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Accepted Manuscript Journal of the Geological Society

The Herefordshire Lagerstätte: fleshing out marine

Derek J. Siveter, Derek E. G. Briggs, David J. Siveter & Mark D. Sutton

DOI: https://doi.org/10.1144/jgs2019-110

Received 9 July 2019 Revised 19 August 2019 Accepted 21 August 2019

© 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

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The Herefordshire Lagerstätte: fleshing out Silurian

Derek J. Siveter1,2*, Derek E. G. Briggs3, David J. Siveter4 and Mark D. Sutton5

1Earth Collections, University Museum of Natural History, Oxford OX1 3PW, UK

2Department of Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK

3Department of and Geophysics and Yale Peabody Museum of Natural History, Yale University, Po Box 208 109, New Haven, CT 06520-8109, USA

4School of Geography, Geology and the Environment, University of Leicester, Leicester LE1 7RH, UK

5Department of Earth Sciences and Engineering, Imperial College London, London SW7 2BP, UK

*Correspondence: [email protected]

Abstract: The Herefordshire Lagerstätte (c. 430 Myr BP) from the UK is a rare example of soft-tissue preservation in the Silurian. It yields a wide range of Silurian 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 , which was located on in the southern subtropics. The specimens are investigated by serial grinding, digital photography and rendering in the round as a ‘virtual ’ 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 Palaeozoic Lagerstätten;ACCEPTED and in providing evidence of theMANUSCRIPT early evolution of crown-group representatives of several groups.

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The Herefordshire Lagerstätte is unique in preserving representatives of many major groups in high fidelity three-dimensions, as calcite in-fills within concretions in a marine volcaniclastic deposit. The Herefordshire fossils are unimpressive at first sight, yet they yield unrivalled evidence about the palaeobiology and evolutionary significance of Silurian . It provides, for example, the earliest evidence for the time of origin of certain major crown-groupsi.e. the living biota), for instance sea (pycnogonids).

The locality lies in the Welsh Borderland, a historic region central to R. I. Murchison’s pioneering research which established the Silurian in the 1830s. In 1990 Bob King, mineralogist and retired Curator in the Department of Geology at the University of Leicester, collected concretions in float from the locality and presented nine of them to the Leicester collections. Curator Roy Clements asked David Siveter to examine one of the concretions when he was accessioning the material in 1994. What David recognised under the microscope, an with preserved limbs (later named kingi), was confirmed by Derek Siveter and, together with Bob King, they identified the stratigraphic horizon and collected more concretions. In 1995 Derek Briggs joined the team and, following more visits to the locality, we published an initial paper flagging the significance of the discovery (Briggs et al. 1996). NERC and Leverhulme Trust funding enabled Patrick Orr and later Mark Sutton to work on the Lagerstätte, initially as postdoctoral researchers. The support and cooperation of the site owners and help from English Nature have been crucial in realizing its scientific potential.

Locality, age and palaeogeographic setting

The Herefordshire site lies in the Old Radnor to Presteigne area of the Welsh Borderland (Fig. 1a) in the vicinityACCEPTED of the Fault Zone. TheMANUSCRIPT fossil-bearing concretions (Fig. 1b, c) occur in a soft, fine-grained, cream-coloured, weathered and largely unconsolidated bentonite (Fig. 1b) that crops out over about 30 metres. The bentonite is unique in the British stratigraphic record, at least, in reaching a thickness of at least a metre in places. The bentonite was deposited within mudstones of the Wenlock Series Coalbrookdale Formation (Fig. 1d), which rests on the slightly older Dolyhir and Nash Scar Formation. In the Dolyhir region these and algal-rich lie with angular on a sedimentary basement (Woodcock 1993). At nearby Nash Scar, laterally equivalent carbonates sit, possibly disconformably (Woodcock 1993), on the shallow Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

marine upper Llandovery Series Folly Sandstone and a hardground is developed locally on the limestones (Hurst et al. 1978).

The host bentonite lies approximately at the boundary between the and stages (Fig. 1d), which is radiometrically constrained to 430.5 ±0.7 Ma (Cohen et al. 2013, updated 2018). Various evidence from , graptolites and indicate a Wenlock age for the Dolyhir and Nash Scar Limestone Formation in addition to the overlying Coalbrookdale Formation (Bassett 1974; Cocks et al. 1992). The Sheinwoodian Ozarkodina sagitta rhenana has been recovered from the limestones at Dolyhir and Nash Scar (Bassett 1974; Aldridge et al. 1981). The presence of Homerian Cyrtograptus lundgreni Biozone graptolites in the Coalbrookdale Formation mudstones immediately above the limestones at Nash Scar (Bassett 1974) suggests that deposition of these mudstones did not commence locally until C. lundgreni times (Hurst et al. 1978). This is consistent with the presence of a hardground that presumably formed during a hiatus in deposition in the late Sheinwoodian.

The recovery of the Margachitina margaritana, Ancyrochitina plurispinosa and Cingulochitina from 20 cm and 50 cm below the bentonite also indicates an upper Sheinwoodian to Homerian age for the Coalbrookdale Formation (G. Mullins, pers. comm, 2000; see also Steeman et al. 2015). That age is unchallenged by (David Siveter, unpublished information) and radiolarians (Siveter et al. 2007a) recovered from concretions in the bentonite.

The volcanic ash in which the Herefordshire Lagerstätte is preserved accumulated in the Welsh Basin (Fig. 1e). This lay on the microcontinent of Avalonia, which included southern Britain (Fig. 1f; Cocks & Torsvik 2011). Avalonia and adjacent were in southern tropical/subtropical palaeolatitudes in the mid-Silurian, separated from by a remnant . Shallow water muds characterised much of the Welsh Borderland and central England east of the Church Stretton Fault Zone (Bassett et al. 1992; Cherns et al. 2006). Coeval fine clastics and turbidites accumulated to the west in the deepest parts of the Welsh Basin. Palaeogeographical and sedimentologicalACCEPTED evidence indicates that the CoalbrookdaleMANUSCRIPT Formation in the Dolyhir-Nash Scar area of the Church Stretton Fault Zone accumulated in a moderately deep, outer shelf/shelf slope environment (Briggs et al. 1996); the presence of the Visbyella community suggests water depths of 100-200 m (Hurst et al. 1978; Brett et al. 1993). Geochemical analyses suggest a destructive plate margin as the source of the volcanic ash (Riley 2012). The closest known volcanic centres of Wenlock age (Cocks & Torsvik 2005) are the in southwest England (Avalonia) and the Dingle peninsula in southwest (Avalonia). The Herefordshire bentonite is geochemically similar to volcanic rocks in both areas (Riley 2012). A more distant candidate is the , then on Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

Perunica, a microcontinent in the centre of the (Fig. 1f).

Box 1. The fauna of the Herefordshire Lagerstätte

(Includes Table 1 (Faunal list) and Fig. 2a, b)

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 ~50% when other biomineralized taxa, including , and the extensive fauna, are described. Arthropods are diverse (Fig. 2a): 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 (uninvestigated). There are 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. ACCEPTEDThe chelicerate Offacolus kingi is theMANUSCRIPT 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 horseshoe 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 Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

tens of specimens and some, such as the bivalve Praectenodonta ludensis, on one specimen only.

The volcanic ash that hosts the Herefordshire biota was deposited on an erosion surface on massive limestone and, in places, muds; the uneven topography likely reflects the original Wenlock seafloor. The ash layer varies from a few tens of cm to over 1 m thick and is highly weathered at least at the surface: no evidence for primary sedimentary structures is preserved. Thus, it is unclear whether the ash represents in situ settling or was reworked, or whether there was one or several ashfalls. Occasional examples of unsuccessful escape traces (Orr et al. 2000) indicate that at least some of the animals were buried alive, which is consistent with the remarkable preservation. The bentonite has yielded at least five thousand randomly distributed calcite-cemented (Fig. 3) concretions 2-25 cm in diameter (Orr et al. 2000; Fig. 1b, c). Unusually for fossils preserved in this way, the organisms do not appear to have influenced the size or shape of the concretions, nor are they usually in the centre (Fig. 1c). Fossils are only preserved where they were incorporated into concretions but it is not clear what determined the locus of concretion formation. The timing and nature of the growth of 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 and the biomineralized taxa preserve remarkable details in addition to features preserved in the normal fossil record. The soft tissues show negligible collapse (Fig. 3a, d) indicating that the ash stiffened and recorded the outer morphology of the animals before significant decay took place. The spherical morphology of the concretions suggests that they formed rapidly, before sediment compaction. The fossils are preserved as voids that were infilled with calcite (Fig. 3) at the same time as the concretions formed (Orr et al. 2000). The biomineralized skeletons of brachiopods, gastropods, trilobites and ostracods are preserved, but organic remains, even non-biomineralized arthropod cuticles, are absent – the peridermACCEPTED of the rare graptolites is an exception. MANUSCRIPT Preservation was investigated in detail in the arthropod Offacolus (Orr et al. 2000). Sparry calcite infilled the external mould of Offacolus, sometimes with an initial phase of finer fibrous calcite around the periphery (Figs 3a, 4c, f). Pyrite precipitated on the boundary between calcite crystals but as a minor constituent of the void fill. Clay minerals precipitated on and underneath the exoskeleton, and in some examples the gut trace was replicated in calcium phosphate. Ferroan dolomite (ankerite) formed at a later perhaps coincident with the transformation on burial of smectite to illite. Abundant radiolarians are evident Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

on the surface of split concretions (Siveter et al. 2007a; Fig. 5x). Their preservation shows parallels to that of Offacolus (Orr et al. 2002). Calcite and pyrite precipitated in the void vacated by the cytoplasm within the radiolarian tests; this space was not invaded by matrix. The precipitation of calcite retained the shape of the test when the opaline silica that formed it was replaced during diagenesis by a mixture of ankerite and diagenetic clay minerals; replacement of silica by ankerite has also been observed in siliceous sponge-spicules (Nadhira et al. 2019). Although there are similarities with the diagenetic sequence displayed by radiolarians in concretions elsewhere (Holdsworth 1967; Orr et al. 2002) the nature of the preservation of soft tissue morphology at the Herefordshire site remains unique.

Releasing the information from the fossils

The Herefordshire concretions are collected in bulk using earth moving equipment. Each scoop is tipped slowly onto a waste-pile; concretions roll out and are collected manually. The concretions are split repeatedly in the laboratory with a manual -splitter; the process is halted if a fossil is found (Fig. 3; ~50% of concretions). Specimens are identified provisionally, photographed with a Leica DFC420 digital camera mounted on a Leica MZ8 binocular microscope with a thin layer of water over the specimen to enhance contrast, and catalogued in a custom online database.

The fossils record copious anatomical information but present challenges for its extraction. Manual or chemical preparation of specimens (see e.g. Sutton 2008) is impractical. Study of the Herefordshire fauna is unique among invertebrate Lagerstätten in relying almost exclusively on virtual reconstructions. Indeed, research on the fossils has played an important role in the development 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 preferable as a basis for virtual reconstructions but are largely ineffective for Herefordshire fossils: X-ray absorption contrast between fossil and matrix is very low. Limited success has been achieved with some specimens by using phase-contrast synchrotron tomography (see Sutton et al. 2014, p. 78) but a destructiveACCEPTED yet highly effective physical-optical MANUSCRIPT tomography ‘serial grinding’ technique is, of necessity, our method of choice. This takes advantage of the strong optical contrast between specimen and matrix and has been used to reconstruct over 100 specimens.

Specimens for reconstruction are trimmed with a fine rock-saw to ~10 mm cuboids and mounted on a Buehler ‘slide holder’ which consists of an inner metal cylinder which can be adjusted to elevate specimens 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 Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

until the entire specimen has been digitised, typically in 200–400 increments. Part and counterpart are ground separately, and large specimens are processed in several pieces which are re-united digitally, following reconstruction, using the custom SPIERS software suite (Sutton et al. 2012b). Reconstruction begins with manual or semi-automatic registering (aligning) of photographs using the cut edges of cuboids as a guide. Data are then prepared for reconstruction by carefully distinguishing pixels representing the fossil from those corresponding to matrix. Software exists to automate this process but an understanding of the taphonomy and likely anatomy of the specimen, and of artefacts generated by the process (e.g. 'out-of-plane' structures visible through bubbles in the encasing resin), is essential to extract maximum morphological information. Data are ‘marked up’ to identify separate structures for visualization, e.g. a particular arthropod appendage. Prepared datasets are reconstructed to triangle-mesh isosurfaces using the Marching Cubes algorithm (Lorenson & Cline 1987). These are visualised and studied using the SPIERSview component of the software suite, which can combine multi-part models (e.g. part and counterpart) and enables stereoscopic viewing, arbitrary rotation, scaling, dissection, and sectioning. Models are exported into VAXML/STL format (Sutton et al. 2012b) for publication, or to the open-source rendering package Blender (http://www.blender.org) 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 of preservation is such that a single specimen can provide abundant data for detailed analysis.

Unexpected character combinations and a possible role for evolutionary development

The reconstruction of phylogenies and the ordering and timing of character-acquisition has been an overarching project in evolutionary biology and palaeontology for over 150 . Reconstructing the based on the living biota alone is attractive as vast quantities of data are obtainable, including the genetic sequences that have revolutionized phylogenetic practice in recent years. However, such data are restricted to crown-groups; no amount of avian sequences would reveal the origin of within Dinosauria, for example. Fossils document otherwise unknown character combinations that did not survive to the present. These combinations can illuminate relationships and evolutionaryACCEPTED history (e.g. in arthropods: LeggMANUSCRIPT et al. 2013), a reminder that the significance of fossils may be out of proportion to their morphological information content. It is no surprise therefore that the high-fidelity soft-tissue anatomy preserved in the Herefordshire fossils provides important new data for determining the course of evolution in many groups, as illustrated by molluscs.

Deep molluscan phylogeny hinges on the position of the Aplacophora which comprise two worm-like groups, Chaetodermomorpha and Neomeniomorpha, united in lacking a shell and fully- Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

developed molluscan foot. Aplacophorans were traditionally interpreted (e.g. by Salvini-Plawen 1991) as the earliest branch of the mollusc lineage (Fig. 4u), but an alternative model places them as sister to the multivalved Polyplacophora () in a clade termed Aculifera, which is in turn the sister group to all other molluscs (Conchifera). This ‘Aculifera hypothesis’ is supported by both molecular and morphological data (Sigwart & Sutton 2007; Wanninger & Wollesen 2018; but see Salvini-Plawen & Steiner 2014). Resolution of the debate is critical for reconstructing molluscan evolution (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 were discovered in the Herefordshire biota. Multivalved molluscs had been described from the Palaeozoic (e.g. Cherns 1998a, 1998b), but were treated as early polyplacophorans (chitons). The discovery of hayae from Herefordshire (Sutton et al. 2001a; Sutton et al. 2004; Dean et al. 2015) revealed an aplacophoran- like form bearing spicules, lacking a true foot and possessing a posterior respiratory cavity (Fig. 4s, t). Nonetheless Acaenoplax shows serialisation and multiple dorsal valves reminiscent of Polyplacophora. The subsequent discovery of Kulindroplax perissokomos (Sutton et al. 2012a) documented an even more aplacophoran-like body with no trace of a foot but bearing a set of typical Palaeozoic ‘polyplacophoran’ valves (Fig. 4l, m). This association appeared chimeric but only because it represents an extinct character combination. Acaenoplax and Kulindroplax provide support for Aculifera (e.g. Vinther 2015), indicating that the ‘simple’ morphology of aplacophorans is derived rather than primitive for the . Furthermore, these Herefordshire fossils reveal a sequence of evolutionary events that cannot be deduced from extant forms: a vermiform body predated the loss of valves, and a diversity of ‘plated aplacophorans’, previously misinterpreted as polyplacophorans, was present during the early Palaeozoic. Modern aplacophorans evolved through a secondary loss of valves and are a remnant of a once larger group (Fig. 4v).

Research on evolutionary development has demonstrated how control genes can initiate major changes in morphology. Modern polyplacophorans (chitons) have a series of eight dorsal valvesACCEPTED associated with the expression of the engrailed MANUSCRIPT gene (Jacobs et al. 2000) whereas Acaenoplax and Kulindroplax (Fig. 4l, m, s, t) have only seven. Such seven-fold seriality is expressed during the ontogeny of living aculiferans (Scherholz et al. 2013) and may reflect an ancestral morphology evident in Kulindroplax (Wanninger & Wollesen 2018). The addition of an eighth shell, which occurs after metamorphosis in Polyplacophora, appears to be an advanced condition (Wanninger & Wollesen 2018). Acaenoplax is unusual in having an obvious gap between valves 6 and 7 where an additional valve might have been placed (Sutton et al. 2001a, 2004). The ‘missing’ valve may reflect Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

the ancestral 7-valve morphology or, given that the space for an additional valve is present, may have been lost secondarily from an 8-valved form.

The remarkable details preserved in some of the Herefordshire arthropods provide evidence of ancestral morphologies and prompt questions as to how they might have been transformed into modern forms. Martin et al. (2016) used CRISPR/Cas9 mutagenesis and RNAi knockdown to show how shifting Hox gene domains in a living amphipod crustacean could contribute to macroevolutionary changes in the body plan. The limbs of the Herefordshire chelicerates Dibasterium (Briggs et al. 2012; Fig. 4d, e, g, i) and Offacolus (Sutton et al. 2002; Fig. 4c, f) can be homologized readily with those of the living but they differ in fundamental ways. Limbs 2 to 5 in the anterior division of the body (prosoma) of the Herefordshire Silurian forms have two branches, an endopod and exopod, both of which are robust and segmented, whereas only one branch is present in Limulus. Unlike the branches of limbs in other arthropods, however, the two branches are not connected to a common basal segment: they insert in different places on the body wall. This arrangement can be interpreted as a stage in the loss of the exopod to yield a limb with a single ramus as in Limulus. The expression of Distal-less (Dll) in larval Limulus (Mittman & Scholtz 2001) suggests that the loss of the exopod may be developmental. Limulus shows a strong expression of Dll associated with the origin of the endopod and a weaker expression, in the early embryo, in an adjacent position corresponding to the insertion of the exopods in Dibasterium. This weaker expression disappears in later embryonic stages echoing the loss of the outer ramus during the evolution of the group (Briggs et al. 2012). The chelicera of Dibasterium (limb 1) is also unusual in being an elongate flexible antenna-like appendage in contrast to the familiar claw of Limulus. Here too knockdown of genes such as Dll and dachsund may have led to the loss of distal podomeres and the evolution of the shorter chelicera in modern horseshoe , echoing experimental results on the harvestman Phalangium (Sharma et al. 2013).

Extended stratigraphic ranges and the calibration of molecular clocks

The Herefordshire Lagerstätte is one of very few windows on soft-bodied organisms during the Silurian.ACCEPTED It is not surprising, therefore, that the MANUSCRIPTfossils provide important examples of stratigraphic range extensions. Thanahita (Fig. 5a, b), for example, falls in a clade with the three known species of the iconic lower to mid- lobopodian (Siveter et al. 2018b). The long slender trunk appendages of Thanahita contrast with those of short-legged lobopodians such as the Cambrian Antennacanthopodia (Ou et al. 2011). Long-legged lobopodians are also represented by Carbotubulus from the late Mazon Creek Lagerstätte of Illinois (Haug et al. 2012) which is some 135 Myr younger than Thanahita. Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

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

Xylokorys is the only known Silurian euarthropod (Siveter et al. 2007b; Fig. 5r, w). It belongs to the Acercostraca which, in contrast to the familiar Cambrian , is characterized by a large shield-like carapace which covers the head and trunk. Xylokorys extends the Cambrian (Primicaris and ) and (Enosiaspis) acercostracan record and heralds the youngest known example, Vachonisia, in the Devonian Hunsrück Slate.

Haliestes (Siveter et al. 2004; Fig. 5n, o) is arguably the best preserved and most complete of just thirteen known species (Sabroux et al. 2019) of fossil pycnogonid. These include a putative larval pycnogonid, Cambropycnogon klausmuelleri, from the upper Cambrian Orsten of Scandinavia (Waloszek & Dunlop 2002), and a basal stem group pycnogonid, Palaeomarachne granulata, from the Ordovician William Lake Lagerstätte of Canada (Rudkin et al. 2013). The Herefordshire pycnogonid Haliestes shows reduction of the body to a small trunk projection beyond the posteriormost limbs. This is a feature of Pantopoda, indicating that crown-group pycnogonids 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 Devonian Hunsrück Slate, of which Palaeopantopus maucheri and Palaeothea devonica show trunk end reduction. Three species from the of La Voulte-sur-Rhȏne, France, may also belong to extant pantopod families (Charbonier et al. 2007).

Where Herefordshire taxa extend the range of particular clades back in time, they provide fossil calibrations for the tree of life, notably in the case of arthropods. Haliestes providesACCEPTED one example (Wolfe et al. 2016) , tMANUSCRIPThe Rhamphoverritor reduncus (Briggs et al. 2003; Fig. 4o, n) provides a calibration for crown-group Thecostraca, and the phyllocarid Cinerocaris magnifica (Briggs et al. 2005, fig. 5c, d) for crown (Wolfe et al. 2016). Herefordshire ostracods (Fig. 4a, b, q, r; 4u, z, zz) provided the calibration for crown Ostracoda (Oakley et al. 2013) prior to the discovery of Luprisca incubi from the late Ordovician Beecher’s Bed of New York State (Siveter et al. 2014b; Wolfe et al. 2016). Invavita piratica from the Herefordshire Lagerstätte (Siveter et al. 2015b; Fig. 4p-r), the earliest adult pentastomid pancrustacean, is arguably a more secure calibration than Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

Boeckelericambria pelturae from the Cambrian Orsten (Walossek & Müller 1994), which is based on a larva (Wolfe et al. 2016). The described Herefordshire biota includes five species of ostracods with preserved appendages, all of which are myodocopes: Colymbosathon ecplecticos, Nymphatelina gravida, Nasunaris flata, 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 cylinderoberidids, which are characterised by the presence of gills. The only other fossil myodocopes with possible gill preservation are Triadocypris from the lower of Spitzbergen (Weitschat 1983) 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; Fig. 4l, m, s, t) also provide molecular clock calibrations. Both represent total group aplacophorans and crown-group Aculifera (Vinther et al. 2017; Wanninger & Wollesen 2018). The Herefordshire polychaete worm Kenostrychus represents crown Aciculata even though its original placement as a stem-group phyllodocid (Sutton et al. 2001c; Fig. 5s) has been revised to stem amphinomid (Parry et al. 2016).

Box 2. Other 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 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, , rostroconch and helcionelloid bivalves, hyolithoids, and .

ACCEPTEDApart from Herefordshire, open marine MANUSCRIPT 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 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 , brachiopods, polychaete annelids, echinoderms, conodonts, and possible . The fauna of the late Llandovery Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

Waukesha Lagerstätte from Wisconsin (Mikulic et al., 1985a, 1985b) shows some similarity to Eramosa but is peritidal. It is dominated by arthropods, especially trilobites (13 genera), and includes a xiphosuran and phyllocarid, and thylacocephalan crustaceans. Conulariids and graptolites are common, but brachiopods, bryozoans, , molluscs and echinoderms are rare or absent. The Přídolí Bertie Group of Ontario and New York State is dominated by eurypterids, xiphosurans, 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 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 from the Priesthill and Waterhead groups of Lesmahagow, and correlatives in the Hagshaw and , represent restricted quasi-marine to lacustrine settings and are typically dominated by eurypterids, phyllocarids and fish (Allison & Briggs 1991; Ritchie 1968, 1985; Rolfe 1973; 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, (Suarez et al. 2017; see also Shillito & Davies 2017 for ichnological evidence). The basal Přídolí 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).

ModeACCEPTED of life and palaeoecology MANUSCRIPT

Herefordshire taxa reveal aspects of gender, larval development and brood care in fossil arthropods. The recognition of hemipenes in the myodocope ostracod Colymbosathon (Siveter et al. 2003; Fig. 4a) and eggs and a juvenile, in situ, within the carapace of the myodocope Nymphatelina (Siveter et al. 2007c, 2015b; Fig. 4b) allows male and female to be distinguished in early Palaeozoic animals. The brood care strategy evident in Nymphatelina remains essentially unchanged in myodocopes today. A more complex and unique brooding behaviour is preserved in the stem mandibulate Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

Aquilonifer; the presumed female carries ten tiny juveniles each within a cuticular capsule tethered by a delicate thread (Briggs et al. 2016; Fig. 4h). Such data help 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 million years (Briggs et al. 2005; Fig. 4o, n). A boring through a valve of Rhampoverritor (Fig. 4n) also provides evidence of 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 marine invertebrates (Siveter et al. 2015b; Fig. 4p-r). Bethia serreticulma is the only recorded fossil rhynchonelliformean brachiopod with preserved soft parts (Fig. 4k). Evidence of its mode of life, shell size and lophophore 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. serreticulma an in vivo epifauna including a tiny unmineralised lophophorate, Drakozoon kalumon (Fig. 4 j), which may allude to a widespread occurrence of similar but unknown lophophorates in the Palaeozoic (Sutton et al. 2010).

The majority of the 63 known Herefordshire species lived on the seafloor (Table 1; Fig. 7), either as sessile or vagile epibenthos. Unless the Herefordshire animals were transported from significantly shallower waters, it appears that the benthic components of the biota lived in dim light conditions at best. Several of the benthic animals lack eyes and there is an absence of photic-zone indicators such as algae. Given the evidence for rapid burial it is likely that the water column biota is underrepresented. Sponges are the dominant sessile element, with brachiopods, echinoderms (the edrioasteroid Heropyrgus and a crinoid), a cirripede, coral, and a possible dendroid graptolite and gastropod (?) as minor components. Most of the vagile epibenthos are arthropods, but this group also includes two echinoderms (the asteroid Bdellacoma and ophiocistioid Sollasina), the aplacophoranACCEPTED mollusc Acaenoplax, a gastropod MANUSCRIPT and a bivalve mollusc. The nektobenthos is made up entirely of arthropods, with ostracods the most species abundant. A nautiloid is the only nektonic element. A graptolite and radiolarians comprise the pelagic plankton. A possible infaunal/semi- infaunal mode of life is interpreted for just three taxa, a lingulid brachiopod, the aplacophoran Kulindroplax and the annelid Kenostrychus. The numerical dominance of Offaculus within the biota is probably real rather than representing an artefact of sampling, as concretions are selected and split randomly and, as one of the smaller taxa, its high yield is unlikely to result from being selectively preserved. The biota includes representatives of many major invertebrate groups but there is no Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

obvious reason why some groups known from the Silurian of the Welsh Basin elsewhere, such as and bryozoans, have not been found in the concretions.

Why is the Herefordshire Lagerstätte apparently unique?

The Herefordshire biota is one of a number of examples of soft tissue preservation in concretions. Concretion formation requires carbonate (or more rarely silica) supersaturation and the presence of a nucleus. In some cases, a buried carcass can provide both. Soft tissues can be preserved within concretions in a number of ways (McCoy et al. 2015): as carbonaceous remains, as a result of replication in a variety of authigenic minerals, or by void fill as in the case of the Herefordshire biota. Exceptional preservation is promoted where concretion growth is rapid, and void fills are more likely to occur where cementation occurs from the inside out, slowing diffusion at an early stage and isolating the organism from its environment (McCoy et al. 2015). An analysis of factors associated with exceptional preservation in concretions (based on 88 concretion sites of which 20% preserve soft-tissues) showed that concretion formation enhances the chances of exceptional preservation only where other conditions are favourable, such as burial in fine grained sediment (McCoy et al. 2015). The volcanic ash that buried the Herefordshire organisms may have played a role in their exceptional preservation. Fine grain size may have promoted anoxia within the sediment and inhibited scavengers.

Predictably perhaps, most examples of exceptional preservation associated with volcanic ash are in terrestrial settings (Briggs et al. 1996), in lacustrine and fluvial environments. Preservation of materials in volcanic ash may be enhanced by rapid burial and anoxic conditions, as in the of Argentina (Lafuente Diaz et al. 2018) and the Clarkia Beds of Idaho, USA (Ladderud et al. 2015). The Triassic Chañares Formation of Argentina yields a diversity of in carbonate concretions in volcanic ash that likely formed where microbial decay promoted carbonate precipitation (Rogers et al. 2001) but concretion biotas are rare in terrestrial environments. Giant radiodontids are preserved in silica-chlorite concretions in the lower Fezouata FormationACCEPTED of Morocco including silica, iron and MANUSCRIPT aluminium thought to be sourced from volcanic ash (Gaines et al. 2012). A volcanic influence has also been reported impacting fossil preservation in other marine settings, 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 are shelly fossils in the Wenlock Ballynane Formation of the Dingle Peninsula, Co. Kerry, Ireland (Ferretti & Holland 1994), but soft tissue preservation is unknown and associated concretions have not been reported. The cuticle of decapod moults and the ligament of bivalves are preserved in concretions in the Cretaceous of Alberta (Bentonite number 5) following rapid Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

burial in volcanic ash (Heikoop et al. 1996) but carbonate concretions associated with volcanics in marine settings are rare.

Preservation of the Herefordshire fossils involved an unusual combination of circumstances: rapid burial of small living animals in thick, fine grained volcanic ash in a marine setting, and very rapid formation of near-spherical carbonate cemented concretions. Other biotas preserved in a similar fashion to the Herefordshire example have yet to be discovered. The tiny crystalline infills are unlikely to attract the interest of a casual collector and the fossils are often off centre and may not be intersected by the plane of splitting. Silurian bentonites are widespread in the UK, but they are generally only a few centimetres thick (Cave & Loydell 1998). Examples of concretions within ashes of any age merit serious scrutiny but early formed concretions in fine grained marine mudstones may also yield exceptionally preserved fossils depending, perhaps, on the clay minerals present.

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, 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.

AcknowledgementsACCEPTED and Funding MANUSCRIPT Carolyn Lewis (Oxford University Museum of Natural History) is thanked for technical assistance. 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 Invertebrate Division, and English Nature. We are grateful for insightful comments from Patrick Orr, Robert Gaines and a third anonymous reviewer. References Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

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Boxes, Table, Figures

Figure 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 cm by 4 cm, concentrically weathered, lacking the blue-grey hearted centre of calcium carbonate that is present in partially weathered examples, and incorporating an eccentric specimen of the aplacophoran Acaenoplax. (d) Local stratigraphy of the Dolyhir-Presteigne area. Radiometric dates are those given in Cohen et al. (2013, updated 2018), the International Commission on Stratigraphy (ICS) Chronostratigraphic Chart. Cramer et al. (2012) give marginally different radiometric dates for some of the boundaries based on sampling from Gotland and the West Midlands, UK, for example 429.5 Ma for the Sheinwoodian-Homerian boundary. However, the majority margin of error on these dates (±0.7 Ma) provides overlap with the dates given in the ICS Chronostratigraphic Chart. (e) Welsh Basin ACCEPTEDpalaeogeography at approximately the SheinwoodianMANUSCRIPT-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).

Box 1. The Faunal composition of the Herefordshire Lagerstätte

Includes Table 1 (Faunal list), Figure 2a, b Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

Table 1, Faunal list

Figure 2a. 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.

Figure 2b. 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%).

Figure 3. Examples of Herefordshire animals on the split surface of nodules. (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-light blue coloured carbonate-rich matrix of the nodule, within and around the fossil, and (b) and (d) show orange-yellow coloured matrix of the nodule, indicating weathered (leached) examples. Scale bars are all 1 mm.

Figure 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 kalumma, ventral view. (k) Bethia serraticulma, with attached Drakozoon kalumma and ?atrypide brachiopod, lateral view. (l, m) Kulindroplax perissokomus, lateral, dorsal views. (n, o) Rhamphoverritor reduncus, attached juvenile, free swimmingACCEPTED stage, lateral views. (p) Invavita MANUSCRIPT 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, dorsal and lateral view; main and posterior parts of body, dorsal and lateral 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

Figure 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) Bdellocoma 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 ravitus, 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.

Box 2. Comparison of Silurian Lagerstätten.

Includes Figure 6.

Figure 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.

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

ACCEPTED MANUSCRIPT Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

Table 1 Faunal composition of the Herefordshire Lagerstätte

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


Carduispongia pedicula (Ardianty et al. 2019; Fig. 4y)

Some 20 other species (uninvestigated)

Cnidarians ? hydroid (uninvestigated colonial organism) Coral (uninvestigated)


Bethia serraticulma (Sutton et al. 2005a; Fig. 3k)

At least two other brachiopod species (both uninvestigated), one of them a lingulid

Lophophorate indet.

Drakozoon kalumon (Sutton et al. 2010; Fig. 3j)


Kenostrychus clementsi (Sutton et al. 2001c; Fig. 4s)



Acaenoplax hayae (Sutton et al. 2001a; Fig. 3s, t)

Kulindroplax perissokomos (Sutton et al. 2012a; Fig. 3l, m)


Praectenodonta ludensis Reed, 1931 (Fu 2016)


Platyceras? sp. (Sutton et al. 2006; Fig. 4v)

A high spired species (uninvestigated)

Cephalopods Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

Nautiloids (uninvestigated)



Thanahita distos (Siveter et al. 2018b; Fig. 4a, b)


Enalikter aphson (Siveter et al. 2014a; Siveter et al. 2015a; Fig. 4e)


Haliestes dasos (Siveter et al. 2004; Fig. 4n, o)


Offacolus kingi (Orr et al. 2000; Fig. 3c, f)

Dibasterium durgae (Briggs et al. 2012; Fig. 3d, e, g, i)

Trilobites sp. (uninvestigated) Tapinocalymene sp. (uninvestigated)


Xylokorys chledophilia (Siveter et al. 2007b; Fig. 4w, r)


Aquilonifer spinosus (Briggs et al. 2016; Fig. 3h)

Tanazios dokeron (Siveter et al. 2007d; Fig. 4p, t)



Cinerocaris magnifica (Briggs et al. 2003; Fig. 4c, d)

Cascolus ravitis (Siveter et al. 2017; Fig. 4l, m)

Cirripedes Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

Rhamphoverritor reduncus (Briggs et al. 2005; Fig. 3o, n)



Colymbosathon ecplecticos (Siveter et al. 2003; Fig. 3a)

Nymphatelina gravida (Siveter et al. 2007c; Fig. 3b, q, r)

Nasunaris flata (Siveter et al. 2010; Fig. 4z)

Pauline avibella (Siveter et al. 2013; Fig. 4zz)

Spiricopia aurita (Siveter et al. 2018a; Fig. 4u)

Podocopes and palaeocopes Fragments of unidentified species belonging to these ostracod groups have been recovered from acid residues.


Invavita piratica (Siveter et al. 2015b, Fig. 3p, q, r)



Bdellacoma sp. (Sutton et al. 2005b; Fig. 4f-i)


Heropyrgus disterminus (Briggs et al. 2017; Fig. 4j, k)

Crinoids (uninvestigated sp.)

Ophiocistioids SollasinaACCEPTED cthulhu (Rahman et al. 2019; 4q) MANUSCRIPT Hemichordates Pterobranchs Graptoloids (uninvestigated) Dendroids? (uninvestigated)

Radiolarians Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

Inanihella sagena (Siveter et al. 2007a; Fig. 4x)

Inanihella sp. of (Siveter et al. 2007a) Haplentactinia armista (Siveter et al. 2007a) Parasecuicollacta hexactinia (Won et al. 2002)


Fragments of unidentified mazuelloid species have been recovered from acid residues.

ACCEPTED MANUSCRIPT Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

ACCEPTED MANUSCRIPT Downloaded from http://jgs.lyellcollection.org/ by guest on September 27, 2021

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