The Herefordshire Lagerstätte: fleshing out Silurian marine life

Derek J. Siveter1,2*, Derek E. G. Briggs3, David J. Siveter4 & Mark D. Sutton51 Earth Collections, University Museum of Natural History, Oxford OX1 3PW, UK2 Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK3 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, UK5 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: derek.siveter@oum.ox.ac.uk

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. Thefossils 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 onAvalonia in the southern subtropics. The specimens are investigated by serial grinding, digital photography and rendering in theround as ‘virtual fossils’ by computer. The fossils contribute much to our understanding of the palaeobiology and early historyof the groups represented. They are important in demonstrating unusual character combinations that illuminate relationships; incalibrating molecular clocks; in variously linking with taxa in both earlier and later Paleozoic Lagerstätten; and in providingevidence 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-tives of many major invertebrate groups in high-fidelity 3-D, ascalcite in-fills within concretions in a marine volcaniclastic deposit.The Herefordshire fossils are unimpressive at first sight, yet theyyield unrivalled evidence about the palaeobiology and evolutionarysignificance of Silurian animals. They provide, for example, theearliest evidence for the time of origin of certain major crown-groups (i.e. the living biota), for instance sea spiders (pycnogonids).

The locality lies in theWelsh Borderland, a historic region centralto R. I. Murchison’s pioneering research which established theSilurian System in the 1830s. In 1990 Bob King, mineralogist andretired Curator in the Department of Geology at the University ofLeicester, collected concretions in float from the locality andpresented nine of them to the Leicester collections. Curator RoyClements asked David Siveter to examine one of the concretionswhen he was accessioning the material in 1994. What Davidrecognized under the microscope, an arthropod with preservedlimbs (later named Offacolus kingi), was confirmed by DerekSiveter and, together with Bob King, they identified thestratigraphic horizon and collected more concretions. In 1995Derek Briggs joined the team and, following more visits to thelocality, we published an initial paper flagging the significance ofthe discovery (Briggs et al. 1996). NERC and Leverhulme Trustfunding enabled Patrick Orr and later Mark Sutton to work on theLagerstätte, initially as postdoctoral researchers. The support andco-operation of the site owners and help from English Nature havebeen crucial in realizing its scientific potential.

Locality, age and palaeogeographical setting

The Herefordshire site lies in the Old Radnor to Presteigne area oftheWelsh Borderland (Fig. 1a) in the vicinity of the Church StrettonFault Zone. The fossil-bearing concretions (Fig. 1b and c) occur in asoft, fine-grained, cream-coloured, weathered and largely uncon-solidated bentonite (Fig. 1b) that crops out over about 30 m. The

bentonite is unique in the British stratigraphic record, at least, inreaching a thickness of more than 1 m in places. The bentonite wasdeposited within mudstones of the Wenlock Series CoalbrookdaleFormation (Fig. 1d), which rests on the slightly older Dolyhir andNash Scar Limestone Formation. In the Dolyhir region these coraland algal-rich limestones lie with angular unconformity on aPrecambrian sedimentary basement (Woodcock 1993). At nearbyNash Scar, laterally equivalent carbonates sit, possibly disconform-ably (Woodcock 1993), on the shallow-marine, upper LlandoverySeries Folly Sandstone Formation and a hardground is developedlocally on the limestones (Hurst et al. 1978).

The host bentonite lies approximately at the boundary betweenthe Sheinwoodian and Homerian stages (Fig. 1d), which isradiometrically constrained to 430.5 ± 0.7 Ma (Cohen et al. 2013,updated 2018). Various evidence from brachiopods, graptolites andconodonts indicates a Wenlock age for the Dolyhir and Nash ScarLimestone Formation in addition to the overlying CoalbrookdaleFormation (Bassett 1974; Cocks et al. 1992). The Sheinwoodianconodont Ozarkodina sagitta rhenana has been recovered from thelimestones at Dolyhir and Nash Scar (Bassett 1974; Aldridge et al.1981). The presence of Homerian Cyrtograptus lundgreni Biozonegraptolites in the Coalbrookdale Formation mudstones immediatelyabove the limestones at Nash Scar (Bassett 1974) suggests thatdeposition of these mudstones did not commence locally untilC. lundgreni times (Hurst et al. 1978). This is consistent with thepresence of the limestone hardground that presumably formedduring a hiatus in deposition in the late Sheinwoodian.

The recovery of the chitinozoans Margachitina margaritana,Ancyrochitina plurispinosa and Cingulochitina cingulata from 20to 50 cm below the bentonite also indicates an upper Sheinwoodianto Homerian age for the Coalbrookdale Formation (G. Mullins,pers. comm. 2000; see also Steeman et al. 2015). That age isunchallenged by ostracods (David Siveter, unpublished informa-tion) and radiolarians (Siveter et al. 2007a) recovered fromconcretions 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

Review Focus Journal of the Geological Society

Published Online First https://doi.org/10.1144/jgs2019-110

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

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 theaplacophoran 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 ofthe 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 Basinpalaeogeography at approximately the Sheinwoodian–Homerian boundary (modified from Cherns et al. 2006, fig. 4.13a). (f ) Eastern Laurussianpalaeogeography during the Wenlock (modified from Cocks & Torsvik 2005, fig. 9).

D. J. Siveter et al.

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

The volcanic ash in which the Herefordshire Lagerstätte ispreserved accumulated in the Welsh Basin (Fig. 1e). This lay on themicrocontinent of Avalonia, which included southern Britain(Fig. 1f; Cocks & Torsvik 2011). Avalonia and adjacent Balticawere in southern tropical/subtropical palaeolatitudes in the mid-Silurian, separated from Laurentia by a remnant Iapetus Ocean.Shallow-water muds characterized much of the Welsh Borderlandand central England east of the Church Stretton Fault Zone (Bassettet al. 1992; Cherns et al. 2006). Coeval fine clastics and turbiditesaccumulated to the west in the deepest parts of the Welsh Basin.Palaeogeographical and sedimentological evidence indicates thatthe Coalbrookdale Formation in the Dolyhir–Nash Scar area of theChurch Stretton Fault Zone accumulated in a moderately deep, outershelf/shelf slope environment (Briggs et al. 1996); the presence ofthe Visbyella brachiopod community suggests water depths of100–200 m (Hurst et al. 1978; Brett et al. 1993). Geochemicalanalyses suggest a destructive plate margin as the source of thevolcanic ash (Riley 2012). The closest known volcanic centres ofWenlock age (Cocks & Torsvik 2005) are the Mendip Hills in SWEngland (Avalonia) and the Dingle peninsula in SW Ireland(Avalonia). The Herefordshire bentonite is geochemically similar tovolcanic rocks in both areas (Riley 2012). A more distant candidateis the Czech Republic, then on Perunica, a microcontinent in thecentre of the Rheic Ocean (Fig. 1f).

Taphonomy

The volcanic ash that hosts the Herefordshire biota was deposited onan erosion surface on massive limestone and, in places, muds; theuneven topography likely reflects the original Wenlock seafloor.The ash layer varies from a few tens of centimetres to over 1 m thickand is highly weathered at least in outcrop: no evidence for primarysedimentary structures is preserved. Thus, it is unclear whether theash represents in situ settling or was reworked, or whether there wasone or several ashfalls. Occasional examples of unsuccessful escapetraces (Orr et al. 2000) indicate that at least some of the animals wereburied alive, which is consistent with the remarkable preservation.The bentonite has yielded at least five thousand randomlydistributed calcite-cemented (Fig. 3) concretions 2–25 cm indiameter (Orr et al. 2000; Fig. 1b and c). Unusually for fossilspreserved in this way, the organisms do not appear to haveinfluenced the size or shape of the concretions, nor are they usuallyin the centre (Fig. 1c). Fossils are only preserved where they wereincorporated into concretions but it is not clear what determined thelocus of concretion formation. The timing and nature of the growthof the Herefordshire concretions merits further investigation.

The fossils show high-fidelity three-dimensional preservation –the threads that attach the juveniles to the arthropod Aquilonifer(Briggs et al. 2016), for example, are less than 50 µm in diameter(Fig. 4h). About half the taxa lacked biomineralized hard parts andthe biomineralized taxa preserve remarkable details in addition tofeatures preserved in the normal fossil record. The soft tissues shownegligible collapse (Fig. 3a and d) indicating that the ash stiffenedand recorded the outer morphology of the animals before significantdecay took place. The spherical morphology of the concretionssuggests that they formed rapidly, before sediment compaction. Thefossils are preserved as voids that were infilled with calcite (Fig. 3)which also dominates the concretion cement (Orr et al. 2000). Thebiomineralized skeletons of brachiopods, gastropods, trilobites andostracods are preserved, but organic remains, even non-biominer-alized arthropod cuticles, are lost – the periderm of the raregraptolites is an exception. Preservation was investigated in detail inthe arthropod Offacolus (Orr et al. 2000). Sparry calcite infilled theexternal mould of Offacolus, sometimes with an initial phase offiner fibrous calcite around the periphery (Figs 3a, 4c and f). Pyriteprecipitated on the boundary between calcite crystals but as a minor

constituent of the void fill. Clay minerals precipitated on andunderneath the exoskeleton, and in some examples the gut trace wasreplicated in calcium phosphate. Ferroan dolomite (ankerite) formedat a later stage perhaps coincident with the transformation on burialof smectite to illite. Abundant radiolarians are evident on the surfaceof split concretions (Siveter et al. 2007a; Fig. 5x). Theirpreservation shows parallels to that of Offacolus (Orr et al. 2002).Calcite and pyrite precipitated in the void vacated by the cytoplasmwithin the radiolarian tests; this space was not invaded by matrix.The precipitation of calcite retained the shape of the test when theopaline silica that formed it was replaced during diagenesis by amixture of ankerite and diagenetic clay minerals; replacement ofsilica by ankerite has also been observed in siliceous sponge-spicules (Nadhira et al. 2019). Although there are similarities withthe diagenetic sequence displayed by radiolarians in concretionselsewhere (Holdsworth 1967; Orr et al. 2002), the nature of thepreservation of soft tissue morphology at the Herefordshire siteremains unique.

Releasing the information from the fossils

The Herefordshire concretions are collected in bulk using earthmoving equipment. Each scoop is tipped slowly on to a waste-pile;concretions roll out and are collected manually. The concretions aresplit repeatedly in the laboratory with a manual rock-splitter; theprocess is halted if a fossil is found (Fig. 3; c. 50% of concretions).Specimens are identified provisionally, photographed with a LeicaDFC420 digital camera mounted on a Leica MZ8 binocularmicroscope with a thin layer of water over the specimen to enhancecontrast, and catalogued in a custom online database.

The fossils record copious anatomical information but presentchallenges for its extraction. Manual or chemical preparation ofspecimens (see e.g. Sutton 2008) is impractical. Study of theHerefordshire fauna is unique among invertebrate Lagerstätten inrelying almost exclusively on virtual reconstructions. Indeed,research on the fossils has played an important role in thedevelopment of such techniques and their wider application(Sutton et al. 2001b, 2014). Non-destructive approaches to data-capture, such as X-ray computed tomography, are clearly preferableas a basis for virtual reconstructions but are largely ineffective forHerefordshire fossils: X-ray absorption contrast between fossil andmatrix is very low. Limited success has been achieved with somespecimens by using phase-contrast synchrotron tomography (seeSutton et al. 2014, p. 78), but a destructive yet highly effectivephysical-optical tomography ‘serial grinding’ technique is, ofnecessity, our method of choice. This takes advantage of thestrong optical contrast between specimen and matrix and has beenused to reconstruct over 100 specimens.

Specimens for reconstruction are trimmed with a fine rock-saw toc. 10 mm cuboids and mounted on a Buehler ‘slide holder’ whichconsists of an inner metal cylinder which can be adjusted to elevatespecimens a precise distance above a hard outer ceramic disc.Specimens are ground to remove a consistent thickness (20–50 µm),washed, and photographed. This process is repeated until the entirespecimen has been digitized, typically in 200–400 increments. Partand counterpart are ground separately, and large specimens areprocessed in several pieces which are re-united digitally, followingreconstruction, using the custom SPIERS software suite (Suttonet al. 2012b). Reconstruction begins with manual or semi-automaticregistering (aligning) of photographs using the cut edges of cuboidsas a guide. Data are then prepared for reconstruction by carefullydistinguishing pixels representing the fossil from those correspond-ing to matrix. Software exists to automate this process but anunderstanding of the taphonomy and likely anatomy of thespecimen, and of artefacts generated by the process (e.g. ‘out-of-plane’ structures visible through bubbles in the encasing resin), is

The Herefordshire Lagerstätte

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

Box 1. The faunal composition of the Herefordshire Lagerstätte

Thirty-two species, 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 mayfall to c. 50% when other biomineralized taxa, including trilobites and the extensive sponge fauna, are described. Arthropods are diverse (Fig. 2b): 19 specieshave been recorded, 17 of which have been fully described. They include representatives of most major euarthropod groups: megacheirans, pycnogonids,chelicerates, trilobites, marrellomorphs, stem mandibulates and pancrustaceans (this last by far the most species-rich, with 9 described species) as well as onelobopodian. Sponges are the most abundant and diverse of the other major groups, probably comprising at least 20 species, although all but one awaitdetailed investigation. Molluscs are represented by two species of aplacophorans and two gastropods (one uninvestigated), one bivalve and an orthoconicnautiloid (uninvestigated). There are at least four echinoderm species, an asteroid, edrioasteroid, ophiocistioid and a crinoid (the last uninvestigated), at leastthree brachiopods (one an uninvestigated lingulid), an annelid, possibly two cnidarians (a coral and a possible hydrozoan, both uninvestigated), and twohemichordates (a graptoloid and a possible dendroid, both uninvestigated). The large number of unidentified specimens (1506; 41.0%) reflects poorlypreserved examples and also the difficulty of identifying specimens on split concretions prior to reconstruction. Radiolarians are not included in the faunalcomposition 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 mandibulateTanazios dokeron and the malacostracan crustacean Cinerocaris magnifica. Most of the arthropods are known from just a few specimens (e.g. themegacheiran Enalikter aphson), or single individuals, such as the horseshoe crab Dibasterium durgae. Of non-arthropod taxa the polychaete annelidKenostrychus clementsi is the most abundant at over 250 specimens. Many of the non-arthropods, however, such as the edrioasteroid Heropyrgusdisterminus, 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

SpongesCarduispongia pedicula (Nadhira et al. 2019; Fig. 5y)Some 20 other species (uninvestigated)Cnidarians? Hydroid (uninvestigated colonial organism)Coral (uninvestigated)BrachiopodsBethia serraticulma (Sutton et al. 2005a; Fig. 4k)At least two other brachiopod species (both uninvestigated), one ofthem a lingulid

Lophophorate indet.Drakozoon kalumon (Sutton et al. 2010; Fig. 4j)AnnelidsKenostrychus clementsi (Sutton et al. 2001c; Fig. 5s)MolluscsAplacophoransAcaenoplax hayae (Sutton et al. 2001a; Fig. 4s and t)Kulindroplax perissokomos (Sutton et al. 2012a; Fig. 4l and m)BivalviaPraectenodonta ludensis Reed, 1931 (Fu 2016)GastropodsPlatyceras? sp. (of Sutton et al. 2006; Fig. 5v)A high spired species (uninvestigated)CephalopodsNautiloids (uninvestigated)PanarthropodsLobopodiansThanahita distos (Siveter et al. 2018b; Fig. 5a and b)MegacheiransEnalikter aphson (Siveter et al. 2014a, 2015a; Fig. 5e)PycnogonidsHaliestes dasos (Siveter et al. 2004; Fig. 5n and o)CheliceratesOffacolus kingi (Orr et al. 2000; Fig. 4c and f)Dibasterium durgae (Briggs et al. 2012; Fig. 4d, e, g and i)TrilobitesDalmanites sp. (uninvestigated)Tapinocalymene sp. (uninvestigated)MarrellomorphsXylokorys chledophilia (Siveter et al. 2007b; Fig. 5r and w)MandibulatesAquilonifer spinosus (Briggs et al. 2016; Fig. 4h)

Tanazios dokeron (Siveter et al. 2007d; Fig. 5p and t)CrustaceansMalacostracansCinerocaris magnifica (Briggs et al. 2003; Fig. 5c and d)Cascolus ravitis (Siveter et al. 2017; Fig. 5l and m)CirripedesRhamphoverritor reduncus (Briggs et al. 2005;Fig. 4n and o)

Ostracods- MyodocopesColymbosathon ecplecticos (Siveter et al. 2003; Fig. 4a)Nymphatelina gravida (Siveter et al. 2007c;Fig. 4b, q and r)

Nasunaris flata (Siveter et al. 2010; Fig. 5z)Pauline avibella (Siveter et al. 2013; Fig. 5zz)Spiricopia aurita (Siveter et al. 2018a; Fig. 5u)- Podocopes and palaeocopes Fragments of unidentifiedspecies belonging to these ostracod groups have beenrecovered from acid residues

PentastomidsInvavita piratica (Siveter et al. 2015b, Fig. 4p, q and r)EchinodermsAsteroidsBdellacoma sp. (of Sutton et al. 2005b; Fig. 5f–i)EdrioasteroidsHeropyrgus disterminus (Briggs et al. 2017; Fig. 5j and k)Crinoids (uninvestigated sp.)OphiocistioidsSollasina cthulhu (Rahman et al. 2019; Fig. 5q)HemichordatesPterobranchsGraptoloids (uninvestigated)Dendroids? (uninvestigated)RadiolariansInanihella sagena (Siveter et al. 2007a; Fig. 5x)Inanihella sp. (of Siveter et al. 2007a)Haplentactinia armista (Siveter et al. 2007a)Parasecuicollacta hexactinia (Won et al. 2002)MazuelloidsFragments of unidentified mazuelloid species have beenrecovered from acid residues

The relevant primary reference is given after the species name. Other relevantreferences are given throughout the text.

D. J. Siveter et al.

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

essential to extract maximum morphological information. Data are‘marked up’ to identify separate structures for visualization, e.g. aparticular arthropod appendage. Prepared datasets are reconstructedto triangle-mesh isosurfaces using the Marching Cubes algorithm(Lorensen&Cline 1987). These are visualized and studied using theSPIERSview component of the software suite, which can combinemulti-part models (e.g. part and counterpart) and enablesstereoscopic viewing, arbitrary rotation, scaling, dissection andsectioning. Models are exported into VAXML/STL format (Suttonet al. 2012b) for publication, or to the open-source renderingpackage Blender (Blender Foundation 2019) to produce maximal-quality ray-traced images and animations (see Sutton et al. 2014,pp. 27–31, 155–158 for details of the process). The quality ofpreservation is such that a single specimen can provide abundantdata for detailed analysis.

Unexpected character combinations and a possible rolefor evolutionary development

The reconstruction of phylogenies and the ordering and timing ofcharacter-acquisition has been an overarching project in evolution-ary biology and palaeontology for over 150 years. Reconstructingthe history of life based on the living biota alone is attractive as vastquantities of data are obtainable, including the genetic sequencesthat have revolutionized phylogenetic practice in recent years.However, such data are restricted to crown-groups; no amount ofavian sequences would reveal the origin of birds within Dinosauria,for example. Fossils document otherwise unknown charactercombinations that did not survive to the present. These combina-tions can illuminate relationships and evolutionary history (e.g. inarthropods: Legg et al. 2013), a reminder that the significance offossils may be out of proportion to their morphological informationcontent. It is no surprise therefore that the high-fidelity soft-tissueanatomy preserved in the Herefordshire fossils provides importantnew data for determining the course of evolution in many groups, asillustrated by molluscs.

Deep molluscan phylogeny hinges on the position of theAplacophora which comprise two worm-like groups,Chaetodermomorpha and Neomeniomorpha, united in lacking ashell and fully developed molluscan foot. Aplacophorans weretraditionally interpreted (e.g. by Salvini-Plawen 1991) as the earliestbranch of themollusc lineage (Fig. 4u), but an alternativemodel placesthem as sister to the multivalved Polyplacophora (chitons) in a cladetermed Aculifera, which is in turn the sister group to all other molluscs(Conchifera). This ‘Aculifera hypothesis’ is supported by bothmolecular and morphological data (Sigwart & Sutton 2007;Wanninger & Wollesen 2018; but see Salvini-Plawen & Steiner2014). Resolution of the debate is critical for reconstructingmolluscanevolution (did the ancestor of crown-group molluscs have a shell and/or foot?) and for determining their position in the tree of life.

Aplacophorans were unknown as fossils before they werediscovered in the Herefordshire biota. Multivalved molluscs hadbeen described from the Paleozoic (e.g. Cherns 1998a, b), but weretreated as early polyplacophorans (chitons). The discovery ofAcaenoplax hayae from Herefordshire (Sutton et al. 2001a, 2004;Dean et al. 2015) revealed an aplacophoran-like form bearingspicules, lacking a true foot and possessing a posterior respiratorycavity (Fig. 4s and t). Nonetheless Acaenoplax shows serializationand multiple dorsal valves reminiscent of Polyplacophora. Thesubsequent discovery of Kulindroplax perissokomos (Sutton et al.2012a) documented an even more aplacophoran-like body with notrace of a foot but bearing a set of typical Paleozoic ‘polyplaco-phoran’ valves (Fig. 4l and m). This association appeared chimaericbut only because it represents an extinct character combination.Acaenoplax and Kulindroplax provide support for Aculifera (e.g.Vinther 2015), indicating that the ‘simple’ morphology ofaplacophorans is derived rather than primitive for the Mollusca.Furthermore, these Herefordshire fossils reveal a sequence ofevolutionary events that cannot be deduced from extant forms: avermiform body predated the loss of valves, and a diversity of‘plated aplacophorans’, previously misinterpreted as polyplaco-phorans, was present during the early Paleozoic. Modern

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 faunalcomposition of the Herefordshire Lagerstätte. Percentage abundance is based on number of specimens (n = 982). The chelicerate Offacolus is by farthe best represented species (833 specimens = 84.8% of the total number); all other species are represented by less than 10 specimens each (= lessthan 1%), except for the mandibulate Tanazios (88 specimens = 9.0%) and the malacostracan Cinerocaris (18 = 1.8%).

The Herefordshire Lagerstätte

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

aplacophorans evolved through a secondary loss of valves and are aremnant of a once larger group (Fig. 4v).

Research on evolutionary development has demonstrated howcontrol genes can initiate major changes in morphology. Modernpolyplacophorans (chitons) have a series of eight dorsal valvesassociated with the expression of the engrailed gene (Jacobs et al.2000), whereas Acaenoplax and Kulindroplax (Fig. 4l, m, s and t)have only seven. Such seven-fold seriality is expressed during theontogeny of living aculiferans (Scherholz et al. 2013) and mayreflect an ancestral morphology evident in Kulindroplax(Wanninger & Wollesen 2018). The addition of an eighth shell,which occurs after metamorphosis in Polyplacophora, appears to bean advanced condition (Wanninger & Wollesen 2018). Acaenoplaxis unusual in having an obvious gap between valves 6 and 7 wherean additional valve might have been placed (Sutton et al. 2001a,2004). The ‘missing’ valve may reflect the ancestral 7-valvemorphology or, given that the space for an additional valve ispresent, may have been lost secondarily from an 8-valved form.

The remarkable details preserved in some of the Herefordshirearthropods provide evidence of ancestral morphologies and promptquestions as to how they might have been transformed into modernforms. Martin et al. (2016) used CRISPR/Cas9 mutagenesis andRNAi knockdown to show how shifting Hox gene domains in aliving amphipod crustacean could contribute to macroevolutionarychanges in the body plan. The limbs of the Herefordshirechelicerates Dibasterium (Briggs et al. 2012; Fig. 4d, e, g and i)and Offacolus (Sutton et al. 2002; Fig. 4c and f) can behomologized readily with those of the living horseshoe crabLimulus but they differ in fundamental ways. Limbs 2 to 5 in theanterior division of the body (prosoma) of the HerefordshireSilurian forms have two branches, an endopod and exopod, both ofwhich are robust and segmented, whereas only one branch is presentin Limulus. Unlike the branches of limbs in other arthropods,however, the two branches are not connected to a common basalsegment: they insert in different places on the body wall. Thisarrangement can be interpreted as a stage in the loss of the exopod toyield a limb with a single ramus, as in Limulus. The expression ofDistal-less (Dll) in larval Limulus (Mittmann & Scholtz 2001)suggests that the loss of the exopod may be developmental. Limulusshows a strong expression of Dll associated with the origin of theendopod and a weaker expression, in the early embryo, in anadjacent position corresponding to the insertion of the exopods in

Dibasterium. This weaker expression disappears in later embryonicstages echoing the loss of the outer ramus during the evolution of thegroup (Briggs et al. 2012). The chelicera of Dibasterium (limb 1) isalso unusual in being an elongate flexible antenna-like appendage,in contrast to the familiar claw of Limulus. Here, too, knockdown ofgenes such as Dll and dachsund may have led to the loss of distalpodomeres and the evolution of the shorter chelicera in modernhorseshoe crabs, echoing experimental results on the harvestmanPhalangium (Sharma et al. 2013).

Extended stratigraphic ranges and the calibration ofmolecular clocks

The Herefordshire Lagerstätte is one of very few windows on soft-bodied organisms during the Silurian. It is not surprising, therefore,that the fossils provide important examples of stratigraphic rangeextensions. Thanahita (Fig. 5a and b), for example, falls in a cladewith the three known species of the iconic lower to mid-Cambrianlobopodian Hallucigenia (Siveter et al. 2018b). The long slendertrunk appendages of Thanahita contrast with those of short-leggedlobopodians, such as the Cambrian Antennacanthopodia (Ou et al.2011). Long-legged lobopodians are also represented byCarbotubulus from the late Carboniferous Mazon CreekLagerstätte of Illinois (Haug et al. 2012) which is some 135 Mayounger than Thanahita.

The discovery of the arthropod Enalikter (Siveter et al. 2014a,2015a; Fig. 5e) revealed that megacheirans (short-great-appendagearthropods) were present in the Silurian. Phylogenetic analysisshowed that Bundenbachiellus from the Devonian Hunsrück Slate,not previously interpreted as a megacheiran, is sister to Enalikter,extending the range of megacheirans nearly 100 Ma beyond thepreviously youngest known short-great-appendage arthropod,Leanchoilia? sp. from the mid-Cambrian of Utah (Briggs et al.2008).

Xylokorys is the only known Silurian marrellomorph euarthropod(Siveter et al. 2007b; Fig. 5r and w). It belongs to the Acercostraca(see Legg 2016) which, in contrast to the familiar CambrianMarrella, is characterized by a large shield-like carapace whichcovers the head and trunk. Xylokorys extends the Cambrian(Primicaris and Skania) and Ordovician (Enosiaspis) acercostracanrecord and heralds the youngest known example, Vachonisia, in theDevonian Hunsrück Slate.

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 areall 1 mm.

D. J. Siveter et al.

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

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) Kulindroplaxperissokomos, lateral, dorsal views. (n, o) Rhamphoverritor reduncus, attached juvenile, free swimming stage, lateral views. (p) Invavita piratica, lateralview. (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 depictingcompeting molluscan phylogenies (simplified from Wanninger & Wollesen 2018): (u) ‘Testaria’ hypothesis, one of several phylogenetic models in whichaplacophorans 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.

The Herefordshire Lagerstätte

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

Haliestes (Siveter et al. 2004; Fig. 5n and o) is arguably the bestpreserved and most complete of just 13 known species (Sabrouxet al. 2019) of fossil pycnogonid. These include a putative larval

pycnogonid, Cambropycnogon klausmuelleri, from the upperCambrian Orsten of Scandinavia (Waloszek & Dunlop 2002), anda basal stem group pycnogonid, Palaeomarachne granulata, from

Fig. 5. Herefordshire Lagerstätte species. (a, b) Thanahita distos, dorsal, right lateral views. (c, d) Cinerocaris magnifica, right lateral, posteriorventrolateral 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) Spirocopiaaurita, 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.

D. J. Siveter et al.

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

the Ordovician William Lake Lagerstätte of Canada (Rudkin et al.2013). The Herefordshire pycnogonid Haliestes shows reduction ofthe body to a small trunk projection beyond the posteriormost limbs.This is a feature of Pantopoda, indicating that crown-grouppycnogonids extended back to the Silurian (Siveter et al. 2004;Arango & Wheeler 2007; Dunlop 2010; but see Charbonier et al.2007). Five pycnogonid species are known from the DevonianHunsrück Slate, of which Palaeopantopus maucheri andPalaeothea devonica show trunk end reduction. Three speciesfrom the Jurassic of La Voulte-sur-Rhȏne, France, may also belongto extant pantopod families (Charbonier et al. 2007).

Where Herefordshire taxa extend the range of particular cladesback in time, they provide fossil calibrations for the tree of life,notably in the case of arthropods. Haliestes provides one example(Wolfe et al. 2016), the barnacle Rhamphoverritor reduncus (Briggset al. 2003; Fig. 4n and o) provides a calibration for crown-groupThecostraca, and the phyllocarid Cinerocaris magnifica (Briggset al. 2005, Fig. 5c and d) for crown Malacostraca (Wolfe et al.2016). Herefordshire ostracods (Figs 4a, b, q, r; 5u, z, zz) providedthe calibration for crown Ostracoda (Oakley et al. 2013) prior to the

discovery of Luprisca incubi from the late Ordovician Beecher’sTrilobite Bed of New York State (Siveter et al. 2014b; Wolfe et al.2016). Invavita piratica from the Herefordshire Lagerstätte (Siveteret al. 2015b; Fig. 4p–r), the earliest adult pentastomid pancrusta-cean, is arguably a more secure calibration than Boeckelericambriapelturae from the Cambrian Orsten (Walossek & Müller 1994),which is based on a larva (Wolfe et al. 2016).

The described Herefordshire biota includes five species ofostracods with preserved appendages, all of which are myodocopes:Colymbosathon ecplecticos, Nymphatelina gravida, Nasunarisflata, Pauline avibella and Spiricopia aurita (Siveter et al. 2003,2007c, 2010, 2013, 2015b, 2018a). All of these, except N. gravida,represent the earliest representatives of crown-group cylindroleber-idids, which are characterized by the presence of gills. The onlyother fossil myodocopes with possible gill preservation areTriadocypris from the lower Triassic of Spitzbergen (Weitschat1983) and Juraleberis from the upper Jurassic of Russia (Vannier &Siveter 1996).

The Herefordshire molluscs Acaenoplax and Kulindroplax(Sutton et al. 2001a, 2004, 2012a; Sigwart & Sutton 2007;

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 WeeksFormation of Utah (Lerosy-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, machaeridianannelids, 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 viaBurgess Shale-type pathways as degraded organic (carbonaceous) compression fossils and almost all represent marginal-marine or ‘stressed’ environments (Orr2014). 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, eurypterids, xiphosurans, diverse crustaceans, brachiopods, polychaete annelids,echinoderms, conodonts and possible fish. The fauna of the late LlandoveryWaukesha Lagerstätte fromWisconsin (Mikulic et al. 1985a, b) shows some similarity toEramosa but is peritidal. It is dominated by arthropods, especially trilobites (13 genera), and includes a xiphosuran and phyllocarid, ostracod and thylacocephalancrustaceans. Conulariids and graptolites are common, but brachiopods, bryozoans, corals, molluscs and echinoderms are rare or absent. The Prí̌dolí Bertie Group ofOntario 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 thesediment after deposition and the Bertie Group represents a shallow subtidal setting (Vrazo et al. 2016). The deeper-marine Herefordshire fauna includes numeroussponges, two trilobite 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 thePriesthill and Waterhead groups of Lesmahagow, and correlatives in the Hagshaw and Pentland Hills, represent restricted quasi-marine to lacustrine settings and aretypically 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 millipedes (Wilson & Anderson 2004), was considered to be of late Wenlock/Ludlow age on the basis ofpalynomorphs (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 Prí̌dolí Ludlow Bone Bed Lagerstätte at Ludford Lane and Corner in the Welsh Borderland yields – in additionto fish bone fragments and thelodonts – centipedes, arthropleurid and kampecarid myriapods, eurypterids and scorpions (Jeram et al. 1990; Siveter 2000; Siveter2010b).

Fig. 6. Stratigraphic occurrence of Silurian Lagerstätten. Theradiometric dates are those given in Cohen et al. (2013, updated2018), the International Commission on Stratigraphy (ICS)Chronostratigraphic Chart.

The Herefordshire Lagerstätte

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

Fig. 4l, m, s and t) also provide molecular clock calibrations. Bothrepresent total group aplacophorans and crown-group Aculifera(Vinther et al. 2017; Wanninger & Wollesen 2018). TheHerefordshire polychaete worm Kenostrychus represents crownAciculata even though its original placement as a stem-groupphyllodocid (Sutton et al. 2001c; Fig. 5s) has been revised to stemamphinomid (Parry et al. 2016).

Mode of life and palaeoecology

Herefordshire taxa reveal aspects of gender, larval development andbrood care in fossil arthropods. The recognition of hemipenes in themyodocope ostracod Colymbosathon (Siveter et al. 2003; Fig. 4a)and eggs and a juvenile, in situ, within the carapace of themyodocope Nymphatelina (Siveter et al. 2007c, 2015b; Fig. 4b)allow male and female to be distinguished in early Paleozoicanimals. The brood care strategy evident in Nymphatelina remainsessentially unchanged in myodocopes today. A more complex andunique brooding behaviour is preserved in the stem mandibulateAquilonifer; the presumed female carries ten tiny juveniles eachwithin a cuticular capsule tethered by a delicate thread (Briggs et al.

2016; Fig. 4h). Such data suggest that complexity of broodingstrategies probably evolved early in the history of the Arthropoda.Specimens representing a free-living ‘cyprid’ larval stage and anattached juvenile of the barnacle Rhampoverritor reduncusdemonstrate that the lifecycle of cirripedes has remained essentiallythe same over 430 myr (Briggs et al. 2005; Fig. 4n and o). A boringthrough a valve of Rhampoverritor (Fig. 4n) also provides evidenceof predation in the biota. The discovery of a pentastomidpancrustacean, Invavita piratica, parasitic on the ostracodNymphatelina gravida is the only known fossil pentastomidpreserved with its host and suggests that the group may haveoriginated as ectoparasites on marine invertebrates (Siveter et al.2015b; Fig. 4p–r). Bethia serraticulma is the only described fossilrhynchonelliformean brachiopod with preserved soft parts (Fig. 4k).Evidence of its mode of life, shell size and lophophore configurationindicates that the only known specimen is an immature example. Itsunusual 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 ofB. serraticulma bears an in vivo epifauna including a tinyunmineralized lophophorate, Drakozoon kalumon (Fig. 4j), whichmay allude to a widespread occurrence of similar but unknownlophophorates in the Paleozoic (Sutton et al. 2010).

The majority of the 63 known Herefordshire species lived on theseafloor (Table 1; Fig. 7), either as sessile or vagile epibenthos.Unless the Herefordshire animals were transported from signifi-cantly shallower waters, it appears that the benthic components ofthe biota lived in dim light conditions at best. Several of the benthicanimals lack eyes and there is an absence of photic-zone indicatorssuch as algae. Given the evidence for rapid burial it is likely that thewater column biota is under-represented. Sponges are the dominantsessile element, with brachiopods, echinoderms (the edrioasteroidHeropyrgus and a crinoid), a cirripede, coral, and a possibledendroid graptolite and gastropod (Platyceras?) as minor compo-nents. Most of the vagile epibenthos are arthropods, but this groupalso includes two echinoderms (the asteroid Bdellacoma andophiocistioid Sollasina), the aplacophoran mollusc Acaenoplax, agastropod and a bivalve mollusc. The nektobenthos is made upentirely of arthropods, with ostracods the most species abundant. Anautiloid is the only nektonic element. A graptolite and radiolarianscomprise the pelagic plankton. A possible infaunal/semi-infaunalmode of life is interpreted for just three taxa, a lingulid brachiopod,the aplacophoran Kulindroplax and the annelid Kenostrychus. Thenumerical dominance of Offaculus within the biota is probably realrather than representing an artefact of sampling, as concretions areselected and split randomly and, as one of the smaller taxa, its highyield is unlikely to result from being selectively preserved. The biotaincludes representatives of manymajor invertebrate groups but thereis no obvious reason why some groups known from the Silurian of

Fig. 7. Ecological types and percentage abundance based on number ofspecies (n = 63). It includes the four radiolarian species, and some speciesbelonging to other major groups that are as yet undescribed andunassigned at species level.

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 otherarthropods, crinoids and other echinoderms, graptolites and various microfossils 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 otherSilurian locality has yet been identified, even within the Welsh Basin.

D. J. Siveter et al.

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

the Welsh Basin elsewhere, such as vertebrates and bryozoans, havenot been found in the concretions.

Why is the Herefordshire Lagerstätte apparentlyunique?

The Herefordshire biota is one of a number of examples of softtissue preservation in concretions. Concretion formation requirescarbonate (or more rarely silica) supersaturation and the presence ofa nucleus. In some cases, a buried carcass can provide both. Softtissues can be preserved within concretions in a number of ways(McCoy et al. 2015): as carbonaceous remains, as a result ofreplication in a variety of authigenic minerals, or by void fill, as inthe case of the Herefordshire biota. Exceptional preservation ispromoted where concretion growth is rapid, and void fills are morelikely to occur where cementation occurs from the inside out,slowing diffusion at an early stage and isolating the organism fromits environment (McCoy et al. 2015). An analysis of factorsassociated with exceptional preservation in concretions (based on88 concretion sites, of which 20% preserve soft-tissues) showed thatconcretion formation enhances the chances of exceptional preser-vation only where other conditions are favourable, such as burial infine-grained sediment (McCoy et al. 2015). The volcanic ash thatburied the Herefordshire organisms may have played a role in theirexceptional preservation. Fine grain size may have promoted anoxiawithin the sediment and inhibited scavengers.

Predictably, perhaps, most examples of exceptional preservationassociated with volcanic ash are in terrestrial settings (Briggs et al.1996), in lacustrine and fluvial environments. Preservation of plantmaterials in volcanic ash may be enhanced by rapid burial andanoxic conditions, as in the Cretaceous of Argentina (Lafuente Diazet al. 2018) and the Miocene Clarkia Beds of Idaho, USA(Ladderud et al. 2015). The Triassic Chañares Formation ofArgentina yields a diversity of tetrapods in carbonate concretions involcanic ash that likely formed where microbial decay promotedcarbonate precipitation (Rogers et al. 2001) but concretion biotasare rare in terrestrial environments. Giant radiodontids are preservedin silica-chlorite concretions in the lower Fezouata Formation ofMorocco, including silica, iron and aluminium thought to besourced from volcanic ash (Gaines et al. 2012). Avolcanic influencehas also been reported impacting fossil preservation in other marinesettings, including some from the Silurian of the UK and Ireland.Coral communities in the Llandovery Kilbride Formation of Co.Mayo, Ireland are buried by volcanic ash (Harper et al. 1995), as areshelly fossils in the Wenlock Ballynane Formation of the DinglePeninsula, Co. Kerry, Ireland (Ferretti & Holland 1994), but softtissue preservation is unknown and associated concretions have notbeen reported. The cuticle of decapod moults and the ligament ofbivalves are preserved in concretions in the Cretaceous BearpawFormation of Alberta (Bentonite number 5) following rapid burial involcanic ash (Heikoop et al. 1996), but carbonate concretionsassociated with volcanics in marine settings are rare.

Preservation of the Herefordshire fossils involved an unusualcombination of circumstances: rapid burial of small living animalsin thick, fine-grained volcanic ash in a marine setting, and very rapidformation of near-spherical carbonate-cemented concretions. Otherbiotas preserved in a similar fashion to the Herefordshire examplehave yet to be discovered. The tiny crystalline infills are unlikely toattract the interest of a casual collector and the fossils are often off-centre and may not be intersected by the plane of splitting. Silurianbentonites are widespread in the UK, but they are generally only afew centimetres thick (Cave & Loydell 1998). Examples ofconcretions within ashes of any age merit serious scrutiny butearly formed concretions in fine-grained marine mudstones mayalso yield exceptionally preserved fossils depending, perhaps, onthe clay minerals present.

Acknowledgements Carolyn Lewis (Oxford University Museum ofNatural History) is thanked for technical assistance. We are grateful for insightfulcomments from Patrick Orr, Robert Gaines and a third anonymous reviewer.

Funding Funding is gratefully acknowledged from the Leverhulme Trust(grant no. EM-2014-068), the Natural Environment Research Council (grant no.NF/F0108037/1), the Yale Peabody Museum of Natural History InvertebratePaleontology Division, and English Nature.

Author contributions DerekJS led funding acquisition and specimencuration and coordinated this review; all authors contributed equally to researchand to the preparation of the paper.

Scientific editing by Philip Donoghue

ReferencesAldridge, R.J., Dorning, K.J. & Siveter, D.J. 1981. Distribution of microfossil

groups across the Wenlock Shelf of the Welsh Basin. In: Neale, J.W. &Brasier, M.D. (eds) Microfossils from Recent and fossil Shelf Seas. BritishMicropalaeontological Society, 18–30.

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 Lockedin the Fossil Record. Plenum Press, New York, 25–70.

Arango, C.P. & Wheeler, W.C. 2007. Phylogeny of the sea spiders (Arthropoda,Pycnogonida) based on direct optimization of six loci and morphology.Cladistics, 23, 255–293, https://doi.org/10.1111/j.1096-0031.2007.00143.x

Bassett, M.G. 1974. Review of the stratigraphy of the Wenlock Series in theWelsh Borderland and South Wales. Palaeontology, 17, 745–777.

Bassett, M.G., Bluck, B.J., Cave, R., Holland, C.H. & Lawson, J.D. 1992.Silurian. In: Cope, J.C.W., Ingham, J.K. & Rawson, P.F. (eds) Atlas ofPalaeogeography and Lithofacies. Geological Society, London, Memoirs, 13,37–56, https://doi.org/10.1144/GSL.MEM.1992.012.01.16

Blender Foundation. 2019. Blender, http://www.blender.orgBrett, 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.tb01507.x

Briggs, D.E.G., Siveter, D.J. & Siveter, D.J. 1996. Soft-bodied fossils from aSilurian volcaniclastic deposit.Nature, 382, 248–250, https://doi.org/10.1038/382248a0

Briggs, D.E.G., Sutton, M.D., Siveter, D.J. & Siveter, D.J. 2003. A newphyllocarid (Crustacea) from the Silurian Fossil-Lagerstätte of Herefordshire,England. Proceedings of the Royal Society of London B, 271, https://doi.org/10.1098/rspb.2003.2593

Briggs, D.E.G., Sutton, M.D., Siveter, D.J. & Siveter, D.J. 2005. Metamorphosisin a Silurian barnacle. Proceedings of the Royal Society of London B, 272,2365–2369, https://doi.org/10.1098/rspb.2005.3224

Briggs, D.E.G., Lieberman, B.S., Hendricks, J.R., Halgedahl, S.L. & Jarrard,R.D. 2008. Middle Cambrian arthropods from Utah. Journal of Paleontology,82, 238–254, https://doi.org/10.1666/06-086.1

Briggs, D.E.G., Siveter, D.J., Siveter, D.J., Sutton, M.D., Garwood, R.J. & Legg,D. 2012. Silurian horseshoe crab illuminates the evolution of arthropod limbs.Proceedings of the National Academy of Sciences, 109, 15702–15705, https://doi.org/10.1073/pnas.1205875109

Briggs, D.E.G., Siveter, D.J., Siveter, D.J., Sutton, M.D. & Legg, D. 2016. Tinyindividuals attached to a new Silurian arthropod suggest a unique mode ofbrood care. Proceedings of the National Academy of Sciences, 113,4410–4415, https://doi.org/10.1073/pnas.1600489113

Briggs, D.E.G., Siveter, D.J., Siveter, D.J., Sutton, M.D. & Rahman, I.A. 2017.An edrioasteroid from the Silurian Herefordshire Lagerstätte of Englandreveals the nature of the water vascular system in an extinct echinoderm.Proceedings of the Royal Society of London B, 284, 20171189, https://doi.org/10.1098/rspb.2017.1189

Cave, R. & Loydell, D.K. 1998. Wenlock volcanism in the Welsh Basin.Geological Journal, 33, 107–120, https://doi.org/10.1002/(SICI)1099-1034(1998040)33:2<107::AID-GJ778>3.0.CO;2-R

Charbonier, S., Vannier, J. & Riou, B. 2007. New sea spiders from the Jurassic LaVouIte-sur-Rhône Lagerstätte. Proceedings of the Royal Society B, 274,2555–2561, https://doi.org/10.1098/rspb.2007.0848

Cherns, L. 1998a. Chelodes and closely related Polyplacophora (Mollusca) fromthe Silurian of Gotland, Sweden. Palaeontology, 41, 545–573.

Cherns, L. 1998b. Silurian polyplacophoran molluscs from Gotland, Sweden.Palaeontology, 41, 939–974.

Cherns, L., Cocks, L.R.M., Davies, J.R., Hillier, R.D., Waters, R.A. &Williams,M. 2006. Silurian: the influence of extensional tectonics and sea-level changeson sedimentation in the Welsh Basin and on the Midland Platform. In:Brenchley, P.J. & Rawson, P.F. (eds) The Geology of England and Wales.Geological Society, London, 5–102, https://doi.org/10.1144/GOEWP.4

Clarke, J.M. & Ruedemann, R. 1912. The Eurypterida of New York. Memoirs ofthe New York State Museum, 14.

Cocks, L.R.M. & Torsvik, T.H. 2005. Baltica from the late Precambrian to mid-Palaeozoic times: the gain and loss of a terrane’s identity. Earth ScienceReviews, 72, 39–66, https://doi.org/10.1016/j.earscirev.2005.04.001

The Herefordshire Lagerstätte

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

Cocks, L.R.M. & Torsvik, T.H. 2011. The Palaeozoic geography of Laurentia andwestern Laurussia: a stable craton with mobile margins. Earth ScienceReviews, 106, 1–51, https://doi.org/10.1016/j.earscirev.2011.01.007

Cocks, L.R.M., Holland, C.H. & Rickards, R.B. 1992. A Revised Correlation ofSilurian Rocks in the British Isles. Geological Society, London, SpecialReports, 21.

Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. 2013 (updated 2018). TheICS International Chronostratigraphic Chart. Episodes, 36, 199–204.

Copeland, M. & Bolton, T.E. 1985. Fossils of Ontario Part 3: The eurypteridsand phyllocarids. Royal Ontario Museum Life Sciences MiscellaneousPublications, 48.

Cramer, B.D., Condon, D.J. et al. 2012. U–Pb (zircon) age constraints on thetiming and duration of Wenlock (Silurian) paleocommunity collapse andrecovery during the ‘Big Crisis’. Geological Society of America Bulletin, 124,1841–1857, https://doi.org/10.1130/B30642.1

Dean, C.D., Sutton, M.D., Siveter, D.J. & Siveter, D.J. 2015. A novel respiratoryarchitecture in the Silurian mollusc Acaenoplax. Palaeontology, 58, 839–847,https://doi.org/10.1111/pala.12181

Dunlop, J.A. 2010. Geological history and phylogeny of Chelicerata. ArthropodStructure&Development, 39, 124–142, https://doi.org/10.1016/j.asd.2010.01.003

Ferretti, A. &Holland, C.H. 1994. Biofacies and palaeoenvironmental analysis ofa limestone lens, unique in the Irish Silurian from the Dingle Peninsula,County Kerry. Irish Journal of Earth Sciences, 13, 83–89.

Fu, H.L.K. 2016. A Silurian bivalve with soft tissues. Newsletter of thePalaeontological Association, 94, 88–89.

Gaines, R.R., Briggs, D.E.G., Orr, P.J. & Van Roy, P. 2012. Preservation of giantanomalocaridids in silica–chlorite concretions from the early Ordovician ofMorocco. Palaios, 27, 317–325, https://doi.org/10.2110/palo.2011.p11-093r

Harper, D.A.T., Scrutton, C.T. & Williams, D.M. 1995. Mass mortalities on anIrish Silurian sea-floor. Journal of the Geological Society, London, 152,917–922, https://doi.org/10.1144/GSL.JGS.1995.152.01.06

Haug, J.T., Mayer, G., Haug, C. & Briggs, D.E.G. 2012. A Carboniferous non-onychophoran lobopodian reveals long-term survival of a Cambrianmorphotype. Current Biology, 22, 1673–1675, https://doi.org/10.1016/j.cub.2012.06.066

Heikoop, J.M., Tsujita, C.J., Heikoop, C.E., Risk, M.J. & Dicken, A.C. 1996.Effects of volcanic ashfall recorded in ancient marine benthic communities:Comparison of a nearshore and an offshore environment. Lethaia, 29,125–139, https://doi.org/10.1111/j.1502-3931.1996.tb01868.x

Holdsworth, B.K. 1967. Dolomitization of siliceous microfossils in Namurianconcretionary limestones. Geological Magazine, 104, 148–154, https://doi.org/10.1017/S0016756800040607

Hurst, J.M., Hancock, N.J. & McKerrow, W.S. 1978. Wenlock stratigraphy andpalaeogeography ofWales andWelsh Borderland.Proceedings of the Geologists’Association, 89, 197–222, https://doi.org/10.1016/S0016-7878(78)80012-1

Jacobs, D.K., Wray, C.G. et al. 2000. Molluscan engrailed expression, serialorganization, and shell evolution. Evolution & Development, 2, 340–347,https://doi.org/10.1046/j.1525-142x.2000.00077.x

Jeram, A.J., Selden, P.A. & Edwards, D. 1990. Land animals in the Silurian:Arachnids and myriapods from Shropshire, England. Science, 250, 658–661,https://doi.org/10.1126/science.250.4981.658

Ladderud, J.A., Wolff, J.A., Rember, J.C. & Brueseke, M.E. 2015. Volcanic ashlayers in the Miocene Lake Clarkia Beds: Geochemistry, regional correlation,and age of the Clarkia Flora. Northwest Science, 89, 309–323, https://doi.org/10.3955/046.089.0402

Lafuente Diaz, M.A., D’Angelo, J.A., Del Fueyo, G.M. & Zodrow, E.L. 2018.Fossilization model for Squamastrobus tigrensis foliage in a volcanic-ashdeposit: Implications for preservation and taphonomy (Podocarpaceae, LowerCretaceous, Argentina). Palaios, 33, 323–337, https://doi.org/10.2110/palo.2017.110

Legg, D.A. 2016. An acercostracan marrellomorph (Euarthropoda) from theLower Ordovician ofMorocco. The Science of Nature, 103, 21, https://doi.org/10.1007/s00114-016-1352-5

Legg, D.A., Sutton, M.D. & Edgecombe, G.D. 2013. Arthropod fossil dataincrease congruence of morphological and molecular phylogenies. NatureCommunications, 4, 2485, https://doi.org/10.1038/ncomms3485

Lerosy-Aubril, R., Gaines, R.R., Hegna, T.A., Ortega-Hernández, J., Van Roy,P., Kier, C. & Bonino, E. 2018. The Weeks Formation Konservat-Lagerstätteand the evolutionary transition of Cambrian marine life. Journal of theGeological Society, London, 175, 705–715, https://doi.org/10.1144/jgs2018-042

Lorensen, W.E. & Cline, H.E. 1987. Marching cubes: a high resolution 3Dsurface construction algorithm. Computer Graphics, 21, 163–169, https://doi.org/10.1145/37402.37422

Marshall, J.E. 1991. Palynology of the Stonehaven Group, Scotland: evidence fora Mid Silurian age and its geological implications.Geological Magazine, 128,283–286, https://doi.org/10.1017/S0016756800022135

Martin, A., Serano, J.M. et al. 2016. CRISPR/Cas9 mutagenesis reveals versatileroles of Hox genes in crustacean limb specification and evolution. CurrentBiology, 26, 14–26, https://doi.org/10.1016/j.cub.2015.11.021

McCoy, V.E., Young, R.T. & Briggs, D.E.G. 2015. Factors controllingexceptional preservation in concretions. Palaios, 30, 272–280, https://doi.org/10.2110/palo.2014.081

Mikulic, D.G., Briggs, D.E.G. & Kluessendorf, J. 1985a. A Silurian soft-bodiedbiota. Science, 228, 715–717, https://doi.org/10.1126/science.228.4700.715

Mikulic, D.G., Briggs, D.E.G. & Kluessendorf, J. 1985b. A new exceptionallypreserved biota from the Lower Silurian of Wisconsin. U.S.A. PhilosophicalTransactions of the Royal Society of London B, 311, 75–85, https://doi.org/10.1098/rstb.1985.0140

Mittmann, B. & Scholtz, G. 2001. Distal-less expression in embryos of Limuluspolyphemus (Chelicerata, Xiphosura) and Lepisma saccharina (Insecta,Zygentoma) suggests a role in the development of mechanoreceptors,chemoreceptors, and the CNS. Development, Genes and Evolution, 211,232–243, https://doi.org/10.1007/s004270100150

Nadhira, A., Sutton, M.D. et al. 2019. Three-dimensionally preserved soft-tissuesand calcareous hexactins in a Silurian sponge: implications for early spongeevolution. Royal Society Open Science, 6, https://doi.org/10.1098/rsos.190911

Oakley, T.H., Wolfe, J.M., Lindgren, A.R. & Zaharoff, A.K. 2013.Phylotranscriptomics to bring the understudied into the fold: MonophyleticOstracoda, fossil placement, and Pancrustacean phylogeny. MolecularBiology and Evolution, 30, 215–233, https://doi.org/10.1093/molbev/mss216

Orr, P.J. 2014. Late Proterozoic–early Phanerozoic ‘taphonomic windows’:the environmental and temporal distribution of recurrent modes ofexceptional preservation. In: Laflamme, M., Schiffbauer, J.D. & Darroch,S.A.F. (eds) Reading and Writing of the Fossil Record: PreservationalPathways to Exceptional Fossilization. The Paleontological Society Papers,20, 289–313.

Orr, P.J., Briggs, D.E.G., Siveter, D.J. & Siveter, D.J. 2000. Three-dimensionalpreservation of a non-biomineralized arthropod in concretions in Silurianvolcaniclastic rocks from Herefordshire, England. Journal of the GeologicalSociety, London, 157, 173–186, https://doi.org/10.1144/jgs.157.1.173

Orr, P.J., Siveter, D.J., Briggs, D.E.G. & Siveter, D.J. 2002. Preservation ofradiolarians in the Herefordshire Konservat-Lagerstätte (Wenlock, Silurian),England, and implications for the taphonomy of the biota. Special Papers inPalaeontology, 67, 205–224.

Ou, Q., Liu, J., Shu, D., Han, J., Zhang, Z., Wan, X. & Lei, Q. 2011. A rareonychophoran-like lobopodian from the Lower Cambrian ChengjiangLagerstätte, southwestern China, and its phylogenetic implications. Journalof Paleontology, 85, 587–594, https://doi.org/10.1666/09-147R2.1

Parry, L.A., Edgecombe, G.D., Eibye-Jacobsen, D. & Vinther, J. 2016. Theimpact of fossil data on annelid phylogeny inferred from discretemorphological characters. Proceedings of the Royal Society of London B,283, 20161378, https://doi.org/10.1098/rspb.2016.1378

Rahman, I.A., Thompson, J.R., Briggs, D.E.G., Siveter, D.J., Siveter, D.J. &Sutton, M.D. 2019. A new ophiocistioid with soft-tissue preservation from theSilurian Herefordshire Lagerstätte, and the evolution of the holothurian bodyplan. Proceedings of the Royal Society of London B, 286, 20182792, https://doi.org/10.1098/rspb.2018.2792

Riley, D.A. 2012. Preservation and taphonomy of the fossils of the Herefordshire(Silurian) Konservat-Lagerstätte. PhD thesis, University of Leicester.

Ritchie, A. 1968. New evidence on Jamoytius kerwoodi White, an importantostracoderm from the Silurian of Lanarkshire, Scotland. Palaeontology, 11,21–39.

Ritchie, A. 1985. Ainiktozoon loganense Scourfield, a protochordate? from theSilurian of Scotland. Alcheringa, 9, 117–142, https://doi.org/10.1080/03115518508618961

Rogers, R.R., Arcucci, A.B., Abdala, F., Sereno, P.C., Forster, C.A. & May, C.L.2001. Paleoenvironment and taphonomy of the Chañares Formation tetrapodassemblage (Middle Triassic), northwestern Argentina: Spectacular preserva-tion in volcanogenic concretions. Palaios, 16, 461–481, https://doi.org/10.1669/0883-1351(2001)016<0461:PATOTC>2.0.CO;2

Rolfe, W.D.I. 1973. Silurian arthropod and fish assemblages from Lesmahagow,Lanarkshire. In: Bluck, B.J. (ed.) Excursion Guide to the Geology of theGlasgow District. Geological Society of Glasgow, 119–126.

Rudkin, D.M., Cuggy, M.B., Young, G.A. & Thompson, D.P. 2013. AnOrdovician pycnogonid (sea spider) with serially subdivided ‘head’ region.Journal of Paleontology, 87, 395–405, https://doi.org/10.1666/12-057.1

Sabroux, R., Audo, D., Charbonnier, S., Corbari, L. & Hassanin, A. 2019. 150-million-year-old sea spiders (Pycnogonida: Pantopoda) of Solnhofen. Journalof Systematic Palaeontology, https://doi.org/10.1080/14772019.2019.1571534

Salvini-Plawen, L.V. 1991. Origin, phylogeny and classification of the PhylumMollusca. Iberus, 9, 1–33.

Salvini-Plawen, L.V. & Steiner, G. 2014. The Testaria concept (Polyplacophora+ Conchifera) updated. Journal of Natural History, 48, 2751–2772, https://doi.org/10.1080/00222933.2014.964787

Scherholz,M.,Redl, E.,Wollesen, T., Todt, C.&Wanninger,A. 2013.Aplacophoranmolluscs evolved from ancestors with polyplacophoran-like features. CurrentBiology, 23, 2130–2134, https://doi.org/10.1016/j.cub.2013.08.056

Sharma, P.P., Schwager, E.E., Giribet, G., Jockusch, E.L. & Extavour, C.G. 2013.Distal-less and dachshund pattern both plesiomorphic and apomorphicstructures in chelicerates: RNA interference in the harvestman Phalangiumopilio (Opiliones). Evolution & Development, 15, 228–242, https://doi.org/10.1111/ede.12029

Shillito, A.P. & Davies, N.S. 2017. Archetypally Siluro-Devonian ichnofaunain the Cowie Formation, Scotland: implications for the myriapod fossil recordand Highland Boundary Fault movement. Proceedings of the Geologists’Association, 128, 815–828, https://doi.org/10.1016/j.pgeola.2017.08.002

Sigwart, J.D. & Sutton, M.D. 2007. Deep molluscan phylogeny: synthesis ofpalaeontological and neontological data. Proceedings of the Royal Society B,275, 1587–1593, https://doi.org/10.1098/ rspb.2007.0701

D. J. Siveter et al.

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from

Siveter, D.J. 2000. Ludford Lane and Ludford Corner. In: Aldridge, R.A.,Siveter, D.J., Siveter, D.J., Lane, P.D., Palmer, D. & Woodcock, N.H. (eds)British Silurian Stratigraphy. Joint Nature Conservation Committee,Peterborough, 432–437.

Siveter, D.J. 2010a. Dunside, South Lanarkshire; Gutterford Burn, East Lothianand Midlothian; Slot Burn, Ayrshire. In: Jarzembowski, E., Siveter, D.J.,Palmer, D. & Selden, P.A. (eds) Fossil Arthropods of Great Britain. JointNature Conservation Committee, Peterborough, 57–71.

Siveter, D.J. 2010b. Ludford Lane and Ludford Corner. In: Jarzembowski, E.,Siveter, D.J., Palmer, D. & Selden, P.A. (eds) Fossil Arthropods of GreatBritain. Joint Nature Conservation Committee, Peterborough, 93–97.

Siveter, D.J. & Palmer, D. 2010. Stonehaven. In: Jarzembowski, E., Siveter, D.J.,Palmer, D. & Selden, P.A. (eds) Fossil Arthropods of Great Britain. JointNature Conservation Committee, Peterborough, 79–85.

Siveter, D.J., Sutton, M.D., Briggs, D.E.G. & Siveter, D.J. 2003. An ostracodecrustacean with soft parts from the Lower Silurian. Science, 300, 1749–1751,https://doi.org/10.1126/science.1091376

Siveter, D.J., Sutton, M.D., Briggs, D.E.G. & Siveter, D.J. 2004. A Silurian seaspider. Nature, 431, 978–980, https://doi.org/10.1038/nature02928

Siveter, D.J., Aitchison, J.C., Siveter, D.J. & Sutton, M.D. 2007a. The Radiolariaof the Silurian Konservat-Lagerstätte of Herefordshire, England. Journal ofMicropalaeontology, 26, 1–8, https://doi.org/10.1144/jm.26.1.87

Siveter, D.J., Fortey, R.A., Sutton,M.D., Briggs, D.E.G. & Siveter, D.J. 2007b. ASilurian ‘marrellomorph’ arthropod. Proceedings of the Royal Society ofLondon B, 274, 2223–2229, https://doi.org/10.1098/rspb.2007.0712

Siveter, D.J., Siveter, D.J., Sutton, M.D. & Briggs, D.E.G. 2007c. Brood care in aSilurian ostracod. Proceedings of the Royal Society of London B, 247,465–469, https://doi.org/10.1098/rspb.2006.3756

Siveter, D.J., Sutton, M.D., Briggs, D.E.G. & Siveter, D.J. 2007d. A newprobable stem lineage crustacean with three-dimensionally preserved soft-parts from the Herefordshire (Silurian) Lagerstätte, UK. Proceedings of theRoyal Society of London B, 274, 2099–2107, https://doi.org/10.1098/rspb.2007.0429

Siveter, D.J., Briggs, D.E.G., Siveter, D.J. & Sutton, M.D. 2010. Anexceptionally preserved myodocopid ostracod from the Silurian ofHerefordshire, UK. Proceedings of the Royal Society of London B, 277,1539–1544, https://doi.org/10.1098/rspb.2009.2122

Siveter, D.J., Briggs, D.E.G., Siveter, D.J., Sutton, M.D. & Joomun, S.C. 2013. ASilurian myodocope with preserved soft parts: cautioning the interpretation ofthe shell-based ostracod record. Proceedings of the Royal Society of London B,280, 20122664, https://doi.org/10.1098/rspb.2012.2664

Siveter, D.J., Briggs, D.E.G., Siveter, D.J., Sutton, M.D., Legg, D. & Joomun, S.C.2014a. A Silurian short-great-appendage arthropod. Proceedings of the RoyalSociety of London B, 281, 20132986, https://doi.org/10.1098/rspb.2013.2986

Siveter, D.J., Tanaka, G., Farrell, ÚC, Martin, M.J., Siveter, D.J. & Briggs,D.E.G. 2014b. Exceptionally preserved 450-million-year-old Ordovicianostracods with brood care. Current Biology, 24, 801–806, https://doi.org/10.1016/j.cub.2014.02.040

Siveter, D.J., Briggs, D.E.G., Siveter, D.J. & Sutton, M.D. 2015a. Enalikteraphson is an arthropod: a reply to Struck et al. (2014). Proceedings of theRoyal Society of London B, 282, 20142663, https://doi.org/10.1098/rspb.2014.2663

Siveter, D.J., Briggs, D.E.G., Siveter, D.J. & Sutton, M.D. 2015b. A 425–million-year-old Silurian pentastomid parasitic on ostracods. Current Biology,25, 1–6, https://doi.org/10.1016/j.cub.2015.04.035

Siveter, D.J., Briggs, D.E.G., Siveter, D.J., Sutton, M.D. & Legg, D.A. 2017. Anew crustacean from the Herefordshire (Silurian) Lagerstätte, UK, and itssignificance in malacostracan evolution. Proceedings of the Royal Society ofLondon B, 284, 20170279, https://doi.org/10.1098/rspb.2017.0279

Siveter, D.J., Briggs, D.E.G., Siveter, D.J. & Sutton, M.D. 2018a. A well-preserved respiratory system in a Silurian ostracod. Biology Letters, 14,20180464, https://doi.org/10.1098/rsbl.2018.0464

Siveter, D.J., Briggs, D.E.G., Siveter, D.J., Sutton, M.D. & Legg, D. 2018b. Athree-dimensionally preserved Silurian lobopodian from the Herefordshire(Silurian) Lagerstätte, UK. Royal Society Open Science, 5, 172101, https://doi.org/10.1098/rsos.172101

Steeman, T., Vandenbroucke, T.R.A. et al. 2015. Chitinozan biostratigraphy ofthe Silurian Wenlock–Ludlow boundary succession of the Long Mountain,Powys, Wales. Geological Magazine, 153, 95–109, https://doi.org/10.1017/S0016756815000266

Suarez, S.E., Brookfield, M.E., Catlos, E.J. & Stöckli, D.F. 2017. A U–Pb zirconage constraint on the oldest-recorded air-breathing land animal. PLoS ONE,12, e0179262. https://doi.org/ 10.1371/journal.pone.0179262

Sutton, M.D. 2008. Tomographic techniques for the study of exceptionallypreserved fossils. Proceedings of the Royal Society B, 275, 1587–1593, https://doi.org/10.1098/rspb.2008.0263

Sutton, M.D., Briggs, D.E.G., Siveter, D.J. & Siveter, D.J. 2001a. Anexceptionally preserved vermiform mollusc from the Silurian of England.Nature, 410, 461–463, https://doi.org/10.1038/35068549

Sutton,M.D., Briggs, D.E.G., Siveter, D.J. & Siveter, D.J. 2001b. Methodologies forthe visualization and reconstruction of three-dimensional fossils from the SilurianHerefordshire Lagerstätte. Palaeontologia Electronica, 4, 1A.

Sutton, M.D., Briggs, D.E.G., Siveter, D.J. & Siveter, D.J. 2001c. A three-dimensionally preserved fossil polychaete worm from the Silurian ofHerefordshire, England. Proceedings of the Royal Society of London B, 268,2355–2363, https://doi.org/10.1098/rspb.2001.1788

Sutton, M.D., Briggs, D.E.G., Siveter, D.J., Siveter, D.J. & Orr, P.J. 2002. Thearthropod Offacolus kingi (Chelicerata) from the Silurian of Herefordshire,England: computer based morphological reconstructions and phylogeneticaffinities. Proceedings of the Royal Society, London B, 269, 1195–1203,https://doi.org/10.1098/rspb.2002.1986

Sutton, M.D., Briggs, D.E.G., Siveter, D.J. & Siveter, D.J. 2004. Computerreconstruction and analysis of the vermiform mollusc Acaenoplax hayae fromthe Herefordshire Lagerstätte (Silurian, England), and implications formolluscan phylogeny. Palaeontology, 47, 293–318, https://doi.org/10.1111/j.0031-0239.2004.00374.x

Sutton, M.D., Briggs, D.E.G., Siveter, D.J. & Siveter, D.J. 2005a. A Silurianarticulate brachiopod with soft parts. Nature, 436, 1013–1015, https://doi.org/10.1038/nature03846

Sutton, M.D., Briggs, D.E.G., Siveter, D.J., Siveter, D.J. & Gladwell, D.J. 2005b.A starfish with three-dimensionally preserved soft parts from the Silurian ofEngland. Proceedings of the Royal Society of London B, 272, 1001–1006,https://doi.org/10.1098/rspb.2004.2951

Sutton, M.D., Briggs, D.E.G., Siveter, D.J. & Siveter, D.J. 2006. Fossilizedsoft tissues in a Silurian platyceratid gastropod. Proceedings of the RoyalSociety of London B, 273, 1039–1044, https://doi.org/10.1098/rspb.2005.3403

Sutton, M.D., Briggs, D.E.G., Siveter, D.J. & Siveter, D.J. 2010. A soft-bodiedlophophorate from the Silurian of England. Biology Letters, 7, 146–149,https://doi.org/10.1098/rsbl.2010.0540

Sutton, M.D., Briggs, D.E.G., Siveter, D.J., Siveter, D.J. & Sigwart, J.D. 2012a.A Silurian armoured aplacophoran and implications for molluscan phylogeny.Nature, 490, 94–97, https://doi.org/10.1038/nature11328

Sutton, M.D., Garwood, R.J., Siveter, D.J. & Siveter, D.J. 2012b. SPIERS andVAXML; a software toolkit for tomographic visualisation and a format forvirtual specimen interchange. Palaeontologia Electronica, 15, 5T, https://doi.org/10.26879/289

Sutton, M.D., Rahman, I.A. & Garwood, R.J. 2014. Techniques for VirtualPalaeontology. Wiley, Chichester.

Vannier, J.M.C. & Siveter, D.J. 1996. On Juraleberis jubata gen. et sp. nov.Stereo-Atlas of Ostracod Shells, 22 [for 1995], 86–95.

Van Roy, P., Briggs, D.E.G. & Gaines, R.R. 2015. The Fezouata fossils ofMorocco; an extraordinary record of marine life in the Early Ordovician.Journal of the Geological Society, London, 172 541–549, https://doi.org/10.1144/jgs2015-017

Vinther, J. 2015. The origins of molluscs. Palaeontology, 58, 19–34, https://doi.org/10.1111/pala.12140

Vinther, J., Parry, L., Briggs, D.E.G. & Van Roy, P. 2017. Ancestral morphologyof crown-group molluscs revealed by a new Ordovician stem aculiferan.Nature, 542, 471–475, https://doi.org/10.1038/nature21055

Vrazo, M.B., Brett, C.E. & Ciurca, S.J. 2016. Buried or brined? Eurypteridsand evaporites in the Silurian Appalachian basin. Palaeogeography,Palaeoclimatology, Palaeoecology, 444, 48–59, https://doi.org/10.1016/j.palaeo.2015.12.011

Walossek, D. & Müller, K.J. 1994. Pentastomid parasites from the LowerPalaeozoic of Sweden. Transactions of the Royal Society of Edinburgh, EarthSciences, 85, 1–37, https://doi.org/10.1017/S0263593300006295

Waloszek, D. & Dunlop, J.A. 2002. A larval sea spider (Arthropoda:Pycnogonida) from the Upper Cambrian ‘Orsten’ of Sweden, and thephylogenetic position of pycnogonids. Palaeontology, 45, 421–426, https://doi.org/10.1111/1475-4983.00244

Wanninger, A. & Wollesen, T. 2018. The evolution of molluscs. BiologicalReviews, 94, 102–115, https://doi.org/10.1111/brv.12439

Weitschat, W. 1983. Myodocopid ostracodes with preserved appendages from theLower Triassic of Spitsbergen. Paläontologische Zeitschrift, 57, 309–323,https://doi.org/10.1007/BF02990320

Wellman, C. 1993. A land plant microfossil assemblage of Mid Silurian age fromthe Stonehaven Group, Scotland. Journal of Micropalaeontology, 12, 47–66,https://doi.org/10.1144/jm.12.1.47

Wilson, H.M. & Anderson, L. 2004. Morphology and taxonomy of Paleozoicmillipedes (Diplopoda: Chilognatha: Archipolypoda) from Scotland. Journalof Palaeontology, 78, 169–184, https://doi.org/10.1666/0022-3360(2004)078<0169:MATOPM>2.0.CO;2

Wolfe, J.M., Daley, A.C., Legg, D.A. & Edgecombe, G.D. 2016. Fossilcalibrations for the arthropod Tree of Life. Earth Science Reviews, 160,43–110, https://doi.org/10.1016/j.earscirev.2016.06.008

Won, M.-Z., Blodgett, R.B. & Nestor, V. 2002. Llandoverian (Early Silurian)radiolarians from the Road River Formation of east-central Alaska and the newfamily Haplotaeniatumidae. Journal of Paleontology, 76, 941–964, https://doi.org/10.1017/S0022336000057796

Woodcock, N.H. 1993. The Precambrian and Silurian succession of the OldRadnor to Presteigne area. In: Woodcock, N.H. & Bassett, M.G. (eds)Geological Excursions in Powys, Central Wales. University of Wales Press,National Museum of Wales, Cardiff, 209–228.

The Herefordshire Lagerstätte

by guest on March 7, 2020http://jgs.lyellcollection.org/Downloaded from