Palaeoscolecids from the Ludlow Series of Leintwardine, Herefordshire (UK): the latest occurrence of palaeoscolecids in the fossil record

The documentation of cuticular micro‐ornament is vital for the taxonomic assignment of palaeoscolecids: vermiform lower Palaeozoic ecdysozoans interpreted as stem‐group priapulans or early diverging panarthropods. This is due to the absence of the character‐rich proboscis and tail hooks in palaeoscolecid material not from Burgess Shale‐type Konservat‐Lagerstätten. Here, the cuticular micro‐ornamentation of palaeoscolecids from the upper Silurian (Ludlow) fauna of Leintwardine (Herefordshire, England), is described using scanning electron microscopy and reflectance transformation imaging. This material is taxonomically unstable because it was included in an effective wastebasket genus (Protoscolex) long before these imaging techniques were developed. The Leintwardine material is shown to be most closely comparable to a palaeoscolecid from the Darriwilian (Middle Ordovician) of the Builth–Llandrindod inlier, Powys, Wales, and is transferred accordingly to Radnorscolex Botting et al. as Radnorscolex latus (Bather). The Leintwardine fauna represents the uppermost stratigraphic occurrence of palaeoscolecids, constrained to the Saetograptus leintwardinensis Zone (lower Ludfordian), and the comparatively sparse Silurian palaeoscolecid record is subsequently discussed. It is hypothesized that palaeoscolecids may have become extinct during the mid‐Ludfordian Lau Event, the onset of which is recorded in the biozone immediately above the Leintwardine fauna (Bohemograptus Zone). Finally, the British palaeoscolecid fauna is summarized, including a new record from the Dapingian (Middle Ordovician) of Carmarthenshire, South Wales.

Llandrindod inlier, Powys, Wales, and is transferred accordingly to Radnorscolex Botting et al. as Radnorscolex latus (Bather).The Leintwardine fauna represents the uppermost stratigraphic occurrence of palaeoscolecids, constrained to the Saetograptus leintwardinensis Zone (lower Ludfordian), and the comparatively sparse Silurian palaeoscolecid record is subsequently discussed.It is hypothesized that palaeoscolecids may have become extinct during the mid-Ludfordian Lau Event, the onset of which is recorded in the biozone immediately above the Leintwardine fauna (Bohemograptus Zone).Finally, the British palaeoscolecid fauna is summarized, including a new record from the Dapingian (Middle Ordovician) of Carmarthenshire, South Wales.Morris & Robison, 1986 is a clade of vermiform marine ecdysozoans known mostly from the Cambrian and Ordovician, wherein they are widespread, persisting to the Silurian where there are far fewer records.They possess a narrow elongate trunk covered by a transversely annulated cuticle ornamented with biomineralized plates, an anterior proboscis comprising a spiny introvert and toothed eversible pharynx, and bilaterally oriented terminal posterior hooks.Palaeoscolecida share these characters with Cricocosmiidae Hou et al., 1999, a strikingly similar clade of worms currently known only from the Chengjiang Biota (Eoredlichia-Wutingaspis Zone, Maotianshan Member, Yu'anshan Formation, Cambrian Series 2, Stage 3) of Yunnan Province, China.Along with their annulated trunks, the proboscises of palaeoscolecids and cricocosmiids are routinely homologized with those of cycloneuralian ecdysozoans, which has previously justified the systematic treatment of these worms among, for example, Priapulida (Conway Morris 1997;Harvey et al. 2010; and see Shi et al. 2022) and Nematomorpha (Hou & Bergstr€ om 1994).However, recent phylogenetic analyses also hypothesize a panarthropod affinity for these worms.Examples of serially repeated ventral trunk structures were identified in both Palaeoscolecida and Cricocosmiidae by Shi et al. (2022).These included paired limb-like projections in cricocosmiids and paired ventral spines in a palaeoscolecid (Shi et al. 2022, fig. 5).These ventral structures are theoretically homologized with panarthropod appendages (Shi et al. 2022;Smith & Dhungana 2022), although the exact systematic relationships are currently unclear regarding whether Palaeoscolecida and Cricocosmiidae form a monophyletic group (Palaeoscolecidomorpha, Shi et al. 2022) or a paraphyletic group (Smith & Dhungana 2022), and where specifically these lineages diverge within total group Panarthropoda, or even total group Cryptovermes (Nematoida + Panarthropoda; Howard et al. 2022).
Palaeoscolecida differ from Cricocosmiidae in that their trunk is adorned with a micro-ornamentation of phosphatic (e.g.Martin et al. 2016) tessellating polymorphic plates and micro-plates, whereas Cricocosmiidae instead possess serially repeated lateral or dorso-lateral groups of larger sclerites with a net-like micro-texture seemingly homologous with those of some fossil lobopodians (Han et al. 2007;Steiner et al. 2012;Shi et al. 2022).The trunk micro-ornamentation is critical in taxonomic assignment among palaeoscolecids because it is commonly preserved, both as individual plates in microfossil samples (e.g.Hinz et al. 1990) and in articulated compression fossils (e.g.Conway Morris 1997), and because micro-ornamentation differs considerably between taxa.By contrast, the details of the proboscis, which also vary greatly between taxa in the arrangement of spines and teeth, is only rarely preserved in the widespread palaeoscolecids, whereas all three cricocosmiid species have proboscis data, being endemic to the exceptional preservation of the Chengjiang biota (Shi et al. 2022, table 3).Only a handful of the many named palaeoscolecid taxa have proboscis data and these are only from Burgess Shale-type Konservat-Lagerst€ atten such as Scathascolex minor Smith, 2015 from the Burgess Shale of British Columbia, Canada, and Mafangscolex yunnanensis Hou & Sun, 1988(Yang et al. 2020) from Chengjiang.Therefore, very few palaeoscolecids are known from both gross morphology and cuticular micro-ornament, resulting in a kind of dual taxonomy of the group that presents problems for phylogenetic analysis.
The present work concerns the late Silurian palaeoscolecid previously known as Protoscolex latus Bather, 1920; transferred here to Radnorscolex Botting et al., 2012 under the new combination Radnorscolex latus (Bather, 1920).Radnorscolex latus is known from a handful of specimens from three localities pertaining to the channel-fill sections of the Lower Leintwardine Formation (Whitaker 1962) around the village of Leintwardine, Herefordshire, England.This material is well-preserved, with the characteristic palaeoscolecid ornamented annulations and three-dimensional gut preservation, but lacking the proboscis and posterior hooks.Radnorscolex latus was one of the first palaeoscolecids to be described (although it was considered an annelid at the time; Bather 1920) and has never had its cuticular micro-ornament fully investigated using modern imagine techniques (Gladwell 2005 reported a preliminary scanning electron microscopy (SEM) study but did not describe or figure the results).As such, a combination of SEM and reflectance transformation imaging (RTI) was used here to enable re-description of R. latus in line with the modern literature and stabilize the taxon through comparison to other taxa described using SEM.This also provides a platform to discuss the evolutionary history of palaeoscolecids and to summarize the British palaeoscolecid fauna.

GEOLOGICAL & STRATIGRAPHICAL CONTEXT
Radnorscolex latus is known only from the Lower Leintwardine Formation of the Leintwardine Group, a unit of laminated siltstones and calcareous siltstones (Whitaker 1962;Siveter et al. 1989) deposited on the shelf or shelf edge of the Palaeozoic microcontinent Avalonia, which was located at southern mid-latitudes, progressively coalescing with the larger continents of Baltica to the east and Laurentia to the west by the late Silurian (Fortey & Cocks 2003).These rocks are exposed across the Anglo-Welsh borderland region of southern Britain, the type area being around the eponymous settlements of Ludlow, Shropshire and neighbouring Leintwardine, Herefordshire.The Leintwardine Group is biostratigraphically constrained by its graptolite fauna to the Saetograptus leintwardinensis Zone (Siveter 2000;Siveter et al. 1989), which is also recognized outside the Anglo-Welsh basin, previously as equivalent to the Saetograpus linearis Zone ( Storch et al. 2014).The base of the Saetograptus leintwardinensis Zone equates to the base of the Ludfordian Stage (<426.5 Ma), designated by the GSSP at Sunnyhill Quarry (Martinsson et al. 1981), 2.5 km southwest of Ludlow (SO 4950 7255).
Around the village of Leintwardine (c. 10 km west of Ludlow), six parallel channels trending NE-SW, sloping off the shelf edge towards the deeper waters of the Welsh Basin, are recognized (Whitaker 1962).These channels, interpreted as submarine canyon-heads, cut into lower Ludlow Series strata in the Leintwardine area (Fig. 1A), and were subsequently infilled with sediments of the Lower Leintwardine Formation during Saetograptus leintwardinensis Zone times.Where these Lower Leintwardine Formation channel-fill deposits are exposed, additional fossil elements (some allochthonous, some autochthonous according to Gladwell 2005) are preserved that are not typically encountered elsewhere across the distribution of the Lower Leintwardine Formation, which normally yields a stratigraphically typical shelly fauna including trilobites, brachiopods, cephalopods, bryozoans and graptolites (Siveter et al. 1989;Gladwell 2005).In addition to R. latus, these channel-fill assemblages include 16 species of asterozoans (including asteroids and ophiuroids; Gladwell 2018), nonbiomineralizing arthropods (eurypterid, bunodid and pseudoniscid euchelicerates and phyllocarid malacostracans; Siveter 2010), as well as beds of graptolites, cephalopods and brachiopods that are palaeocurrent-aligned parallel to the NE-SW axis of the channel (Siveter et al. 1989;Gladwell 2005; Fig. 1B).Three specific localities exposing the channel-fill sediments of the Lower Leintwardine Formation in the Leintwardine area have yielded specimens of R. latus: Church Hill Quarry (Fig. 1C) (pertaining to the Church Hill Channel), Martin's Shell Quarry (Todding Channel) and Mocktree Quarry (Mocktree Channel).Church Hill is located c. 1 km SE of the centre of Leintwardine, and the Mocktree and Martin's Shell sites are c. 2 km NE of the centre of Leintwardine (Fig. 1A).

MATERIAL AND METHOD
Five specimens of R. latus were available for study (Table 1; Fig. 2).This includes four from the Natural History Museum, London (NHMUK), including the holotype, and one from the Cole Museum of Zoology, Reading (CMZ), a topotype.Further specimens were figured in the PhD thesis of Gladwell (2005, pl. 2.19).Additionally, one indeterminate palaeoscolecid from the upper Arenig Series (Dapingian, Middle Ordovician) of South Wales is reported (Discussion and Fig. 2I).Specimens were initially examined with a Wild M5A stereomicroscope and then photographed under low-angled light with a Canon EOS 750D equipped with a 105 mm Sigma f2.8 EX DG Macro OS lens before being studied using RTI and SEM.
RTI was used to enhance the contrast between the specimens and surrounding matrix (Fig. 2) or to better illustrate areas of interest with relief on key specimens (Figs 3,4).A FlyDome RTI dome and Canon EOS 5D mkiv were used to produce polynomial texture maps (PTM) using either a Canon MP-E 65 mm or a Canon EF 100 mm macro lens.Where possible, RTI made use of 54 available LEDs but for close-up images using the MP-E 65 mm lens the ring of LEDs providing the highest angle of illumination was obstructed by the lens and excluded, therefore using a total of 42 images.PTM were constructed using RelightLab (Ponchio et al. 2019) and then visualized using RTIviewer.The figures used the diffuse gain and normals visualization imaging modes to enhance features of interest, and lighting directions are provided in figure panels and in the supporting data files (Parry et al. 2024).Close-up images were rotated prior to processing and separate RTI datasets were generated such that all normals visualization images share a common colour scheme.
SEM was used (Figs 5,6) to show the pattern of cuticular ornamentation on the trunk of R. latus at the Natural History Museum's Imaging & Analysis Centre using a JEOL IT500 and, for higher resolution at greater magnification, an FEI Quanta 650 FEGSEM.Samples were viewed in variable pressure mode and specimens were not coated.All specimens were investigated, and SEM photomicrographs were captured from CMZ R4 (Fig. 5) and NHMUK PI A 1946a and 1946b (Fig. 6), because these specimens had the highest quality of preservation.Diagnosis.Radnorscolex with diffusely tuberculate plates with no central node.Tessellating hexagonal microplates comprise both unornamented forms and some with marginal tubercles and show no consistent distinction in size within and between annuli.
Remarks.The original definition for R. bwlchi (Botting et al. 2012) stated that tubercles on the large plates may be arranged around a central node and that the microplates are distinguished by size between those within an annulus and those in between annuli.These character states distinguish the type species R. bwlchi from R. latus as defined here.
Description.No specimen is complete in overall profile, therefore it is not possible to ascertain a reliable average body length or total number of annuli for the specimens, but the holotype NHMUK PI A 1946 shows that the maximum body length was >70 mm.Trunk width varies between specimens that are more laterally compressed (e.g.c. 3 mm in NHMUK PI A 1946 and NHMUK PI A 7895) and those that retain more three-dimensionality (e.g.c. 1.5 mm in topotype CMZ V3, V4).Nevertheless R. latus is considerably wider than the only specimen of R. bwlchi (c.0.8 mm across, Botting et al. 2012).Cuticular micro-ornament was consistent across the five individuals studied under SEM.Terminology follows Whitaker et al. (2020), with plates being the large diffusely tuberculate units that are transversely arranged in rows and microplates being the smaller units occupying the space between rows that are either unornamented or with marginal tubercles.No platelets are present as defined by Whitaker et al. (2020).Each annulus of the trunk consists of two rows of approximately 20 larger (c. 100 lm) plates (Fig. 5A).The plates of these two rows are not aligned parallel to each other, instead being offset laterally from one another (Figs 5A-D, 6A-C), resulting in a diagonal alignment of plates within annuli.The large plates take the form of raised mounds with a circular outline, covered with at least 8 F I G . 3 .Reflectance transformation imaging (RTI) of CMZ V3.A, whole specimen RTI image rendered using diffuse gain to enhance contrast, specimen illuminated from the northwest; white boxes indicate areas where further RTI (B-D) and scanning electron microscopy images (see below) were prepared.B, anterior end of specimen, RTI image rendered using diffuse gain to enhance contrast, specimen illuminated from the northwest (corresponding to upper right box in A).C, anterior end of specimen, RTI image rendered using normals visualization (corresponding to upper right box in A).D, posterior end of specimen, RTI image under normals visualization (corresponding to lower left box in A).Scale bars represent: 5 mm (A); 2 mm (B, C); 1.5 mm (D).and as many as 12 diffuse tubercles (Figs 5E-F, 6C, E) with no consistent organization.Tubercles are simple apical structures (Fig. 5E-F).In some areas the plates are smooth and flattened rather than tuberculate, however these have been altered or recrystallized because tuberculate plates are present among the areas of smooth, flat plates in CMZ V3 (Fig. 5A).In NHMUK PI A 1946 it can be observed that the plates preserved in negative relief as concave internal moulds where the internal material has weathered away retain the tubercles, while those in positive relief are smooth (Fig. 6B, C, E, F).The interspace both between the large tuberculate plates and the annuli is covered with a mesh of smaller tessellating hexagonal microplates (Fig. 5G, H).These microplates were reported to be unornamented in the type species Radnorscolex bwlchi (Botting et al. 2012), but at least some ornamentation is observed in R. latus.At least some microplates bordering the larger plates are ornamented with 3-6 tubercles around their margins (Fig. 5H).There is variation in the size of the microplates (c.3-6 lm), but no higher order of organization is observed (in contrast to R. bwlchi in Botting et al. 2012).A similar overall arrangement of round, tuberculate plates and hexagonal microplates is observed in some specimens assigned to the microfossil form taxon Milaculum longmyndium van den Boogaard, 1988 from the Aeronian (Llandovery) of Shropshire (van den Boogaard 1988, figs 5B, 6B).These specimens of M. longmyndium can be distinguished from Radnorscolex by their greater number of tubercles, which are more uniformly arranged in rows, but this taxon appears to provide further evidence for the phylogenetic continuity of the broader palaeoscolecid lineage containing Radnorscolex species.
A three-dimensional gut is observed in most specimens and may be preserved in conjunction with overlaid annuli and plates (Fig. 6A).This indicates that the specimens represent the preserved remains of whole animals (rather than moults) that were buried either alive or shortly following death given that the guts of cycloneuralian worms have a low resilience to decay (Sansom 2016).Posterior hooks are not preserved in any specimen.The posterior end of NHMUK PI A 1946 a has two apparent outgrowths, but these could be an artefact of the artificial whitening of the specimen (Fig. 1B) historically used for photography and they are not consistent with posterior hooks observed elsewhere (e.g.Smith 2015, fig. 1J;Martin et al. 2016, fig. 3).

Evolutionary context of Silurian palaeoscolecids
Silurian records of palaeoscolecids are notably sparse in comparison with the Cambrian and Ordovician, where they occur across much of the world.Cambrian palaeoscolecids are extremely widely distributed, being reported from Antarctica, Argentina, Australia, China, Greenland, Iran, Mongolia, Siberian Russia, Spain, T€ urkiye and the USA, while Ordovician palaeoscolecids are represented in Canada, China, Czechia, Estonia, Morocco, Peru, Sweden, the UK, and the USA (see references in table 1 of Wang et al. (2014); and see Goñi et al. (2023) for the Mongolian record).Indeed, palaeoscolecid diversity during the Ordovician may have been greater than previously anticipated.For example, high species diversity noted in a single region (Avalonia) over a comparably short duration (three graptolite biozones) was reported in the Middle Ordovician by Botting et al. (2012).This supports a hypothesis that palaeoscolecids were a successful post-Cambrian group that radiated as part of the benthic component of the Great Ordovician Biodiversification Event (GOBE; Servais et al. 2010Servais et al. , 2021;;Servais & Harper 2018;Deng et al. 2021).This is further compounded by SEM investigations into palaeoscolecid cuticular microstructure, which identify Ordovician architectures that are often highly distinct from Cambrian taxa, implying a turnover of groups from the Cambrian to the Ordovician (Botting et al. 2012;Wang et al. 2014), although at least some Cambrian palaeoscolecid genera are known to occur in the Ordovician as well (e.g.Wronascolex; Garc ıa-Bellido & Guti errez-Marco 2023).
Silurian palaeoscolecids by contrast are known from only a handful of occurrences for both articulated compression fossils and microfossils.These occur only in China, the UK and North America, and most lack sufficient description and illustration.The most reliable records are microfossils from the Llandovery of Shropshire (UK) (van den Boogaard 1988) and South China (Wang 1990), in addition to undescribed body fossils from the soft-tissue-preserving Waukesha Biota of the Brandon Bridge Formation (Llandovery) of Wisconsin, USA; illustrated clearly in Mikulic et al. (1985), Westberg (2019), Wendruff et al. (2020) and Braddy et al. (2023).Most further examples are from central-eastern regions of the USA, in addition to forms 'resembling Palaeoscolex or Protoscolex' from the Eramosa Formation (Sheinwoodian) of southern Ontario (Tetreault 2001, fig.37B, C).The US occurrences are somewhat patchy; some may not be palaeoscolecids and all are referred erroneously to Protoscolex.'Protoscolex' batheri Ruedemann, 1925, from the Lockport Group (Wenlock) is recorded from the Medusaegraptus epibole of Gasport, New York (Ruedemann 1925, p. 40, figs 27-29;LoDuca & Brett 1997).The figures illustrating 'Protoscolex' batheri in Ruedemann (1925) are crude, but it is reported to be elongate and with annuli containing two rows of 'papillae', and therefore it is probably a palaeoscolecid, although it will require SEM investigation to determine an appropriate taxonomic assignment.'Protoscolex' ruedemanni was also described from a temporary exposure of the Lockport Group in Illinois during the construction of a canal and was collected in association with the sipunculan Lecthaylus gregarius, but the illustrations and description are insufficient to determine whether it is really a palaeoscolecid (Roy & Croneis 1931, pl. 44, fig. 4), although it was referred to as such more recently by Frest et al. (1999).The association with Lecthaylus gregarius is mirrored by another example of 'Protoscolex' sp., which is reported from the Mosalem Formation (Llandovery) of Iowa (Frest et al. 1999).
No quantitative macroevolutionary analysis of the palaeoscolecid fossil record has yet been undertaken and therefore it is not possible to draw empirical conclusions on the long-term diversity patterns of Palaeoscolecida.However, hypotheses are required to instigate data-driven analyses, and there are clear parallels that can be drawn between the perception of diversity change within palaeoscolecids and the global climatic, oceanographic and biotic events of the early Palaeozoic.An appealing explanation for the comparative scarcity of Silurian palaeoscolecids is that, as a benthic or endobenthic shelf-dwelling group, they were greatly affected by the protracted glacially induced sealevel extremities and associated catastrophes of the Late Ordovician Mass Extinction (e.g.Sheehan 2001;Finnegan et al. 2012;Zou et al. 2018).It could then be argued that the group subsequently failed to re-establish previous levels of diversity and biogeographic distribution because they were unable to radiate during the palaeoceanographically turbulent Silurian, wherein sealevel and shelf conditions were in a frequent state of flux (i.e.Primo-Secundo states; Jeppsson et al. 1995).The correlation of faunal extirpations and extinctions, oxygen and carbon isotope excursions, and sedimentation and sealevel changes has demonstrated that the Silurian was punctuated by many perturbations to the ocean-atmosphere-biosphere system (sometimes known as Silurian oceanic episodes, SOEs).At least three of these were of certain great ecological impact: the early Sheinwoodian Ireviken event, the mid Homerian 'Big Crisis' (also known as the Mulde or lundgreni event from the perspective of conodonts or graptolites) and the mid-Ludfordian Lau (or kozlowskii graptolite) Event.These biotic crises have been under great scrutiny over the last 30 years and are generally accepted to have occurred (Jeppsson et al. 1995;Jeppsson & Calner 2002;Munnecke et al. 2003;Cramer et al. 2012;Jarochowska & Munnecke 2016;Bowman et al. 2019;Fr yda et al. 2021).The most severe of these is the Lau event, in which a major glaciation with associated cooling and sealevel drop (Fr yda et al. 2021) caused a rate of faunal turnover comparable to the 'big five' mass extinctions (Bowman et al. 2019).The onset of the Lau event is recognized in the Bohemograptus Zone, immediately above the Saetograptus leintwardinensis Zone wherein Radnorscolex latus occurs, tentatively suggesting that R. latus may have been among the final surviving palaeoscolecids before the group ultimately succumbed to the palaeoceanographic catastrophes of the mid-Ludfordian Lau Event.Additionally, it is notable that Ludfordian strata of the Prague Synform show a separate more moderate extinction at the top of the Saetograptus leintwardinensis Zone among pelagic graptolites and cephalopods, although the authors concluded that current data were insufficient to determine whether the 'leintwardinensis Event' affected benthic macrofauna in addition ( Storch et al. 2014).

The British record of palaeoscolecids
In addition to R. latus described here and the Milaculum-type plates described by van den Boogaard (1988) from the Llandovery of the Anglo-Welsh Basin, the remainder of British palaeoscolecids are reported from Ordovician localities spanning the Tremadoc (Tremadocian, Lower Ordovician), Arenig (Floian-Dapingian, Lower-Middle Ordovician) and Llanvirn Series (Darriwilian, Middle Ordovician) of England and Wales.
The Tremadoc of England contains the best known of these: Palaeoscolex piscatorum Whittard, 1953 from the Shumardia pusilla (= Shumardia (Conophrys) salopiensis; Fortey & Owens 1991) Zone of the Shineton Shales Formation, Shineton Inlier, Shropshire.This taxon is also reported from the stratigraphically lower Rhabdinopora flabelliformis Zone of the Breadstone Shales Formation of the Tortworth Inlier (Curtis 1968), Gloucestershire.The Shineton material was re-described using SEM by Conway Morris (1997) in a significant early demonstration of associating palaeoscolecid macro-and microfossils.SEM work is necessary, however, to confirm the affinity of the Breadstone material to P. piscatorum.Further Tremadoc examples that may also have affinity to Palaeoscolex are reported from Wales.These include material reported from lower Tremadoc strata (probably the Clonograptus tenellus Zone) at Clyn-côch in the Llangynog area, Carmarthenshire (Owens et al. 1982) and another report from the Afon Gam biota of the Dol-cyn-Afon Formation (basal Tremadoc) of Gwynedd (Botting et al. 2015).F I G . 5 .Scanning electron microscopy images of the three dimensionally preserved trunk of CMZ V3.A-E captured using JEOL IT500; F-H using FEI Quanta.A, trunk annuli showing both tuberculate annuli plates and altered smooth annuli plates.B, broken anterior limit of specimen.C, trunk annuli from posterior end showing mostly intact, tuberculate plates.D, posterior end of specimen with no sign of posterior hooks.E, two tuberculate plates within an annulus surrounded by microplates.F, individual tuberculate plate surrounded by microplates.G, area of microplates in between larger tuberculate plates showing microplates in both positive and negative relief.H, ornamented microplates located on the margin of a tuberculate plate.(See Fig. 3   Records from the Arenig are sparser, limited to three occurrences.Botting et al. (2012) recorded an indeterminate palaeoscolecid with an unusual non-annular plate organization from the Bergamia rushtoni Zone of the Pontyfenni Formation in Carmarthenshire, South Wales (Fortey & Owens 1987).Hearing et al. (2016) reported a well-preserved almost complete but unidentified specimen from the slightly younger Llanfallteg Formation; and an additional indeterminate record from a different locality of the same formation is reported here (Fig. 2I) from the Dionide levigena Zone.This specimen, which preserves the annuli but not the plates, was collected from an old railway cutting, north of Llanfallteg by R. A. Fortey and R. M. Owens in 1987.
The Llanvirn strata of the Builth-Llandrindod inlier of Powys contain the remainder of the British fauna.From this region, Botting et al. (2012) described seven taxa across three graptolite biozones and Botting et al. (2023) reported at least five taxa present in the Castle Bank biota (Gilwern Volcanic Formation, Didymograptus murchisoni Zone), at least some of which differ from those described in Botting et al. (2012).From oldest to youngest, the taxa described by Botting et al. (2012) include Wernia exinia and Loriciscolex cuspidus from the Didymograptus artus Zone of the Camnant Mudstones Formation, Radnorscolex bwlchi from either the upper Didymograptus artus or lower Didymograptus murchisoni Zone of the Gilwern Volcanic Formation, Aggerscolex murchisoni from the Didymograptus murchisoni Zone of the Gilwern Volcanic Formation, Ulexiscolex ormrodi from the basal Hustedograptus?teretiusculus Zone of the Llanfawr Mudstones Formation, and Pluoscolex linearis and Bullascolex inserere from the mid-upper Hustedograptus?teretiusculus Zone of the Llanfawr Mudstones Formation.

CONCLUSION
The palaeoscolecids from the lower Ludfordian Lower Leintwardine Formation in the Anglo-Welsh Basin belong to the genus Radnorscolex, which was first recorded from the same palaeogeographic region in Darriwilian (Middle Ordovician) strata c. 40 myr older (Botting et al. 2012).This presents Radnorscolex as potentially a relict lineage left over from the greater Ordovician diversity of palaeoscolecids, many of which may have perished in the Late Ordovician Mass Extinction.Further support for the continuity of this clade in the Anglo-Welsh basin comes from the early Silurian (Llandovery, Aeronian) microfossil form taxon Milaculum longmyndium (van den Boogaard, 1988), which has a broadly similar kind of cuticular ornament as both the Darriwilian R. bwlchi and Ludfordian R. latus.As discussed here, no palaeoscolecids are known to occur in the stratigraphic record above the Saetograptus leintwardinensis Zone, which precedes the onset of the major biotic turnover of the mid-Ludfordian Lau Event in the Bohemograptus Zone, which potentially marks Radnorscolex as among the last palaeoscolecid lineages to exist if indeed this oceanic event caused their eventual demise.However, the hypotheses discussed here are limited by two major factors that are fruitful for future investigations.First, more Silurian palaeoscolecids must be precisely identified using SEM methods, because currently only Radnorscolex latus in the present study has been afforded this.This is necessary to gain a better grasp of Silurian palaeoscolecid diversity, which may only appear greatly reduced due to a paucity of occurrence data (which is subject to the biases of taphonomy and collection).It will be intriguing to compare further Silurian examples to Ordovician taxa and to see whether the case in Radnorscolex of long-ranging relict genera is repeated, or whether there are new innovations unique to a Silurian radiation.Second, a data-oriented approach to studying palaeoscolecid evolutionary history is required to validate or discard hypotheses discussed here.Currently, there is no phylogenetic tree of palaeoscolecids on which to conduct comparative analyses, and a comprehensive dataset precisely constraining the record of Palaeoscolecida in a phylogenetic and stratigraphical context is wanting.

F
I G . 1 .Geological setting, stratigraphy, and localities.A, inset map.B, NHMUK PI Q 6399 a, palaeocurrent-aligned graptolites (Saetograptus leintwardinensis) collected from Church Hill Quarry showing the NE-SW axial trend of the Church Hill Channel; white arrow indicates flow direction.C, photograph of Church Hill Quarry, August 2023.Scale bars represent: 500 m (A); 5 mm (B).

F
I G . 4 .Reflectance transformation imaging (RTI) of NHMUK PI A 1946 a. A, middle and posterior of specimen rendered using diffuse gain and illuminated from the northwest.White boxes indicate areas where further RTI images (B-D) and scanning electron microscopy images (see below) were prepared.B, middle section of specimen with three-dimensional gut preservation (corresponding to upper right box in A).C, posterior end of specimen rendered using diffuse gain and illuminated from the southeast (corresponding to lower left box in A).D, posterior end of specimen under normals visualization (corresponding to lower left box in A).Scale bars represent: 3 mm (A); 1.5 mm (B-D).

F
I G .6 .Scanning electron microscopy images showing the flattened trunk of NHMUK PI A 1946, all captured using a JEOL IT500.A, section of the trunk showing annuli with altered smooth plates and gut tract.B, trunk annuli showing both altered plates in positive relief and weathered out internal moulds of plates with tubercles in negative relief.C, trunk annuli showing both altered plates in positive relief and weathered out internal moulds of plates with tubercles.D, posterior end of specimen.E, weathered out internal mould of plate with tubercles in negative relief surrounded by microplates.F, altered smooth plate lacking tubercles in positive relief surrounded by microplates.(See Fig. 4 for specimen context.)Scale bars represent: 500 lm (A); 100 lm (B, C); 200 lm (D); 20 lm (E, F).
Morris (1977)List of specimens in this study.ConwayMorris (1977)supposed therefore that of these, only Pr. ornatus was related to Palaeoscolex (the only other known palaeoscolecid genus at the time) and that other Protoscolex species would require taxonomic revision.A handful of further species (including R. latus) with palaeoscolecid-like ornamented annulations were added to Protoscolex in the late 19th and early 20th centuries (summarized in Conway Morris 1977, pp.85-86), as well as two unillustrated species from the Ordovician Beecher's Trilobite Bed later by Cisne (1973).However, this was done erroneously, because the type species Pr. covingtonensis is not a palaeoscolecid.As such, this work transfers the Leintwardine material to the genus Radnorscolex, which it closely resembles, and demonstrates that other examples erroneously assigned to Protoscolex require reassignment based on SEM investigation to determine their affinity to palaeoscolecid taxa.