation. Fine grain size may have promoted anoxia Bassett, M.G., Bluck, B.J., Cave, R., Holland, C.H. & Lawson, J.D. 1992. within the sediment and inhibited scavengers. Silurian. In: Cope, J.C.W., Ingham, J.K. & Rawson, P.F. (eds) Atlas of Palaeogeography and Lithofacies. Geological Society, London, Memoirs, 13, Predictably, perhaps, most examples of exceptional preservation 37–56, https://doi.org/10.1144/GSL.MEM.1992.012.01.16 associated with volcanic ash are in terrestrial settings (Briggs et al. Blender Foundation. 2019. Blender, http://www.blender.org 1996), in lacustrine and fluvial environments. Preservation of Brett, C.E., Boucot, A.J. & Jones, B. 1993. Absolute depths of Silurian benthic assemblages. Lethaia, 26,24–40, https://doi.org/10.1111/j.1502-3931.1993. materials in volcanic ash may be enhanced by rapid burial and tb01507.x anoxic conditions, as in the of Argentina (Lafuente Diaz Briggs, D.E.G., Siveter, D.J. & Siveter, D.J. 1996. Soft-bodied fossils from a et al. 2018) and the Miocene Clarkia Beds of Idaho, USA Silurian volcaniclastic deposit. Nature, 382, 248–250, https://doi.org/10.1038/ (Ladderud et al. 2015). The Triassic Chañares Formation of 382248a0 Briggs, D.E.G., Sutton, M.D., Siveter, D.J. & Siveter, D.J. 2003. A new Argentina yields a diversity of tetrapods in carbonate concretions in phyllocarid (Crustacea) from the Silurian Fossil-Lagerstätte of Herefordshire, volcanic ash that likely formed where microbial decay promoted England. Proceedings of the Royal Society of London B, 271, https://doi.org/ carbonate precipitation (Rogers et al. 2001) but concretion biotas 10.1098/rspb.2003.2593 Briggs, D.E.G., Sutton, M.D., Siveter, D.J. & Siveter, D.J. 2005. 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Chelodes and closely related Polyplacophora (Mollusca) from the Silurian of Gotland, Sweden. Palaeontology, 41, 545–573. biotas preserved in a similar fashion to the Herefordshire example Cherns, L. 1998b. Silurian polyplacophoran molluscs from Gotland, Sweden. have yet to be discovered. The tiny crystalline infills are unlikely to Palaeontology, 41, 939–974. attract the interest of a casual collector and the fossils are often off- Cherns, L., Cocks, L.R.M., Davies, J.R., Hillier, R.D., Waters, R.A. & Williams, centre and may not be intersected by the plane of splitting. Silurian M. 2006. Silurian: the influence of extensional tectonics and sea-level changes on sedimentation in the Welsh Basin and on the Midland Platform. In: bentonites are widespread in the UK, but they are generally only a Brenchley, P.J. & Rawson, P.F. (eds) The Geology of England and Wales. few centimetres thick (Cave & Loydell 1998). 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