A diverse lower Cambrian (series 2) non - trilobite fauna containing a burgess shale type element, from the Llanberis Slate of Penrhyn Quarry, North Wales

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Proceedings of the Geologists’ Association xxx (2019) xxx–xxx.

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A diverse lower Cambrian (series 2) non - trilobite fauna containing a burgess shale type element, from the Llanberis Slate of Penrhyn Quarry, North Wales Richard Birch 102 Nant-y-Felin, Pentraeth, Anglesey, Wales LL75 8YA, United Kingdom

A R T I C L E I N F O

A B S T R A C T

Article history: Received 25 June 2019 Accepted 24 January 2020 Available online xxx

Fossils of trilobites and hyolithids were first identified in the Llanberis Slate of Penrhyn Quarry, Bethesda, North Wales, in 1888, and were reviewed in 1950, leading to the assignment of an early Cambrian (Stage 3, series 2) age. Recent collections from the Upper Green Slate Member of the Llanberis Slate include organisms associated with Burgess-Shale-type Konservat Lagerstätten. Despite low - grade metamorphism of the deposit, a substantial diversity of taxa is recognisable, including sponges, non - trilobite arthropods, vetulicolians, putative sipuncula and other worms. The value of Lagerstätten is the insight they provide into the composition of infaunal and epifaunal communities, including weakly - sclerotised organisms whose identity may only be inferred where the taphonomy is restricted to biomineralised elements or ichnofossils. Exceptional preservation from Cambrian localities including the Burgess Shale of British Columbia and the Maotianshan Shale of Southern China, have provided a benchmark against which formerly unidentified material can be assessed. This fauna is now known to occur World - wide, but good examples from the Avalonian terrane, comprising Southern Britain and Eastern Canada in Cambrian times, have not previously been identified. This is the first record of a Burgess Shale - type fauna in the Cambrian strata of Wales, where it represents the oldest comprehensive early Cambrian biota from the historical type area on which the Cambrian System was originally based. Many of the organisms described also represent the first records from North Wales, and, in an international context, it is also the first record of a Cambrian Burgess Shale type assemblage from Avalonia. © 2020 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.

Keywords: Vetulicolia Isoxys North Wales Burgess Shale fauna Cambrian sponge Bradoriid

1. Introduction Wales is the historical type locality for the Cambrian (Sedgwick and Murchison, 1835) and the name derives from the Latinised Welsh Cymru. Fossils from the Upper Green Slate Member of the Llanberis Slate in Snowdonia, North Wales, were first described by Woodward (1888). He produced a description of the trilobite Conocoryphe viola Woodward from Penrhyn Slate Quarry in North Wales (Location: 53.1552  N, 4.0755  W). Howell and Stubblefield (1950) later reclassified this trilobite as Pseudatops viola, thereby establishing the early Cambrian (series 2) age of this deposit. They briefly described other non - trilobite fossils, hinting at a more diverse community, which included Hyolithus hathaweyi, with one of the earliest references to ‘helens’. They also mentioned, but did not illustrate, ‘two articulating joints of a large crustacean’ (Howell and Stubblefield, 1950, p. 13), from the Green Slate formation at

E-mail address: [email protected] (R. Birch).

Nantlle, 28 km to the southeast of Penrhyn Quarry (Fig. 1). Wood (1969) records the discovery of Pseudatops from there, providing correlation between the two sites. No further in - depth studies of the Green Slate fauna have been made. Since the discovery and description of the Burgess Shale fauna (BSF) of British Columbia (Walcott and Resser, 1931; Conway Morris, 1979), a remarkable diversity of soft - bodied and lightly sclerotised organisms have been revealed from Cambrian deposits there and elsewhere. In addition, it has been possible to interpret and reconstruct previously unsuspected biotas in Cambrian strata worldwide by comparison with the BSF. These faunas are now known to be cosmopolitan, and many of the organisms can be identified from poorly preserved or incomplete skeletal elements. So far, these have not been reported from the Cambrian of Wales, although the early Cambrian rocks of Comley, Shropshire (UK), have yielded exceptionally preserved non - trilobite fossils (Cobbold, 1921; Siveter et al., 2003), and diverse faunas from higher Palaeozoic strata have been recovered from other Welsh localities (Botting et al., 2011, 2015; Hearing et al., 2016). The

https://doi.org/10.1016/j.pgeola.2020.01.004 0016-7878/© 2020 The Geologists' Association. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: R. Birch, A diverse lower Cambrian (series 2) non - trilobite fauna containing a burgess shale type element, from the Llanberis Slate of Penrhyn Quarry, North Wales, Proc. Geol. Assoc. (2020), https://doi.org/10.1016/j.pgeola.2020.01.004

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Fig. 1. Outcrop of Cambrian rocks in North Wales, showing the location of slate quarries where the Green Slate is exposed.

Llanberis Slate Formation is the first record in Avalonia of an early Cambrian assemblage fauna that includes trilobites, hyolithids, sponges, worms, vetulicolians and other non - trilobite arthropods. This paper documents the diversity of non - trilobite fauna from the Green Slate horizon of Penrhyn quarry. Its main elements are summarised and illustrated and the taphonomic and ecological context is discussed.

2. Geological setting The structure and stratigraphy of the Llanberis Slate horizon in Nantlle was first described by Morris and Fearnsides (1926), and later revised by Cattermole and Jones (1970), who estimated the thickness of the slate belt in Nantlle as 750 m, with the uppermost Green Slate as 90 m thick.

Table 1 Stratigraphic table for the Llanberis Slate Formation (GWP Consultants 2017). Formation Bronllwyd Grit Llanberis Slates Formation

Unconformable above Upper Green Slate (Not exposed in working South Quarry) Hard Greys Domain

Purple Domain

Red and Blues Domain

Fachwen Formation

Not exposed Not exposed

Description (colloquial definitions)

Thickness

Massive sandstones and conglomerates.

NA

Green Slate – Fossiliferous green slates occurring in 4 bands between 1 - 22 m thick.

30 m

Hard Grey Slate – Hard silty slates showing the following succession from bottom to top: Hard, silty, purple/grey slates with spaced green bands and sandstones, occasional bright red bioturbated units. Spotted and Striped Purple Slate – Purple slate, common green “reduction” spots and rare amorphous green bands c. 10 - 30 cm thick. Soft Grey - Purple Slate. Soft Blue Slate (‘Penrhyn Blue’) Purple Slate Sandstone – Brown dolomitic sandstone or “Red Grit” or “Black Dyke”. Red slate – bright red slates, occasional green bands surrounding very thin sandstones.

81 m

Hard Blue Slate – blue/grey slates, clear bedded, thin sandstones spaced every 1.5 to 2 m. Striped Blue Slate – slates with closely spaced green sandstones. Basal sandstones and conglomerates.

25 m 28 m 22 - 40 m 10 m 2-8m 21 m (32 m in north pit) 21 m 22 m NA

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The total thickness of the Llanberis Slate in Penrhyn Quarry was estimated to be 2060 feet (628 m) by Smith and Neville - George (1961) and more recently from the working quarry to be 290 m (Table 1), although neither the base nor the Upper Green Slate is exposed there. Fitches and Wood (2005) described the active Penrhyn quarry in Bethesda in a publication for the UK Regionally Important Geological Sites (RIGS) network. That work was restricted to the geomorphic features rather than the palaeontological or stratigraphical elements. Limited access to suitable exposures within Penrhyn Quarry inhibits an accurate assessment of the stratigraphy of the Green Slates, which grades upward from the commercially important purple slate and is unconformably overlain by the Bronllwyd Grit. Sampling from Green Slate spoil indicates that there is a bradoriid dominated horizon lying towards the base. These bradoriids appear to be aligned in the same plane on bedding surfaces, suggesting deposition from currents or turbidites. This phenomenon is also indicated in the underlying purple slate of the working quarry, where distinctive thin layers of coarse - grained material in boudinage structures occur along - strike between green - stained zones, often for tens of metres (Fig. 2). The sediments may have originated from the Irish Sea Horst, which lay to the west in early Cambrian times (Brasier et al., 1992). Between the lower and upper boundaries of the Green Slate, the trilobite fossils by which the lower Cambrian is recognised are [sic.] ‘scanty and difficult to interpret’ (Thomas et al., 1984 p. 8). However, fossiliferous horizons with well-preserved material that includes a soft - bodied element occur within strongly laminated interfacies of the Green Slate at Penrhyn. Fossils show little sign of being transported, leading Howell and Stubblefield (1950) to suggest that deposition was in calm water. The green colouration and relative scarcity of burrowing infauna suggests fluctuations in available oxygen, permitting only episodic colonisation by burrowing animals. Throughout the deposition of the Llanberis Slate, there were occasional influxes of sediment as a result of storm events (Griffiths, 1935; Williams and Ramsey, 1959; Howells et al., 1985; Bennett, 2007), although these have not been recognised within the Upper Green Slate horizon. The stratigraphy, determined by the trilobites collected from the Green Slate horizon exposed at Penrhyn, is sufficient to place the Upper Green Slate member of the Llanberis Slate towards the top of International Stage 3 (series 2) of the Cambrian chronostratigraphy. This is equivalent to upper middle Comley in British regional stratigraphic terms (Harvey et al., 2011). At this

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time, the Welsh Basin was becoming progressively shallower during a period of marine regression that lasted until at least the end of the Cambrian (Brasier et al., 1992; Harvey et al., 2011). Neither the taphonomic style nor fossil quality is uniform, due to the high degree of tectonic deformation. In iron - rich laminated layers of Green Slate at Penrhyn, shelly debris occurs as casts and moulds. Taxa with mineralised skeletal elements (including trilobites and bradoriids) have been replaced by a chlorite - like mineral, which is also deposited on the surfaces of micro - faults and slickensides across the cleavage planes. Soft - bodied organisms are frequently preserved as a film of pyritic framboids. A major factor affecting the quality of the specimens is that cleavage in the slate varies from 5 to 45 to bedding, and individual specimens may be spread across several cleavage planes. On faulted surfaces, soft - bodied organisms are occasionally preserved in a green material identified as a chlorite mineral formed under metamorphic conditions of this type. Mineral replacement represents a later pseudomorph of original preservation fabrics. Other minerals, including a late - stage film of manganese dendrites around individual specimens, are occasionally observed. Sclerotised organisms are occasionally preserved with relief, albeit often crushed and distorted. The nature of the animal can be sometimes be determined from voids within the mould, or fragments that occur adjacent to the body fossil, as in the more robust porifera. This is unlike the typical Burgess Shale mode of preservation as a flattened biofilm (Butterfield, 1990), even though a suite of other minerals may be present in these deposits. Thus, the Green Slate assemblage is an example of a Burgess Shale - type fauna but does not strictly represent a typical Burgess Shale - type preservation (Paterson et al., 2016). 3. Material and methods The Green Slate tends to shatter without favouring the fossils present within it, necessitating some occasional repair. Few of the featured specimens received extensive preparation, the exceptions being the bradoriids featured in Fig. 3A, which were partially excavated using a Parkside© rotary engraver (Chesterfield, UK), and specimens that shattered and had to be reassembled. Photographic techniques varied: high relief specimens were photographed without any preparative treatment; three - dimensional specimens were coated with ammonium chloride; those preserved as pyritic or chloritic films were immersed in a water bath. Specimens were drawn using overlays in the GIS programme MapInfo© v.7 (New York, USA). 4. Results A collection of fossils made between 2008 and 2013 from spoil pits of the Green Slate around Penrhyn Quarry consists of 191 individual trilobite specimens and 201 fossils of other types of organism. These fossils form the basis of the collection deposited in the National Museum of Wales, Cardiff, UK, under reference numbers NMW 2014.29 G. The proportions in Table 2 reflect an initial bias towards the collection of trilobites, but as more of the features of the Green Slate became recognisable as organic in origin, the numbers of non trilobite fossils increased. 4.1. Class Trilobita

Fig. 2. Green - banded zones (arrowed) in the purple slate of Penrhyn south quarry, with boudinage structures (inset). Photograph taken in 2017.

No. of specimens. 191 specimens of at least seven species in the NMW collection (Table 3): Howell and Stubblefield (1950) identified two trilobites from Penrhyn to the specific level, notably Pseudatops viola (Woodward,

Please cite this article in press as: R. Birch, A diverse lower Cambrian (series 2) non - trilobite fauna containing a burgess shale type element, from the Llanberis Slate of Penrhyn Quarry, North Wales, Proc. Geol. Assoc. (2020), https://doi.org/10.1016/j.pgeola.2020.01.004

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Fig. 3. A) NMW 2014.29 G.191. Penrhyn bradoriid Indiana cf. lentiformis external surface. Ai). The same specimen photographed in a water bath. B) NMW 2014.29 G.195. Paired valves. C) NMW 2014.29 G.192. Specimens aligned on slab with internal surfaces uppermost. D) NMW 2014.29 G.234. Single valve of Isoxys sp. E) Camera lucida image with dorsal and internal measurements. Scale bar = 10 mm.

Table 2 Faunal composition of fossils collected from the Green Slate at Penrhyn (where n = 392). Taxon

Totals

%

Pseudatops viola Strenuella cf. strenuella Serrodiscus bellimarginatus Protolenus indet. Unidentified trilobite Miscellaneous non - trilobite arthropod Bradoriids Hyolithids Worms Sponges Trace fossils

27 88 23 21 32 6 33 48 60 28 26 392

7 22 6 5 8 2 8 12 15 7 7 100

1888) which is endemic to the Llanberis Slate, and Eodiscus (Serrodiscus) bellimarginatus (Shaler and Foerste, 1888). These two species provide the stratigraphical age for the Llanberis Slate. Neither possesses eyes. Howell and Stubblefield (1950) also, tentatively, described two additional species (Protolenus (Geyerorodes) howleyi (Matthew, 1894) and Strenuella strenua (Billings, 1872), both of which do possess eyes. A comprehensive reassessment of the trilobite fauna contained in the National Museum of Wales’s collection 2014. 29 G, including additional species, will be described and published elsewhere. Proportionally, trilobites represent the most abundant arthropod fossils from the Green Slate, even allowing for collection bias. They are generally disarticulated, occurring as isolated cranidia and associated fragments. Articulated moults are rare, and soft tissue has not been detected.

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Table 3 Trilobite reference numbers. Species

National Museum Wales (NMW) Reference Number

Pseudatops viola Strenuella cf. strenua

2014.29 G. 2014.29 G.

Serrodiscus bellimarginatus Protolenus Undet Trilobite Undet

2014.29 G. 2014.29 G. 2014.29 G.

1,2, 4–12, 14–16, 35, 39, 40, 47, 98, 99, 219, 220, 230, 263, 267, 270 18-21, 24, 26–29, 31, 33–35, 37, 38, 41, 51, 54, 57, 77–90, 93–97, 100–102, 106–109, 112–118, 123, 124, 133–138, 212, 213, 215–218, 224–229, 255, 256, 259-265 55, 58–67, 69–76, 92, 232, 268 17, 22, 23, 25, 30, 44, 45, 52, 103–105, 110, 130, 266, 229 32, 33, 42, 43, 46, 68, 91, 111, 113, 119–122, 125–129, 131, 135, 137, 139, 178, 179, 221 - 113, 227, 271, 297

4.2. Class bradorida No. of specimens. 51 separate samples in the NMW collection: NMW 2014. 29 G.140, 163–169, 190–203, 246–257, 327. Many specimens contain multiple individuals on the same slab. This is an uncommon phenomenon in the Green Slate Horizon, where fossils are widely spread and frequently occur singly. Only bradoriids regularly manifest this distribution (see Table 4) which is occasional in hyolithids and wormlike organisms resembling sipunculans. Three - dimensional valves are preserved with concave internal surfaces uppermost. They are rarely articulated (two examples) and rarely with the external surface exposed. Specimens are half as wide as long, with external surface ornamented with tubercles. Internal cavity between valves 4.5 5 mm deep. The majority have dimensions of 12 mm on the long axis (= 8 mm wide) with few smaller or larger individuals. Bradoriids are exclusive to one particular zone at Penrhyn. Although the stratigraphic position of this zone cannot be positively verified because sampling was from quarried spoil, the level is

recognisable because the slate in which they occur is speckled with microcrystals of chlorite not found in other layers. The horizon appears to be from the lowest part of the Green Slate because a single specimen was collected from the underlying transitional purple slate. The majority of individuals are preserved with the internal surface of the valve exposed once the rock has been split (Fig. 3B - C). This is regarded as an atypical position by Hou et al. (2002), but specimens with the external surface visible are rare at Penrhyn. When they are collected, the exterior is seen to have an irregular surface with grooves and ridges that have a lateral orientation (Fig. 3A - Ai). The appearance of ornamentation may be exacerbated by crystallisation of chlorite, which occurs throughout the surrounding matrix at regularly spaced intervals and also lines the internal cavity. The specimens are close to Indiana (synonymous with Leperditia) lentiformis of Cobbold (1921, pl. XXIV, fig. 46b, c) in both size and shape. When aggregations are collected, Penrhyn bradoriids are all orientated with the long axes of the carapaces parallel (Fig. 3C), indicating that this zone may have been subjected to strong currents.

Table 4 Total number of bradorids on 34 separate slabs. NMW Ref. No.

No. on slab

2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29

G. G. G. G. G. G. G. G. G. G. G. G.

140 163 164 165 166 167 168 169 190 191 192 193

4 4 2 1 1 1

2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 2014.29 TOTALS

G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G.

194 195 196 197 198 199 200 201 202 203 246 247 248 249 250 251 251 252 253 254 257 327

1 2 1 1 1 1 1 1 1 1 2 4 1 3 4 2 2 1 1 3 1 1 51

1 3 2 6 3

Notes Two on each side of slab. Articulated along umbo Part and counterpart on separate slab, both with internal surface uppermost.

The largest specimen (= 18 mm). Two specimens with convex surface uppermost. Two specimens parallel with convex surface exposed, on large plate (Fig. 3A). Five valves on one plate (all parallel). Single valve on reverse (Fig. 3C). Three parallel valves with convex surface exposed. One has tendrils of iron staining suggesting it decayed in situ. Articulated along umbo (Fig. 3B).

Single specimen. Concave outer surface exposed.

Three on one side, one on another. Spread through different cleavage planes. One smaller individual (= 6 mm). Four small (= 6 mm) specimens. One standard, one small (= 6 mm) specimen.

Equal size, aligned in same plane. Possible bradoriid.

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4.3. Bivalved arthropoda No. of specimens. One large arthropod valve in counterpart, with concave (inner) surface uppermost, NMW 2014.29 G.234 (Fig. 3D). Ellipsoid, almost as deep as wide. Cardinal spines extending beyond the borders of the carapace with a raised umbo where the two halves of the carapace were hinged. No internal features present. This specimen has been tentatively identified as one of the paired valves of Isoxys (Mark Williams, pers. comm., 2017) based on the combined width of the cardinal spines exceeding that of the

carapace (Fig. 3E). In unrelated taxa that also possess similar paired valves, cardinal processes are generally contained within the width of the carapace itself. The specimen most closely approximates to Isoxys auritus from the Chengjiang biota (Legg and Vannier, 2013), other species being longer than wide. However, in the absence of appendages or internal organs, assignment to any known taxon can at best be generic. Isoxys is an arthropod with a worldwide distribution that includes 16 species from 14 localities (Vannier and Chen, 2007). It has not previously been recorded from the Cambrian of Wales.

Fig. 4. A) NMW 2014.29 G.56. Indet. large disarticulated arthropod. B) Close - up of appendage with fringe of setae (arrowed). C) Camera lucida drawing showing internal void, where se.= setae, exo.= exodermal plates. Scale bar = 10 mm.

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4.4. Unidentified non - trilobite arthropoda - 1 No. of specimens. 1, part and counterpart. NMW 2014.29 G.56 (Fig. 4) The matrix of the counterpart is damaged, and some elements are missing. A robust, three - dimensionally - preserved disarticulated array of seven fragmented plates of different sizes, representing a crushed carapace. The plates expose an internal void 9 mm deep between surfaces in the part and counterpart of the (presumed) thorax. Individual plates patterned with longitudinal striations on the (presumed) outer surface. Long appendage, possibly two overlaying each other, with indication of one joint and terminating in bifurcating prongs, damaged and not clearly defined. Appendage is equivalent to the length of the thorax. One surface of appendage (presumed ventral margin) is lined with setae along its length (Fig. 4B). The scattered plates at the (presumed) anterior end have fine longitudinal striae unlike the surface of any other Green Slate fossil, confidently enabling any form of trilobite to be dismissed. The plates are considered to be part of the external surface of a carapace, which may have fractured as a result of compression at burial. There is no defined cephalic region, although

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the disassembled plates are tentatively considered to represent the thorax of an animal, and arthropods with a comparable body plan (e.g. the Burgess Shale arthropod Waptia fieldensis) have the head contained within the paired valves of the thorax. However, an incomplete and disarticulated organism known from a single specimen cannot be accurately determined further. 4.5. Unidentified non - trilobite arthropoda - 2 No. of specimens. 1, part and counterpart. Specimen no. NMW 27.110.G41 (Fig. 5A–B), the latter having undergone some repair. A large, unidentified arthropod from the Green Slate horizon at Nantlle, held in the collection of the National Museum of Wales as specimen no. NMW 27.110.G41, was described but not illustrated or identified by Howell and Stubblefield (1950, p. 13). As it has not been illustrated elsewhere, we feature the material here to provide context, but the location from where it originates features a suite of organisms requiring separate analysis. There are no obviously similar examples from other known lagerstätten of similar age, but without additional material, no further comparison is possible.

Fig. 5. A–B) NMW 27.110.G41 Part and counterpart of arthropod indet. as described (but not illustrated) in Howell and Stubblefield (1950). Photographed under polarising filter. C/D) NMW 2014.29 G.48 – 49. Part and counterpart of carapace with marginal serrations (arrowed), from a large arthropod suggestive of the telson from Helmetia expansa (inset). Scale bar = 10 mm.

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4.6. Unidentified non - trilobite arthropoda - 3

4.7. Class vetulicolia

No. of specimens: 1, in part and counterpart. NMW 2014.29 G.48, 49. A large carapace, much fragmented around the perimeter but with some areas retaining marginal serrations (Fig. 5C). Wider than long, but incomplete; most of the specimen is absent. The surface is marked with folds and it is not clear whether these are the result of distortion of a thin carapace (the specimen lies at approximately 5 to cleavage) or represent fused tergites. There are no visible sutures, all cracks in the carapace resulting in asymmetrical fragments, and it is concluded that the structure was undivided and entire. With the exception of bradoriids, non - trilobite arthropods from the Green Slate are represented by large, fragmentary specimens, and all attempts at identification of the featured specimens are necessarily tentative. The featured carapace most closely resembles a concilitergan (helmetiid) trilobitomorph telson, being undivided and suggesting a broad triangular structure with serrated, possibly convex borders. It is similar to Helmetia expansa Walcott from the Burgess Shale (inset Fig. 5D) and H. fastigiata from the Middle Cambrian of the Czech Republic (Chulpac and Kordule, 2002, Fig. 8, p. 175). The presence of fused tergites may indicate an Arthroaspis - like helmetid arthropod from the Sirius Passet Lagerstätte (Stein et al., 2013).

No. of specimens. One (partial part and counterpart). NMW 2014.29 G.233. Incomplete, presumed dorsal segment of large quadrate organism confirmed as part of the quadrate carapace of a vetulicolian similar to Vetulicolia rectangulata or V. cuneata in size and shape (Li et al., 2015; Li Yu-Jing, pers. comm., 2017). Lateral groove and two (of five) lateral pits visible on outer cuticle. Double skin inner cuticle visible, with lateral groove offset. The specimen is well preserved but incomplete and partly disarticulated. Only a portion of the carapace is present, and it is not possible to confirm the dorso - ventral orientation of this portion, but the dorsal half appears more likely. The specimen is also collapsed on itself in a postero-anterior orientation, and the possibility that part of the ventral section is present has been assumed in the reconstruction in Fig. 6B. Nevertheless the lateral groove and two of the lateral pits (referred to as pharyngeal slits by Ou et al. (2012) are well preserved and can be seen in both the part and more extensive counterpart (Fig. 6A). The carapace demonstrates partial relief and is not preserved as a mere film, as are those from Sirius Passet (Vinther et al., 2011) or as a simple mould like those from Emu Bay (García - Bellido et al., 2014). It has two well - defined layers: epidermal and endodermal,

Fig. 6. A) NMW 2014.29 G.233. Penrhyn vetulicolian with ‘pharyngeal slits’ (arrowed). Photographed in a water bath. B) Generic reconstruction of vetulicolian showing the approximate position of elements. Scale bar = 10 mm.

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as indicated by Shu et al. (2001). Where parts of the exoskeleton are missing, the endodermal layer is exposed, showing both the external parts of a pore and the inner lateral groove, which are not in alignment because the two layers parted and subsequently became offset with respect to each other. In the upper part of Fig. 6A, the outer layer of the cuticle is folded back to expose the inner one. The articulated appendage, generally referred to as the tail, is absent, and the specimen cannot be assigned to any known vetulicolian species. However, the fossil appears to be morphologically similar to Vetulicolia cuneata or V. rectangulata from the equivalent lower Cambrian (Series 2, Stage 3) Chengjiang region of Yunnan, China, and less like other members of this enigmatic group (Aldridge et al., 2006). This is the first example of a vetulicolian from the Cambrian Avalonian terrane.

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4.8. Class hyolitha (and related taxa) No. of specimens. 48 specimens of at least 4 forms. NMW 2014.29 G.141–162, 184, 204–208, 325, 346–356, 359–366. Conical conches considered to represent hyolithids are relatively common fossils in the Green Slate at Penrhyn. They represent multiple species with a conservative morphology varying from long slender forms to truncate wedge - shaped cones. Howell and Stubblefield (1950) described Hyolithes hathaweyi from Penrhyn (specimen number A1171 in the Sedgwick museum collection) and illustrated a conical form 60 mm long and 25 mm wide at the proximal opening. They observed’ fins’ (i.e., helens) on these specimens but did not give measurements. Two very similar specimens matching the description, and of proportional size are present in the material of this study, but helens are not preserved (Fig. 7F).

Fig. 7. A) NMW 2014.29 G.149. Large three - dimensional indet. cnidarian (?Cambrorhytium). B) NMW 2014.29 G.158 Counterpart of fragmented cf. Hyolithus americanus with longitudinal striations on internal surface, where Ve = ventrum. Bi) Part of B. C/D) NMW 2014.29 G.153 and 161. Hyolithid operculae with dorsal depression (arrowed). E) NMW 2014.29 G.155. Narrowly - conical form photographed in a water bath. F) NMW 2014.29 G.142. Hyolithus hathaweyi of Howell and Stubblefield (1950). G) NMW 2014.29 G.290. Pyritised poriferan with scattered spicules at base. Gi) Close - up of spicules with cruciform stauractins. H) NMW 2014.29 G.292. Three - dimensional thick - walled poriferan. Hi) Close - up of margin with cruciform and uniaxial spicules. I) NMW 2014.29 G.291. Thick - walled poriferan with broad - based apertural marginalia at opening (arrowed). Photographed in a water bath. Scale bar = 10 mm.

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One of the most common forms has a truncate, tapered conch, triangular in cross - section at the proximal end, with concave flanks. Frequently, the conch will have disintegrated into two or three sections along the angular keel, ventral and dorsal ridges that give it the triangular cross - sectional structure. This is illustrated in Fig. 7B/Bi, where an interpretation of the profile is shown (Fig. 7B inset). The ventrum has a longitudinal, inflated central region, tapering posteriorly (best seen in Fig. 7Bi). This form often occurs communally in the Green Slate at Penrhyn, sometimes found as an aggregate of many disarticulated plates. As a result of this fragmentation, the internal surface of the conch is revealed as having longitudinal striations Fig. 7B). This hyolithid morphology is similar to that of Burithes (=Hyolithes) americanus (Billings), a poorly described species known from New York and Greenland (Malinky and Skovsted, 2004). Slender forms with faintly defined lateral edges (Fig. 7E) are similar to Gracilitheca (=Hyolithus) bayonet (Matthew) as depicted in Cobbold (1921, pl. XXIV, fig. 12), which was considered synonymous with Gracilitheca by Malinky and Skovsted (2004). Opercula are rarely collected from Penrhyn, and there are only four in the collection, two of which are illustrated in Fig. 7C, D. These show a depressed dorsal rim and the grooves representing the rooflet, as depicted in Marti Mus and Bergstrom (2005, Fig. 1B, p. 1140). Their size and rounded shape suggests they belong to a larger form, not readily corresponding to any conches in the collection. A large and particularly well - preserved three - dimensional cone - shaped organism with internal void is shown in part in Fig. 7A). It has a narrowly tapering conch which has both fine annulations and regularly spaced prominent ones but lacks divisions into facets or a keel and may be a cnidarian (e.g. Cambrorhytium), which elsewhere are more frequently preserved as flattened films (Zhang and Hua, 2005).

4.9. Class Porifera No. of specimens. 24: NMW 2014.29 G.272–279, 290–294, 295, 299–304, 312, 314, 369, 414. Robust, three - dimensional sponge fossils are common body fossils in the Llanberis Green Slate, although many have poor preservation of skeletal detail that makes confirmation of their nature difficult. Large flask - shaped organisms, widest above the middle, have a reinforced external layer, but detailed internal morphology beyond an indication of a porous structure is destroyed by pyritisation. Specimens that have partly disintegrated show both cruciform stauract and monaxial spicules around the main body in such a way as to indicate they were derived from it (Fig. 7G, Gi, H, Hi), thus distinguishing them from calcified algae or inorganic mineral growths. A close - up of the surface reveals a thickened and prominently tuberculate outer wall that was rigid enough (or rapidly enough mineralised) to survive sedimentary compression. No specimens show visible root tufts. The osculum opens apically, and many individuals possess visible apertural prostalia marginalia (arrowed, Fig. 7I). These are most common among reticulosan and hexactinellid sponges, but details of the hexactine - based spiculation are difficult to confirm in situ. The classification of early Cambrian sponges was revised by Botting and Muir (2018) and shown to be much more complex than a simplistic characterisation based on the characters of extant taxa alone. Further classification of the material is beyond the scope of the current work, but numerous complete sponges of this type are a feature of the Green Slate at Penrhyn and are not recorded anywhere else in the lower Cambrian strata of the UK.

4.10. Miscellaneous Porifera and Annelida No. of specimens. 82: NMW 2014.29 G.281, 285, 286, 311, 368, 370, 409, 410, 412, 418, 420–422, 425–429 (additional specimens examined but not assigned a reference no.) Linear, globular and flask - shaped fossils preserved as pyritic films, often spread over several cleavage planes and probably representing multiple taxa including sponges, worm - like organisms and cryptic phyla. Superficial similarity of preservation requires that fossils in this category are collectively included under miscellany. Variation may be due to alignment in the sediment when the organism perished, distortion of the matrix or that multiple taxa preserve similarly as pyritic framboids under the taphonomic conditions present at the time. Fig. 8 shows a selection of superficially similar lanceolate to flask - shaped organisms. Fig. 8A has marginal spines (spicules) extending to the terminus (considered as the anterior of the organism). The form is reminiscent of the sponge Pirania (Walcott, 1917). Fig. 8B has a similar blade - like outline, and abundant stauracts are visible along the margin of the fossil (inset, Fig. 8B), but there are indications of structures suggesting internal organelles. Classification of Fig. 8C is equally conjectural: although well preserved in part and counterpart, no spicules are present internally or in the adjacent matrix, and it may therefore represent a priapulid worm similar in form to Ottoia prolifica from the Burgess Shale (Conway Morris, 1977). Fig. 8D–F are all examples of organisms sharing a flask - shaped structure with a terminal orifice (Or.) at the presumed apex. This bares some resemblance to the osculum of a soft sponge, but spicules are not observed within or near any specimens sharing this form. They also appear to have the ability to compress into a stout pear - shaped trunk with compression annulations Fig. 8D) and a slender introvert which may be retractable (Fig. 8F), indicating some flexibility of movement. This suggests a sipunculan - like organism, and they demonstrate all the positions that might be adopted by a sipunculan, from a fully extended introvert to a retracted one, and sometimes both together. The lack of internal tissue or well - defined characteristics like a nephridiopore at the base of the introvert, hooks or tentacles around the oral disc, characters typical of sipunculans as described by Gibbs (1977) leaves positive identification equivocal. However, Fig. 8F has a darker ring at the base of the introvert, and small hook - like structures can be seen in the matrix nearby. Examination of examples of Phascolosoma granulatum and Golfingia elongata in the National Museum of Wales, Cardiff, UK, shows that these features are equally difficult to observe in extant preserved specimens and visibility of internal organelles is hampered by the coriaceous nature of the sipunculan derma. Of the eight similar specimens in the collection, two have more than one specimen preserved in proximity, as in Fig. 8D. (Fig. 8E has another specimen on the underside of the slab). Given the relative scarcity of fossils in the Green Slate, this is significant. Lecthaylus gregarius from the Upper Niagaran (Silurian) of Blue Island, Illinois (Weller, 1925) is also, as the name indicates, a social organism, occurring as a black film in which the characteristics of sipunculans are equally difficult to determine. Sipunculans with visible internal organs are known from the early Cambrian of China (Huang et al., 2004). Other examples are known from later in the Palaeozoic (Muir and Botting, 2007), but fossil sipuncula are not well represented in the literature, and those that are show considerable taphonomic differences between the respective localities. Fig. 9 shows a complete but damaged organism resembling a finely annulated worm with everted proboscis (Ev.Pr.) which may have affinity with the Sipuncula. There is a prominent opening

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Fig. 8. A) NMW 2014 29 G.418. Sponge (Pirania sp.) with marginal monaxons. B) NMW 2014.29 G.414. Blade - like sponge with cruciform stauractins around margin (inset). C) NMW 2014.29 G.417. Possible priapulid worm with proboscis or alimentary tract (arrowed). D) NMW 2014.29 G.286. Paired sipunculan - like soft - bodied organism with Or. = apical orifice (close - up inset). Photographed in a water bath. E) NMW 2014.29 G.420. Similar specimen spread across multiple cleavage planes. Photographed in a water bath. F) NMW 2014.29 G.412. Sipunculan - like organism with retracted introvert defined by dark line (arrowed). Photographed in a water bath. Scale bar = 10 mm.

with a visible rim in Fig. 9A. An alternative suggestion is that the opening represents a holdfast – in which case the specimen is depicted upside - down. If so, Fig. 9D represents the anterior end of a robust, relatively thick - skinned organism of unknown affinity. The counterpart (Fig. 9B), which shows the posterior half of the trunk displaced by a break at the half - way point, is atypical in that it is not pyritic, but is preserved in a chlorite mineral. This shows that the dermal surface appears to be made up of embedded granular sclerites or raised tubercles typical of sipunculans.

5. Discussion Since the qualitative elaboration of the Burgess Shale Fauna as first described by Walcott (1914), there has been great interest in the non - trilobite fauna of the lower Palaeozoic, and localities have now been discovered worldwide. Significant examples are the lower Cambrian Chengjiang fauna in China (Hou et al., 1991; Zhang et al., 2001; Hou et al., 2002; Shixue et al., 2012; Fu et al., 2019), the lower Cambrian Sirius Passet Formation in Greenland (Taylor, 2002; Peel, 2010), the Wheeler Shale of Utah (Robison and

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Fig. 9. A) NMW 2014.29 G.284. Anterior region of segmented worm - like organism with fine annulations (inset Ai) and an everted structure representing proboscis or holdfast. B) NMW 2014.29 G.283 Close - up of counterpart with annulations and possible embedded sclerites (inset Bi). C) Overlay drawing and interpretation as a densely annulated worm where ev. pr. = everted proboscis. D) NMW 2014.29 G.284. Posterior portion of A descending into matrix, photographed in a water bath. Scale bar = 10 mm.

Richards, 1981; Briggs and Robison, 1984; Conway Morris and Robison, 1986; Conway Morris, 1988) and the lower Cambrian Emu Bay Shale of Kangaroo Island, South Australia (Paterson et al., 2008; García - Bellido et al., 2014). Despite the geographical and temporal differences of these faunas, the similarities of the assemblages are sufficient to suggest that elements of the Cambrian non - trilobite community had a wider geographical spread than did the trilobites (Álvaro et al., 2003) or archaeocyathans (McKerrow et al., 1992). The presence of a non - biomineralised fauna in the Llanberis Green Slate horizon extends the known geographic range of a biota, some

of which had a pan - global distribution, as far as Avalonia in Cambrian times. The Llanberis slate fauna is significant because its early Cambrian age extends, temporally, the enigmatic faunas made famous from the Burgess Shale and elsewhere into the early Cambrian of the Avalonian terrane. While there is currently much interest on how late in the Palaeozoic these faunas persisted (Orr et al., 2000; Briggs et al., 2004; Siveter et al., 2010, 2017), it is important to determine the root of the ‘Cambrian explosion’. Stratigraphically, the Llanberis slate appears to be younger than the

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Table 5 Stratigraphical sequence of Lower and Middle Cambrian Konservat - Lagerstätten (IUGS International Chronostratigraphic Chart v2018/08).

CAMBRIAN

SERIES (EPOCH)

STAGE

UPPER - LOWER LIMIT

LAGERSTATTEN

Furongian

10 Jiangshanian Paibian Guzhangian Drumian Wuliuan

485.4 +/ - 1.9 489.5 494 497 500.5 504.5 509

Alum Shale Formation

Miaolingian

Series 2 Terreneuvian

4 3 2 Fortunian

521 529 541.0 +/ - 1

Chengjiang fauna, and broadly equivalent to the Sirius Passet Formation (Table 5). See also Zhu et al., 2006, Fig. 1, p. 218), a correlation that may be confirmed in future by detailed comparisons of the pelagic element of the fauna with those from other sites. The North Wales assemblage is also significant because exceptional preservation has persisted despite the destructive forces to which the original mudstone was subjected during the Caledonian Orogeny (Ordovician to Devonian). Despite the local effects of this tectonic event, non - biomineralised elements of the fauna remain identifiable. The featured taxa do not represent a comprehensive fauna from the Llanberis Green Slate, but only selected elements that relatively common or are repeated in the fauna. Many more occur singly (including larger arthropods), are fragmentary, or cannot readily be assigned. Nevertheless, there is sufficient diversity of material to make a judgement about the nature of the Penrhyn biota. Bradoriids in the lower part of the formation notwithstanding, non - trilobite arthropods are genuinely rare and those identified appear to represent pelagic forms. Where they have been recovered, preservation is three - dimensional, indicating that at least the more robust taxa would be preserved if they were present in quantity. The in situ arthropods are represented by trilobites, which are relatively common. 35 % of samples represent species which lack eyes, pointing to turbid conditions at depths not exceeding 200 m. Hyolithids are also fairly frequent and sometimes occur communally. Sponges are abundant, but not all forms that appear sponge - like can be confirmed as such, owing to the destruction of diagnostic features by pyritization. In other lagerstätten, priapulids are frequent, but these have not been confirmed in the Green Slate (but see Fig. 8C). The percentage of soft - bodied and weakly - sclerotised organisms that remain unidentified is considerable. This indicates a substantial diversity of scarce species in an environment that was, at least at some stage, dominated by a single form of bradoriid, then succeeded by an endemic demersal fauna of trilobites and sponges, perhaps adapted to poorly oxygenated conditions. One of the prerequisites for the preservation of weakly sclerotised and soft - bodied organisms, and the preservation of articulated moults is that bioturbation is rare or absent (Hou et al., 1991). Among the scenarios proposed to account for variations in the level of bioturbation, and subsequently the quality of preservation, is the presence of clay - sized particles that reduce the porosity of sediment and inhibit a burrowing fauna of scavengers (Gaines et al., 2005). This may also have been facilitated by the influx of sediment incurring rapid burial, a visible phenomenon in the underlying purple slate, but not so obvious in the Green Slate horizon where no intrusive layers of coarse sediment have been observed. The integration of oxygen into the sediment is further reduced in an environment in which the partial

Burgess Shale Wheeler Shale Formation Murero Formation Llanberis Slate Formation Sirius Passet Formation Maotianshan (Chengjiang) Shales Emu Bay Shale Pioche Shale Formation Kalmarsund Sandstone Formation

pressure of oxygen was lower than the modern value, suggested by the relative paucity of bioturbation. This explanation may also account for the preponderance of sponges (Sperling et al., 2013) and the Protolenid-Strenuellid trilobite association, correlating elsewhere with dysaerobic sediments (Westrop and Landing, 2000). The green colouration of the rock is also indicative of a reducing environment. However, evidence suggests levels of O2 were on the rise by the early Cambrian (Chen et al., 2015) so the local conditions in the Welsh Basin at this time may have been markedly different from other localities. The apparent absence of brachiopods, echinoderms and other elements of the early Cambrian benthic biota, is not unique to the Llanberis Green Slate. It is also a phenomenon of the lower Cambrian Emu Bay Lagerstätte, where it has been attributed to unfavourable conditions at the sediment/water interface (Paterson et al., 2016). Howell and Stubblefield (1950) noted that ‘deposition at Penrhyn [was] in calm waters relatively free from wave action’. Turbidity currents at intermittent intervals, suggestive of cyclic storm events as indicated in the underlying purple slates, transported sediment capable of burying and preserving sponges, but of insufficient depth to preserve a high grade lagerstätte, including internal tissue preservation, trilobite appendages, etc. There are no visible indicators of rapid flow in the form of flute clasts or graded bedding, but conversely, bradoriids are always aligned parallel, and occasional disarticulated trilobite exoskeletons have been found with the cephalon preserved perpendicular to the thorax. It is difficult to account for these examples in an environment that was not subject to some level of mass movement that buried soft - bodied organisms, and introduced ex situ organisms, but it was not of the type that resulted in mass mortality deposits, or aggregations of fossil material, as these are not a feature of the Llanberis Green Slate. Nevertheless, it is apparent from the presence of trace fossils that there was an indigenous fauna, with sponges forming a major component. (Fig. 10). Exceptionally well preserved, sponge - dominated faunas have been described from other Welsh localities of Ordovician age (Botting et al., 2011; Botting and Muir, 2012; Botting et al., 2015), from the latter of which well - preserved worm fossils were also recovered. None yield an abundance of non - trilobite arthropods. The predominance of chelicerates in Burgess Shale - type faunas has led to the conclusion that Cambrian communities were typically dominated by arthropods (Hagadorn, 2002). This does not appear to have been the case in the Welsh Basin. The dominance of sponges is known to correlate with environmental stress in modern, marine ecosystems (Bell et al., 2013) as well as some Palaeozoic examples (Botting et al., 2017, 2018) and may be characteristic of post - mass - extinction events (Brayard et al., 2011). However, it is notable that other Ordovician sponge dominated faunas from Wales occur with diverse, normal marine

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Fig. 10. Variable intensity influx of sediment bringing the remains of parautochthonous and pelagic organisms into an environment dominated by sponges and trilobites typical of dysaerobic sediments.

Appendix A. Supplementary data

faunas, and show no evidence of environmental stress (Botting et al., 2018). This dominance may, therefore, be an ecological signature of the Welsh Basin, persisting from the early Cambrian through to at least the late Ordovician.

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.pgeola.2020.01.004.

6. Conclusion

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Although the Llanberis Green Slate does not demonstrate the consistent quality of the internationally - renowned Burgess Shale and Chengjiang biotas (Gabbott et al., 2004) and has been subject to considerable distortion by regional geomorphism, it does preserve elements of Burgess Shale - type communities. It is the best example of an in situ lower Cambrian ecosystem in Wales. The relative abundance of sponges and other non - scerotised organisms among the fauna suggests a stressed environment in which non - trilobite arthropods were genuinely rare, probably representing an allochthonous component of the community. Accessibility of the sites has restricted precise stratigraphical definition, and systematic sampling of the Green Slate would improve understanding of the sedimentary conditions. However, the exposure stretches intermittently for many kilometres and undoubtedly much more remains to be discovered.

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Declaration of Competing Interest No conflict of interest. Acknowledgements I was assisted in photography of material featured in Figs. 4, 6 and 7 by Lucy McCobb and James Turner from the National Museum of Wales, Cardiff, and Stephen Pates permitted use of Fig. 5A and B. Welsh Slate, owners of Penrhyn quarry, allowed a North Wales Geological Association Field Trip to Penrhyn Quarry in 2017, during which images featured in Fig. 2 were taken. I thank Adrian Rushton, Mark Williams, Li Yu - Jing, and Keith Nicholls for helpful discussion. I am grateful to Joseph Botting and Lucy Muir for assistance and comments on an earlier draft of this manuscript and am indebted to the reviewers of the draft of this manuscript for some last-minute observations and identifications.

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Please cite this article in press as: R. Birch, A diverse lower Cambrian (series 2) non - trilobite fauna containing a burgess shale type element, from the Llanberis Slate of Penrhyn Quarry, North Wales, Proc. Geol. Assoc. (2020), https://doi.org/10.1016/j.pgeola.2020.01.004