Chitinozoan biostratigraphy of the regional Arenig Series in Wales and correlation with the global Lower–Middle Ordovician series and stages

Following establishment of the Ordovician System in England and Wales by Lapworth (1879), the Anglo-Welsh Ordovician succession and its constituent series (Tremadoc, Arenig, Llanvirn, Caradoc, Ashgill; Fortey et al. 2000) served as a standard reference for Ordovician correlation for more than a century (e.g. Williams et al. 1972, fig. 2; Fortey et al.1991). Nevertheless, as a type area for the system, the Anglo-Welsh region has its limitations. Structural complexity is common and much of the succession, particularly in the Welsh Basin and NW England, has been subjected to low-grade metamorphism. In addition, there is a lack of long continuous sections and a predominance of clastic sedimentary rocks that hampers the recovery of some stratigraphically useful fossil groups, notably conodonts (Fortey et al.1991). Despite efforts to redefine the Anglo-Welsh series in accordance with modern stratigraphic practice and more appropriate stratotypes (Fortey et al.1995, 2000), the global Ordovician series (Lower, Middle and Upper) and stages (Tremadocian, Floian, Dapingian, Darriwilian, Sandbian, Katian and Hirnantian; Bergström et al. 2009) are now all defined outside the British Isles (Cooper & Sadler, 2012). Nevertheless, as the historic type region for the system, England and Wales remain a key reference area for Ordovician stratigraphy.

Graptolites and conodonts are the quintessential tools of Ordovician biostratigraphy, although trilobites and other shelly fossils have also been used (e.g. Fortey & Owens, 1987). Chitinozoans have the potential for global Ordovician biostratigraphic correlation, and chitinozoan biozonations have been developed for Baltica (Nõlvak, 1999; Nõlvak et al. 2006), different parts of Gondwana (Paris, 1990; Paris et al.1995, 2007; Grahn, 2006; Quintavalle & Playford, 2006; Videt et al. 2010; de la Puente & Rubinstein, 2013; Nowak et al. 2016), South China (X Wang et al. 2005, 2009; Chen et al. 2009; W Wang et al. 2013; Liang et al. 2017) and Laurentia (Achab, 1989, Vandenbroucke et al.2003). However, a comparable biozonation covering the entire Ordovician succession for Avalonia, including England and Wales, has yet to be established. Data are available from Belgium (J Vanmeirhaeghe, unpub. PhD thesis, Univ. Ghent, 2006; Vanmeirhaeghe, 2007) and the upper Middle and Upper Ordovician of England and Wales (Jenkins, 1967; Vandenbroucke, 2008 a, b; Vandenbroucke et al.2005, 2008, 2009 a; Challands et al. 2014), but the Lower Ordovician and the lower part of the Middle Ordovician in England and Wales have remained largely unstudied until now.

The aims of this paper are (i) to describe chitinozoan assemblages from the upper Tremadoc, Arenig and lowermost Llanvirn series in Wales, including the historical type area of the Arenig Series in North Wales and complementary sections in South Wales; (ii) to establish a biostratigraphical framework; and (iii) to assess biostratigraphic ages of sampled successions based on species ranges in Gondwana, Baltica and Laurentia, and therefore to correlate the British regional series with the equivalent global Tremadocian, Floian, Dapingian and lower Darriwilian stages. The new data are also expected to contribute towards tracking biogeographical affinities with Gondwana, Baltica, South China and Laurentia, and to help constrain the stratigraphy of a time interval that is becoming increasingly pivotal in understanding the evolution of Ordovician climate (e.g. Trotter et al.2008; Vandenbroucke et al. 2009 b; Turner et al.2011; Dabard et al. 2015; Amberg et al. 2016, Pohl et al.2016 a, b; Rasmussen et al.2016; Elrick, 2022).

The term ‘Arenig’ was first used by Sedgwick (1852) for rocks that crop out on Arenig Fawr Mountain in North Wales, where the Ordovician System extends around the Cambrian Harlech Dome from the Llⓨn Peninsula in the west to the Bala area in the east and the Arenig Mountains in central Wales (Fig. 1). The Arenig Fawr section is stratigraphically incomplete (Zalasiewicz, 1984), however, and more complete sections are to be found in South Wales, where lower and upper Arenig successions are well developed in the Carmarthen area (Fortey & Owens, 1978) and around Whitland (Fortey & Owens, 1987) respectively. The Ordovician System in South Wales extends over 160 km from SW Wales to east central Wales (Fig. 1), following the Tywi lineament (Fortey et al. 2000). The Arenig Series is at the centre of this Ordovician tract and extends from Ramsey Island, off the westernmost point of the Pembrokeshire coast, towards Llandeilo, east of Carmarthen.

The Arenig Series in South Wales has a relatively rich and diverse macrofauna comprising trilobites, brachiopods, other shelly fossils and graptolites (Fortey & Owens, 1978, 1987; Cope, 1996, 2005; Cocks & Popov, 2019) and has also yielded acritarchs and chitinozoans (Molyneux, 1987). Fortey and Owens (1987) proposed a subdivision of the Arenig Series in South Wales into the Moridunian, Whitlandian and Fennian stages, although the base of the series itself was not defined there (Fortey et al. 1995). Wider correlations have been hindered, however, by the provincialism of many of the macrofossil species present (Cocks & Fortey, 1982), the generally poorly preserved and sparse acritarchs with low species richness, and the preliminary nature of the published chitinozoan record. Graptolites are poorly represented in the Moridunian and Whitlandian stages, but become more numerous, diverse and stratigraphically useful in the Fennian Stage. The lithostratigraphy and biostratigraphy of the areas sampled are summarized below.

Fortey and Owens (1978) provided a modern account of the stratigraphy of the Carmarthen area where the lower Arenig Moridunian Stage is best exposed (Figs 2, 3). The ‘Login beds’, consisting of siltstone, shale and sandstone, comprise the lowest exposed unit in the succession and were dated by acritarchs as latest Tremadoc to earliest Arenig (Molyneux & Dorning, 1989; Molyneux et al. 2007).

The Login beds are overlain by the Moridunian Ogof Hên Formation, although the contact between the two units is not exposed (Fortey & Owens, 1978). The Ogof Hên Formation includes conglomerate, sandstone and siltstone of the Allt Cystanog Member overlain by micaceous mudstone and shale of the Bolahaul Member. The overlying Carmarthen Formation comprises, in upwards succession, black mudstone of the Pibwr Member, turbidite beds and shale of the Cwmffrⓦd Member, and grey mudstone of the Cwm yr Abbey Member. The Carmarthen Formation passes up into the Afon Ffinnant Formation, consisting of turbidite deposits. The base of the middle Arenig Whitlandian Stage is placed 40 m above the base of the Afon Ffinnant Formation (Fortey & Owens, 1987, p. 87). Traynor (1988) assigned the Ogof Hên Formation to his facies 3, comprising fluviodeltaic deposits. The mudstones of the Pibwr and Cwm yr Abbey members are placed in facies 6 (deep water mudstone), and the Cwmffrⓦd Member and Afon Ffinnant Formation are placed in facies 5, representing deep water sediment-gravity flows in which turbidites are interbedded with facies 6 mudstone (Traynor, 1988).

The acritarch assemblage from the Login beds (Molyneux & Dorning, 1989) is similar to the messaoudensis–trifidum acritarch assemblage from the English Lake District and has been recorded as an occurrence of that assemblage in South Wales (Molyneux et al. 2007). Molyneux and Dorning (1989) concluded that the Login assemblage was from the uppermost Tremadoc or lowest Arenig series. Fortey et al. (2000, fig. 7), however, considered the acritarchs to indicate the upper Tremadoc Migneintian Stage and suggested correlation with the Araneograptus murrayi graptolite Biozone.

Above the Login beds, much of the Arenig succession in the Carmarthen area has yielded shelly faunas, rare graptolites and acritarchs (Fortey & Owens, 1978, 1987; Molyneux, 1987; Cope, 1996, 2005; Cocks & Popov, 2019; Ebbestad & Cope, 2021). No shelly fossils or graptolites have been recorded from the Allt Cystanog Member, but shelly faunas from the rest of the succession are varied. Echinoderm fragments, brachiopods and gastropods occur in the Bolahaul Member, bivalves in the Bolahaul, Pibwr and Cwmffrⓦd members, orthoconic nautiloids in the Cwm yr Abbey Member, and trilobites throughout. Fortey and Owens (1987) established a succession of seven trilobite biozones in the Arenig succession of South Wales, of which the lower three occur in the Carmarthen area: the Merlinia selwynii Biozone in the upper Ogof Hên and lower Carmarthen formations (Bolahaul and Pibwr members), the Merlinia rhyakos Biozone in the upper part of the Carmarthen Formation (Cwmffrⓦd and Cwm yr Abbey members), and the Furcalithus radix Biozone in the lower Afon Ffinnant Formation.

Graptolites recorded from the Carmarthen area include Phyllograptus cf. densus and Pseudophyllograptus aff. angustifolius from the Pibwr Member, dendroid graptolites from the the Cwm yr Abbey Member, including Callograptus cf. tenuis, Callograptus salteri and Palaeodictyota sp., and Azygograptus eivionicus and A. hicksii from the Whitlandian part of the Afon Ffinnant Formation (Fortey & Owens, 1978, 1987).

Molyneux (1987) described four acritarch assemblages from the Moridunian Stage in the Carmarthen area, informally designated Assemblage I to Assemblage IV in upwards succession. All comprise relatively sparse and poorly preserved specimens in assemblages of generally low diversity, however, and do not currently provide enough evidence to assist correlation.

The Whitlandian and Fennian stages are better exposed in the Whitland area (Fig. 2), where the succession was described by Fortey and Owens (1987). The Whitlandian Blaencediw Formation at the base of the succession consists of turbidite deposits, shale and siltstone, and is correlated with the upper part of the Afon Ffinnant Formation in the Carmarthen area (Fig. 2). The overlying Colomendy Formation, also Whitlandian, is divided into the sandy shale of the Rhyd-Henllan Member, the grey shale of the Castelldraenog Member and the black shale of the Whitland Abbey Member. The base of the Fennian Stage is placed at the base of the Cwmfelin Boeth Formation, which consists of turbidite beds and black shale. The overlying Pontyfenni Formation comprises black to dark grey shale and mudstone, and the Llanfallteg Formation at the top of the Arenig succession, passing up into the Llanvirn Series, comprises light grey mudstone and shale. The Colomendy, Pontyfenni and Llanfallteg formations were classed as facies 6 deep water mudstone units by Traynor (1988), interbedded with two coarse facies 5 deep water turbidite units represented by the Blaencediw and Cwmfelin Boeth formations.

Trilobites have been collected from all lithostratigraphical units in the Whitland area except the Castelldraenog Member of the Colomendy Formation, graptolites from all except the Rhyd-Henllan Member, and brachiopods, chordates and ostracods are known from some levels (Fortey & Owens, 1987; Cocks & Popov, 2019). The Rhyd-Henllan and Whitland Abbey members contain the typical Whitlandian trilobite Bohemopyge scutarix. The latter also contains the graptolite Expansograptus simulans (recorded as Didymograptus simulans), which is present throughout the Whitlandian Stage (Fortey et al.2000; Cooper et al.2004; Zalasiewicz et al. 2009). Fossils are numerous in the upper part of the Cwmfelin Boeth Formation and include brachiopods and the trilobite Asaphellus. Fortey and Owens (1987) considered them likely to be derived from a relatively shallow source.

The Pontyfenni Formation yielded a rich fauna of graptolites, trilobites, chordates and ostracods (Fortey & Owens, 1987; Jefferies in Fortey & Owens, 1987). A diverse graptolite fauna includes Expansograptus? uniformis lepidus, Expansograptus hirundo and Undulograptus cumbrensis (=‘Glyptograptus’ dentatus of Fortey & Owens, 1987; Owens, 1999). Fortey and Owens (in Fortey et al.2000) placed the Pontyfenni Formation of the Whitland area in the Isograptus gibberulus graptolite Biozone. Trilobites are nowhere common but comprise diverse cyclopygid and atheloptic assemblages with Pricyclopyge binodosa eurycephalathe, Placoparia cambriensis and Selenopeltis buchii macrophthalma, the last two species being typical of the Fennian Stage (Fortey & Owens, 1987).

Fortey et al. (2000, fig. 7) depicted the Llanfallteg Formation as straddling the Arenig–Llanvirn boundary, corresponding to the upper Fennian Didymograptus hirundo graptolite Biozone (since replaced by the Aulograptus cucullus graptolite Biozone: Rushton in Cooper et al. 2004; Zalasiewicz et al.2009) and the lower Abereiddian (lowest Llanvirn) Didymograptus artus graptolite Biozone. Graptolites include Undulograptus cumbrensis (recorded in part as ‘Glyptograptus’ dentatus by Fortey & Owens, 1987; see Owens, 1999), found only in the uppermost Arenig section at Llanfallteg but spanning the Arenig–Llanvirn boundary elsewhere (Zalasiewicz et al. 2009, fig. 4), and Acrograptus acutidens and U. austrodentatus, which range across the Arenig–Llanvirn boundary at Llanfallteg. A diverse trilobite fauna is also present, with Dionide levigena, the eponymous species of the uppermost Arenig trilobite biozone, although it also ranges into the Llanvirn Series, and Ectillaenus perovalis, Barrandia homfrayi, Stapeleyella inconstans, Amphyx linleyensis and P. cambriensis, all of which have ranges that span the Arenig–Llanvirn boundary at Llanfallteg (Fortey & Owens, 1987).

Trilobite biozones established by Fortey and Owens (1987) in the Whitland area comprise, in upwards succession, the Gymnostomix gibbsii Biozone, Stapeleyella abyfrons Biozone, Bergamia rushtoni Biozone and Dionide levigena Biozone. Gymnostomix gibbsii occurs in the Rhyd-Henllan and Whitland Abbey members of the Whitland area, meaning that the gibbsii Biozone coincides with most of the upper part of the Whitlandian Stage. The other three zones all occur in the Fennian Stage, the Stapeleyella abyfrons Biozone in the basal Pontyfenni Formation immediately overlying the Cwmfelin Boeth Formation, the Bergamia rushtoni Biozone through an estimated two-thirds of the Pontyfenni Formation, and the Dionide levigena Biozone in the upper Fennian part of the Llanfallteg Formation.

Molyneux (1987) described one Whitlandian acritarch assemblage, Assemblage V from the Whitland Abbey Member. The assemblage, like those from the Moridunian succession in the Carmarthen area, is of low diversity and comprises mainly small acanthomorph acritarchs (Micrhystridium spp.) that do not assist correlation. More diverse acritarch assemblages of Fennian age, with stratigraphically useful forms, were described from the Pontyfenni Formation (Molyneux, 1987). Assemblage VI, from just above the base of the formation, is distinguished by the presence of Coryphidium bohemicum Vavrdová, ?Frankea hamata Burmann, Orthosphaeridium sp., Stellechinatum uncinatum (Downie) Molyneux, ?Striatotheca mutua Burmann, S. rarirrugulata (Cramer et al.) Eisenack et al. and species of Uncinisphaera. Assemblage VII, from higher in the formation at Pont-y-Fenni, includes Coryphidium bohemicum Vavrdová, Dasydorus cirritus? Playford & Martin, Orthosphaeridium ternatum (Burmann) Eisenack et al., Stellechinatum papulessum Molyneux, and species of Solisphaeridium, Stelliferidium and Uncinisphaera. Chitinozoans assigned to species of Belonechitina, Conochitina and Lagenochitina were recorded from the Pontyfenni Formation at Pont-y-Fenni (Molyneux, 1987).

The Arenig Series is represented in the Arenig Fawr area by a single formation, previously referred to as the Carnedd Iago Formation, with unconformities at the base and top (Fig.e 2; Fortey et al.2000). The Carnedd Iago Formation is now considered to be equivalent to and has been superseded by the Allt Lⓦyd Formation throughout North Wales (Rushton & Howells, 1998), but the older terminology is retained here for better comparison with work reported in the literature.

The Carnedd Iago Formation was established by Lynas (1973) and described by Zalasiewicz (1984) as extending throughout the Arenig with three members: the Garth Grit Member, consisting of quartzo-feldspathic sandstone, the Llyfnant Member, consisting of laminated dark siltstone and pale sandstone, and the Henllan Ash Member, comprising variably bioturbated feldspathic sandstone and sandy mudstone. Fortey et al. (2000) revised the stratigraphy of the Carnedd Iago Formation, restricting it to the lower Moridunian to middle Whitlandian stages and including only the Garth Grit and the Henllan Ash as separate members.

The Llyfnant Member contains common Expansogratus aff. simulans, which suggests correlation with the E. simulans Biozone, the Isograptus victoriae victoriae Biozone, or possibly the Isograptus gibberulus Biozone of the English Lake District (Zalasiewicz, 1984; Zalasiewicz et al. 2009). Specimens of Corymbograptus aff. deflexus and Azygograptus cf. eivionicus have also been recorded. The ranges in England and Wales of the three nominal species, A. eivionicus, C. deflexus and E. simulans, overlap in the simulans Biozone, which is correlated with the lower Whitlandian Stage (Zalasiewicz et al. 2009).

Fortey and Owens (1978, 1987) suggested correlation of the Henllan Ash Member with the Bolahaul Member or the Pibwr Member, or both, based on its abundant trilobite fauna (Whittington, 1966), and therefore with the middle Moridunian Stage. Graptolites from the uppermost Henllan Ash Member were re-examined by Zalasiewicz (1984), who identified a fauna characterized by Expansograptus cf. praenuntius and Tetragraptus reclinatus. Fortey et al. (2000) placed the Henllan Ash in the upper Moridunian Stage, but Zalasiewicz et al. (2009) depicted E. cf. praenuntius as present in the simulans, victoriae and gibberulus graptolite biozones, the last with some doubt. This suggests a possible slightly younger, Whitlandian age than that indicated by Fortey et al. (2000), albeit still Floian.

Samples were collected from the Login beds, Carmarthen Formation and Afon Ffinnant Formation in the Carmarthen area (Fig. 3). No samples were collected from the Ogof Hên Formation.

Ten samples from the Login beds, originally collected by SGM (Molyneux & Dorning, 1989) and curated in the British Geological Survey’s collections at Keyworth, Nottingham, UK (BGS sample registration numbers MPA 26829 to MPA 26838), were resampled. The samples are from a section along Heol Login (‘Login road’), about 2 km SE of Carmarthen (British National Grid References SN 4352 1873–SN 4364 1870; ‘Login’ in Fig. 3; Molyneux & Dorning, 1989, figs 1, 2).

Allt Pen-y-Coed [SN 4425 1823–SN 4446 1803] (Fig. 3a) is a steep-sided, wooded stream section, oriented NW–SE, about 3 km SE of Carmarthen. It exposes the Pibwr and Cwmffrⓦd members of the Carmarthen Formation. The succession dips steeply to the south or SE so that the older beds are to the north. At the southern end of the section, the Cwmffrⓦd Member is unconformably overlain by upper Silurian (Pridoli) beds of Old Red Sandstone facies (https://mapapps2.bgs.ac.uk/geoindex/home.html, accessed 26 March 2020; see also Bedrock map of Wales and adjacent area in Howells, 2007).

Eight samples (CA 13-042 to CA 13-048 and TVDB 11-025) were collected from the Pibwr Member and ten samples (CA 13-006 to CA 13-013, CA 13-049, CA 13-050) from the Cwmffrⓦd Member, upstream from the point at which the stream passes under a bridge on a minor road (Fig. 3a).

The upper Cwmffrⓦd Member, the Cym yr Abbey Member and the lower Afon Ffinnant Formation are exposed in the stream section of Cwm yr Abbey [SN 5002 1988–SN 5013 1943], about 9 km east of Carmarthen (Fig. 3). The section is oriented approximately N–S with the succession generally dipping northwards, although there is much minor folding and faulting (Owens, 1999, fig. 8.7).

One sample (TVDB 11-009) was collected from the upper Cwmffrⓦd Member, seven samples (TVDB 11-004 to TVDB 11-008, CA 13-059, CA 13-060) from the Cwm yr Abbey Member, and three samples (TVDB 11-001 to TVDB 11-003) from the base of the Afon Ffinnant Formation, downstream from the bridge crossing the stream on the minor B4300 road [SN 5002 1978] (Fig. 3b).

Samples were collected from the Castelldraenog Member of the Colomendy Formation and from the Cwmfelin Boeth, Pontyfenni and Llanfallteg formations in the Whitland area (Fig. 4). No samples were taken from the Blaencediw Formation or the Rhyd-Henllan and Whitland Abbey members of the Colomendy Formation.

Four samples (CA 13-055 to CA 13-058; Fig. 4a) were collected from the Castelldraenog Member south of Castell Draenog [SN 2077 2139]. Other samples collected in the vicinity of Castell Draenog are CA 13-053, collected about 750 m SE of Castell Draenog in the vicinity of Pantygroes, about 50 m west of the minor road from Whitland Abbey to Llanboidy on the east side of Nant Colomendy, and CA 13-054, collected from the track approximately 260 m north of Castell Draenog. CA 13-053 is from beds mapped as Pontyfenni Formation by Fortey and Owens (1987, fig. 2), and CA 13-054 from beds now placed in the Abergwilli Formation of Abereiddian (early Llanvirn) age (British Geological Survey, 2010; Burt et al. 2012).

Six samples (CA 13-040, CA 13-041, CA 13-051, CA 13-052, TVDB 11-019, TVDB 11-020) were collected from the Cwmfelin Boeth Formation at Cwm Banau [SN 2123 1862] (Fig. 4), NE of Whitland Abbey.

Eight samples (CA 13-036 to CA 13-039, TVDB 11-021 to TVDB 11-024) were collected from the Pontyfenni Formation in a disused quarry at Pont-y-Fenni [SN 2379 1690–SN 2381 1693], the formation’s effective type locality, on the east bank of the Afon Fenni (Fig. 4b). The beds at Pont-y-Fenni comprise black to dark grey shale and blocky mudstone dipping northwards at about 60°, either on the northern limb of a fold that is subsidiary to an anticlinal area to the north, or on the overturned southern limb of the latter (Owens, 1999). The latter interpretation accords with the British Geological Survey’s (1975) map of the area and is adopted here. The southernmost sample, TVDB 11-021, is therefore placed highest in the succession.

The Llanfallteg section is along a disused and dismantled railway line, oriented NNE–SSW on the east bank of the Afon Taf (Fig. 4c). Beds at the northern end of the section are placed in the Pontyfenni Formation (locality 52Z of Fortey and Owens, 1987) but pass southwards into light grey mudstone and shale of the Llanfallteg Formation. The contact between the two formations is mapped as a fault north of locality 52Y of Fortey and Owens (1987, figs2, 8; British Geological Survey, 1976). The Arenig–Llanvirn series boundary is placed towards the southern end of the section, within the Llanfallteg Formation, and is marked by pendent didymograptid graptolites that include Didymograptus artus, the eponymous index of the lowest Llanvirn graptolite biozone.

Twenty-seven samples (CA 13-001–CA 13-005, CA 13-014–CA 13-035) were collected from the Llanfallteg section over a distance of about 530 m [SN 1592 2058–SN 1567 2012] (Fig. 4c). Two samples at the northern end of the section (CA 13-014, CA 13-015) are from the Pontyfenni Formation. Twenty samples are from the Llanfallteg Formation, the most northerly of which, CA 13-021 (Fig. 4c), is from around locality 52Y of Fortey & Owens (1987). Of these, 18 are from the Arenig part of the formation (CA 13-001–CA 13-003, CA 13-021–CA 13-035) and two from the Llanvirn (CA 13-004, CA 13-005). The remaining five samples (CA 13-016–CA 13-020) were collected between localities 52Y and 52Z of Fortey and Owens (1987) and are therefore from either the upper Pontyfenni Formation or lower Llanfallteg Formation, most likely the former.

Seven samples (TVDB 12-050 to TVDB 12-056; Fig. 5) were collected along a stream section southeast from Hafotty Ffilltirgerig [SH 8184 3857], corresponding to the Llyfnant and Henllan Ash members of Zalasiewicz (1984).

Ninety-three samples were prepared using standard palynological techniques. Between 40 and 60 g of rock were dissolved per sample. See Amberg et al. (2016) for the full procedure. The organic residues were sieved at 51 µm, and the top fraction was hand-picked under a stereomicroscope at ×50 magnification. More than 4400 specimens were obtained, and identifications are based on images taken with a FEI Scanning Electron Microscope (SEM) and a ZEISS LEO SEM. All figured material is stored and available for consultation in the collections of the UMR 8198 at the University of Lille. Ten genera and 44 species were identified from the sections and localities studied. Occurrences and ranges are shown in Figure 6.

R version 3.6.2 (R Core Team, 2019) was used for data analysis and visualization. Stratigraphically constrained hierarchical cluster analysis was carried out on a distance matrix using the ‘rioja’ package (Juggins, 2017) and the CONISS method. The distance matrix was computed from presence–absence data of species occurrences per sample using the Jaccard index for binary data in the ‘vegan’ package (Oksanen et al. 2019). Ranges, stratigraphic columns and the dendrogram were plotted using the ‘ggplot2’ and ‘gridExtra’ packages (Wickham, 2016; Auguié, 2017). A broken-stick model (Bennett, 1996) was applied to the cluster analysis to determine the significance of each cluster. Clusters identified as significant using this method form the basis of assemblages identified in the succession.

Preservation is variable between localities and sections, depending on the degree of metamorphism. Robinson and Bevins (1986, fig. 2) and Merriman (2006) delineated zones of incipient metamorphism in the Welsh Basin based on clay mineral assemblages and illite crystallinity, with the Carmarthen–Whitland area in the diagenetic zone to low anchizone and Arenig Fawr at a higher grade in the epizone. Intense small-scale folding affects the shale and mudstone units in the Carmarthen area (Fortey & Owens, 1978), and small-scale reverse faults occur in the Whitland area (Fortey & Owens, 1987), especially around Castelldraenog, where a NNE–SSW fault along Nant Colomendy pushes a block of the Colomendy Formation up in contact with the Pontyfenni Formation, and around Llanfallteg. The deformation results in generally only slight distortion of the macrofauna and mineralization (Fortey & Owens, 1987).

Abundance and species richness also vary between sections and within sections themselves. For this study, we consider a number of 1 to 50 specimens per sample of dissolved rock (about 40 g) to be low abundance, 51 to 150 specimens to be moderate abundance and over 150 specimens to be high abundance.

Five of the 10 samples from the Login beds yielded chitinozoans, mainly large lagenochitinids, belonging to between one and four species in a maximum of two genera. They are not abundant, but are relatively well preserved, although flattened.

Six of the 18 samples from the Carmarthen Formation at Allt Pen-y-Coed were unproductive, two from the Pibwr Member and four from the Cwmffrⓦd Member. The remainder, six each from the Pibwr and Cwmffrⓦd members, yielded abundant and diverse chitinozoans. Productive samples from the Pibwr Member consistently yielded five to six species belonging to two to four genera. Most of the productive samples from the Cwmffrⓦd Member were similar, with four or five species belonging to one or two genera, mostly conochitinids in the lower part, but desmochitinids higher in the succession. There is a marked increase in species richness in the uppermost sample from the Cwmffrⓦd Member, CA 13-006, however, with seven species of five genera. Both three-dimensional and flattened specimens were found, some being pyritized. Except for the largest specimens, chitinozoans are generally complete.

The samples from the upper Carmarthen Formation (Cwmffrⓦd and Cwm yr Abbey members) and lower Afon Ffinnant Formation in the Cwm yr Abbey section produced moderately abundant and diverse faunas dominated by small lagenochitinids and a few conochitinids. Three of the 11 samples were barren. The rest yielded between two and five species belonging to between one and five genera, the most diverse assemblages being from the Cwm yr Abbey Member. All specimens are relatively poorly preserved, however, the majority being flattened, although a few specimens of Conochitina are preserved in 3D.

The samples from the Castelldraenog Member at Castell Draenog and the Cwmfelin Boeth Formation at Cwm Banau were less productive. Five out of the 12 samples were barren and the rest yielded very sparse faunas, although large rock samples were dissolved (about 100 g). Most productive samples contained two or three species belonging to one or two genera. The exception is sample CA 13-051 from the Cwmfelin Boeth Formation, which yielded eight species of six genera.

The most diverse and abundant faunas are from the samples of the Pontyfenni Formation collected at Pont-y-Fenni. All eight samples yielded chitinozoans, and the specimens are well preserved, although often flattened. Yields are variable, from a minimum of three species in three genera, to a maximum for the area of 14 species belonging to seven genera in sample CA 13-038. Assemblages are dominated by large cyathochitinids with lagenochitinids and desmochitinids.

Abundance and species richness are variable throughout the Llanfallteg section. The six samples (CA 13-014 to CA 13-019) from the northern part of the section, and therefore definitely or probably from the Pontyfenni Formation, yielded assemblages of moderate abundance and diversity, with five to seven species and two to six genera. In contrast, the middle part of the section (11 samples, CA 13-020 to CA 13-030) is almost barren, despite some samples being re-processed twice to enhance recovery (totalling up to about 150 g of dissolved rock for these samples). Only one sample from this part of the section (CA 13-024) yielded chitinozoans, albeit moderately diverse with five species and four genera. Nine of the ten samples from the southern part of the section, however, including those from the lowermost Llanvirn Series, yielded assemblages of moderate abundance and diversity, with between three and eight species in two to six genera. The faunas from the Llanfallteg section are dominated by desmochitinids, belonechitinids and cyathochitinids. The specimens are often pyritized or broken, and both 3D and flattened specimens were found.

The samples from the Arenig Fawr section yielded the poorest assemblages with low abundance and diversity. Only two of the seven samples, both from the Henllan Ash, yielded chitinozoans and the specimens are severely broken. Three species belonging to three genera were determined, with most specimens being lagenochitinids.

The broken-stick model applied to the constrained hierarchical cluster dendrogram distinguished six significant chitinozoan assemblages, here numbered 1–6 from the base of the succession upwards (Fig. 6). Assemblages 2–5 are further subdivided, based on stratigraphic changes in the composition of their faunas. Correlation of the assemblages and sub-assemblages with global series and stages, Anglo-Welsh series and stages, and graptolite, chitinozoan and conodont biozones, is shown in Fig. 7.

Assemblage 1 is restricted to the Login beds and comprises Conochitina decipiens, Lagenochitina brevicollis (Fig. 8s, t), L. conifundus (Fig. 9, o), L. destombesi (Fig. 8a, b) and L. ovoidea (Fig. 9a, b). The species of Lagenochitina have only been recorded from the Login beds in South Wales, whereas C. decipiens ranges at least as high as the basal Pontyfenni Formation (Fig. 6).

The species of Lagenochitina all have Tremadocian records or affinities with Tremadocian species. Lagenochitina ovoidea, described from the Ordovician of the Sahara by Benoît and Taugourdeau (1961), has perhaps the longest range, with records from the Dapingian and Darriwilian stages of Baltica (Nõlvak et al. 2019) as well as the Tremadocian of South China (Liang et al. 2017). The other three Lagenochitina spp. are all zonal indicators for Tremadocian biozones in Gondwana.

Lagenochitina destombesi has its lowest occurrence in the middle Tremadocian of Morocco (Elaouad-Debbaj, 1988; Paris, 1990) and South China (Chen et al. 2008; Wang et al. 2013), and ranges into the upper Tremadocian Araneograptus murrayi graptolite Biozone in Morocco (Nowak et al. 2016), South China (Wang et al. 2013) and NW England (Amberg et al. 2017). It gives its name to the Lagenochitina destombesi chitinozoan Biozone of high palaeolatitude Gondwana (Fig. 7), stated in its original description as having a likely late early Tremadoc – early late Tremadoc age (Paris, 1990, p. 188).

The destombesi Biozone is succeeded in Paris’s (1990) Gondwanan scheme by the L. conifundus chitinozoan Biozone (previously known as the Amphorachitina conifundus Biozone). Lagenochitina conifundus was depicted as having an upper Tremadocian to lower Floian range by Paris (1990), but the lower Floian record was based on the occurrence of abundant specimens, recorded as Amphorachitina conifundus, in sample 2 of Paris and Mergl (1984; Paris, 1990) from the lowermost Klabava Formation of the Prague Basin. The locality was reported by Paris and Mergl (1984) to be from ‘the lowermost part of the Corymbograptus v. similis Zone; assemblage with Clonograptus’ and was correlated with the Tetragraptus approximatus Graptolite Zone. Fatka (1993) reported L. conifundus from the same locality (his sample KL-7) but pointed out that an absence of graptolites at the level of his sample precluded direct correlation with the graptolite zonation.

In revisions of the Gondwanan biozonation scheme, the conifundus Biozone has been replaced either wholly (Paris et al. 2007; Videt et al. 2010) or in part (Cooper & Sadler, 2012) by the brevicollis Biozone. Where it replaces the conifundus Biozone entirely, the brevicollis Biozone is correlated with the upper Tremadocian Araneograptus murrayi and Hunnegraptus copiosus graptolite biozones (Paris et al. 2007; Videt et al. 2010). In other instances (Cooper & Sadler, 2012, fig. 20.1), the brevicollis Biozone is correlated with the uppermost Tremadocian Hunnegraptus copiosus Biozone and the conifundus Biozone with the Araneograptus murrayi Biozone (Fig. 7). In both instances, the brevicollis Biozone is overlain by the symmetrica Biozone. Recent revision of the symmetrica Biozone has placed its base below instead of at the base of the Floian Stage (i.e. below the base of the Arenig Series; Nowak et al. 2016; Amberg et al. 2017; Liang et al. 2017; Achab & Maletz, 2021) so that the conifundus and brevicollis biozones now lie entirely within the upper Tremadocian Stage (Webby et al.2004; Paris et al. 2007; Videt et al. 2010; Cooper & Sadler, 2012).

The lowest occurrence of Lagenochitina brevicollis in South China is in the Araneograptus murrayi graptolite Biozone of the Jiangnan Slope (Wang et al. 2013) and the upper Tremadocian part of the Paroistodus proteus conodont Biozone on the Yangtze Platform (Liang et al. 2017). In both cases, correlation of the lowest occurrence with a level in the upper Tremadocian Stage Slice Tr3 is indicated (Fig. 7). The species’ highest occurrence there is in the Acrograptus filiformis graptolite Biozone of the Yangtze Platform (Liang et al. 2018), which correlates with a level in the middle Floian, either in the upper part of Stage Slice Fl1 (Zhang et al.2007, 2010; Wang et al. 2013) or in Fl2 (Zhang et al. 2010).

Lagenochitina brevicollis is possibly also present in the upper Tremadocian of South America (Fig. 7). De la Puente and Rubinstein (2009) recorded a single specimen from the Tremadocian A. murrayi graptolite Biozone of the Parcha Formation, NW Argentina. They also noted that the holotype of Lagenochitina brevicollis Taugourdeau and de Jekhowsky (1960) was likely to be a specimen of Lagenochitina with a broken neck whereas the paratype displayed characteristics of Desmochitina. Consequently, they referred to their specimen as Desmochitina sp. cf. L. brevicollis. Furthermore, de la Puente and Rubinstein (2009, 2013) regarded specimens recorded by Heuse et al. (1999) as Desmochitina sp. gr. minor from the upper Tremadocian A. murrayi and H. copiosus biozones of south Bolivia to be conspecific with the paratype of L. brevicollis and with their Desmochitina sp. cf. L. brevicollis.

The specimen of Desmochitina sp. cf. L. brevicollis from the Parcha Formation is accompanied by Euconochitina paschaensis, Lagenochitina conifundus and L. cf. longiformis. Wang et al. (2013) suggested that the last species might be equivalent to L. destombesi. Specimens referred to Lagenochitina cf. longiformis also occur in an assemblage from the Leetse Formation, in the proteus conodont Biozone of the Hunneberg Stage of Estonia, close to the Tremadocian–Floian stage boundary (Hints & Nõlvak, 2006). The specimens of L. cf. longiformis from the Leetse Formation are similar to the specimens of L. aff. destombesi from Assemblage 1 and are suggested here to be conspecific.

Conochitina decipiens has widespread lowest occurrences in the Floian Stage (Fig. 7), perhaps as low as the basal Floian approximatus graptolite Biozone in Bolivia (Heuse et al. 1999) and Bohemia (Paris & Mergl, 1984). Achab and Maletz (2021), however, recorded its lowest occurrence in Québec at a level in the symmetrica chitinozoan Biozone that is now placed in the highest Tremadocian. Amberg et al. (2017) recorded a similar form as Conochitina aff. decipiens from the murrayi graptolite Biozone in the Tremadocian of NW England (Fig. 7).

The co-occurrences of Conochitina decipiens and Lagenochitina destombesi suggest a latest Tremadocian, Tr3 age for Assemblage 1 (Figs 6, 7), confirming and perhaps further restricting the latest Tremadocian or earliest Floian age indicated for the same beds by acritarchs (Molyneux & Dorning, 1989; Molyneux et al. 2007).

Assemblage 2 occurs in Allt Pen-y-Coed, where it extends from sample CA 13-042 in the Pibwr Member to CA 13-013 in the lower part of the Cwmffrⓦd Member (Fig. 6). Species of Conochitina dominate the faunas. Conochitina decipiens and C. ordinaria are present throughout, and C. gueddichensis (Fig. 8h–j, l, u, v) and C. pseudocarinata (Fig. 8g) occur in all except the lowest sample. Conochitina raymondii (Fig. 8d) occurs more sporadically.

The assemblage is subdivided into a lower Assemblage 2a, comprising the fauna from the lowest sample of the Pibwr Member in Allt Pen-y-Coed (CA 13-042), and a higher Assemblage 2b comprising chitinozoans from the rest of the samples from the Pibwr Member and the lower Cwmffrⓦd Member in the section (Fig. 6). Neither subdivision contains index species of Gondwanan chitinozoan biozones, which precludes direct correlation and introduces some uncertainty regarding their ages.

Assemblage 2a is characterized by the association of Euconochitina fenxiangensis (Fig. 9j) with Conochitina decipiens, C. ordinaria, C. raymondii (Fig. 8d), Lagenochitina aff. cylindrica (Fig. 8k) and Rhabdochitina gracilis. All range into Assemblage 2b, although E. fenxiangensis and L. aff. cylindrica are restricted to its lower part.

Euconochitina fenxiangensis was described by Chen et al. (2008) from the upper Fenxiang and lower Honghuayuan formations on the Yangtze Platform of South China. Chen et al. (2008) indicated that it ranged from the upper Lagenochitina destombesi Biozone into the Euconochitina symmetrica Biozone and therefore, based on correlations accepted at the time, from upper Tremadocian into lower Floian strata. Liang et al. (2017), however, recalibrated the symmetrica Biozone on the Yangtze Platform, placing it entirely within the uppermost Tremadocian Stage (Fig. 7). This might be taken to indicate that Euconochitina fenxiangensis is restricted to the Tremadocian Stage in South China, but Chen et al. (2008) also noted its coexistence with conodonts of the Paroistodus proteus and Prioniodus elegans biozones, which supports its occurrence in the lower Floian Stage. Subsequent records, however, have been from the upper Tremadocian of South China (Wang et al. 2013; Liang et al.2017, 2018), including beds of the murrayi graptolite Biozone, and from the upper Tremadocian copiosus graptolite Biozone of NW Argentina (Toro et al. 2010; E. cf. fenxiangensis).

The species of Conochitina in Assemblage 2a all have lower Floian or upper Tremadocian lowest occurrences. That of Conochitina decipiens is noted under discussion of Assemblage 1 above. Conochitina raymondii was described by Achab (1980) from the Lévis Formation of Québec, where its lowest occurrence is in zone A of Raymond (1914) with Tetragraptus approximatus (Achab & Maletz, 2021, fig. 2). Chen et al. (2009) designated a Conochitina raymondii Biozone in the Yichang area of South China at a level that correlates with the Oepikodus communis conodont Biozone. This in turn equates with the lower Floian Stage Slice Fl1 (pre-evae Biozone; Wang et al.2005, 2009). Chen et al. (2009) further noted that the lowest occurrence of C. raymondii was in Floian strata older than their sampled level, indicating an earlier Floian age.

Conochitina ordinaria was also described by Achab (1980) from the Lévis Formation, Québec, but its lowest occurrence there is a little higher than that of C. raymondii, in Zone B of Raymond (1914) with the graptolites Phyllograptus typus, Tetragraptus quadribrachiatus and Dichograptus octobrachiatus. Raymond’s (1914) Zone B has been correlated with the Tshallograptus fruticosus graptolite Biozone (Achab & Maletz, 2021, fig. 2) in the middle Floian Stage. The lowest occurrence of Conochitina ordinaria in the Yichang area of South China is at about the base of the Lagenochitina lata Sub-biozone of the Clavachitina langei chitinozoan Biozone. Conodonts of the lower Oepikodus evae Biozone and graptolites of the Didymograptus [Didymograptellus] bifidus Biozone provide independent evidence for a middle to late Floian age (Chen et al. 2009; Fig. 7. See Toro & Herrera Sánchez (2019), for correlation of the D. bifidus Biozone with Stage Slice Fl3 and with the eobifidus, deflexus and probably the lower part of the suecicus biozones in South China).

Of the other two forms included in Assemblage 2a, Lagenochitina aff. cylindrica has affinities with a species that ranges in South China from the Euconochitina symmetrica chitinozoan Biozone (Liang et al. 2017) into the Sagenachitina dapingensis chitinozoan Biozone (Chen et al. 2009), respectively of latest Tremadocian and Dapingian age. Rhabdochitina gracilis is a long-ranging and widespread Ordovician species that has lowest occurrences in the upper Tremadocian of Morocco (Nowak et al. 2016) and close to the Tremadocian–Floian boundary in the Paraistodus proteus conodont Biozone and Hunneberg Regional Stage of Estonia (Hints & Nõlvak, 2006), but ranges into the Upper Ordovician (e.g. Grahn & Nõlvak, 2007).

The base of Assemblage 2b is marked by the lowest occurrences of Conochitina gueddichensis and C. pseudocarinata (Fig. 6). Desmochitina ovulum and Laufeldochitina sp. 1 (Fig. 8f, m, n, q, r, w) have lowest occurrences midway through the interval (Fig. 6). Paris (1981) described Conochitina pseudocarinata from the Armorican Massif, where it occurs in the same samples as Desmochitina ornensis and is one of the index species of his middle Arenig Desmochitina ornensis – Conochitina pseudocarinata Biozone. The latter biozone was not included in Paris’s (1990) scheme, but C. pseudocarinata is listed as one of the associated species of his Desmochitina ornensis Biozone (Fig. 7).

In South China, Conochitina pseudocarinata gives its name to the Conochitina pseudocarinata Biozone of Wang et al. (2005, 2009) and the Conochitina pseudocarinata Sub-biozone of Chen et al. (2009). In both instances, the base of the unit, defined by the lowest occurrence of C. pseudocarinata, is in the upper part of the Oepikidus evae conodont Biozone and the lower part of the Azygograptus suecicus graptolite Biozone. These correlations indicate that the lowest occurrence of C. pseudocarinata in South China is in the upper part of the Floian Stage (Stage Slice Fl3; Fig. 7). Zhang et al. (2010, Fig. 2) correlated the base of the suecicus Biozone with the base of the Australian Castlemainian Stage, which in turn lies in the upper part of Stage Slice Fl3 (Bergström et al.2009; Cooper & Sadler, 2012, fig. 20.9). Records of the species from Belgium (Samuelsson & Verniers, 2000; Herbosch & Verniers, 2014) and NW France (Paris, 1981, 1990) are also from the ‘middle’ Arenig or higher.

Conochitina gueddichensis and Desmochitina ovulum support a later Floian age for Assemblage 2b. Conochitina gueddichensis was described by Oulebsir and Paris (1993) from the Eremochitina brevis chitinozoan Biozone of Algeria, which is correlated with the upper Floian Stage, equivalent to the upper part of Stage Slice Fl2 and most of Fl3 (Cooper & Sadler, 2012, figs 20.1, 20.9) (Fig. 7). Desmochitina ovulum is generally found in deposits of Darriwilian and younger age (Paris, 1981; Nõlvak & Grahn, 1993; Oulebsir & Paris, 1995; Tammekand et al. 2010; Nõlvak et al. 2019), but was recorded by Liang et al. (2018) from the lower Azygograptus suecicus graptolite Biozone in the upper Floian Stage (Stage Slice Fl3) of South China.

It seems unlikely that Assemblage 2b is much older than Fl3 given the First Appearance Datums (FADs) of Conochitina gueddichensis, C. pseudocarinata and Desmochitina ovulum (Fig. 7). The possibility then is that Assemblage 2a is not much older than Assemblage 2b, either Fl3 or possibly Fl2. This suggestion is supported by the continuity of lithostratigraphy and lithofacies between assemblages 2a and 2b and records of the graptolites Phyllograptus cf. densus and Pseudophyllograptus aff. angustifolius from lower in the Pibwr Member at Glan Pibwr (Fortey & Owens, 1978; Owens, 1999). Pseudophyllograptus angustifolius has been reported in England and Wales from the middle Floian jacksoni graptolite Biozone to the middle Darriwilian artus graptolite Biozone, and Phyllograptus densus only from the Dapingian victoriae graptolite Biozone (Zalasiewicz et al. 2009). Webby et al. (2004) correlated the Phyllograptus densus and Pseudophyllograptus angustifolius elongatus biozones of Baltoscandia with their Time Slice 2c, which correlates in turn with the upper Floian Stage. Against this, the occurrence of Euconochitina fenxiangensis in Assemblage 2a and its co-occurrence with C. pseudocarinata and C. gueddichensis in the lower part of Assemblage 2b introduces some uncertainty, given that E. fenxiangensis has not been recorded with confidence from such a high level in the Floian Stage. Nevertheless, a middle to late Floian, Fl2–Fl3 age is suggested here for Assemblage 2a, albeit with a degree of uncertainty, and an Fl3 age for Assemblage 2b (Figs 6, 7). The suggestion of a middle to late Floian age for Assemblage 2a implies the possibility of significant hiatuses lower in the succession. Either the Ogof Hên Formation covers a significant amount of time (Fl1–Fl2?), or there are breaks at the base, top and/or within that formation.

Assemblage 3 covers much of the middle Arenig succession in South Wales, from the upper Moridunian Stage to the basal Fennian Stage, and is subdivided into assemblages 3a, 3b and 3c by successive lowest occurrences of chitinozoan species (Fig. 6). Assemblages 3a and 3b are found in the Carmarthen area and Assemblage 3c in the Whitland area.

Assemblage 3a is restricted to four samples from the upper part of the Cwmffrⓦd Member in Allt Pen-y-Coed and Cwm yr Abbey (Fig. 6). It is characterized by a change from conochitinid to desmochitinid-dominated faunas, with the lowest occurrences of Desmochitina elongata (Fig. 9r), D. minor (Fig. 9c, m) and D. papilla (Fig. 9e) at its base. Desmochitina ovulum ranges up from Assemblage 2b and is relatively common in the lowest two samples. Conochitina decipiens, C. ordinaria, C. raymondii and Laufeldochitina sp. 1 also range up into Assemblage 3a, but the species of Conochitina only occur in the highest two samples and are the only forms recorded in those samples.

The records of Desmochitina ovulum documented above indicate an Fl3 age or younger for Assemblage 3a. There is nothing in the assemblage to limit it to the upper Floian, but it is most likely to correlate with the Fl3 Stage Slice (Figs 6, 7) given the ages suggested below for assemblages higher in the South Wales succession. Of the other desmochitinids in the assemblage, records of D. elongata are from the Darriwilian and Sandbian stages (Nõlvak & Grahn, 1993; Tammekand et al. 2010; Nõlvak et al. 2019), and Desmochitina papilla was described by Grahn (1984) from Estonia where its lowest occurrence is at the base of the Vaana Substage in the middle of the regional Volkhov Stage. This in turn suggests correlation with a level in the lower Dapingian Stage (Cooper & Sadler, 2012, fig. 20.9). In South China, the lowest occurrence of D. papilla is in the Didymograptellus bifidus graptolite Biozone and the lower part of the Oepikidus evae conodont Biozone (Wang et al. 2005), and therefore in the upper Floian Stage (Cooper & Sadler, 2012).

Assemblage 3b covers the rest of the sampled succession in the Carmarthen district, from the upper Cwmffrⓦd Member into the basal Afon Ffinnant Formation in Allt Pen-y-Coed and Cwm yr Abbey (Fig. 6). Its base is marked by the lowest occurrences of Belonechitina micracantha, Lagenochitina obeligis (Fig. 9s, t), Laufeldochitina protolardeuxi (Fig. 8p) and Rhabdochitina magna (Fig. 10l). Conochitina decipiens (Fig. 10g, h), C. ordinaria, Desmochitina ovulum, Laufeldochitina sp. 1 and Rhabdochitina gracilis are also present in the lower part of the assemblage, some ranging through. Short forms of Lagenochitina esthonica (Fig. 9f, g) are consistently present at the top of the succession in samples collected across the contact of the Carmarthen and Afon Ffinnant formations.

Of the four species with lowest occurrences at the base of Assemblage 3b, Lagenochitina obeligis occurs in all samples and is particularly common in those from the upper part of the Cwm yr Abbey Member and the Afon Ffinnant Formation in Cwm yr Abbey. It is a characteristic species of Assemblage 3b but is nevertheless long-ranging. It was described by Paris (1981) from Brittany and depicted by Paris (1990) as ranging from the middle Arenig Eremochitina brevis Biozone into the Llanvirn Series in southern Gondwanan terranes. Other records are from the Tremadocian Stage of South China (Wang et al. 2013; Liang et al.2017, 2018) and the Darriwilian Stage in South America (Grahn 2006), Belgium (Herbosch & Verniers, 2014) and Oman (Sansom et al. 2009; Heward et al. 2018), with similar forms (L. cf. obeligis) recorded from the upper Tremadocian of Morocco (Nowak et al. 2016) and NW England (Amberg et al. 2017) and the Darriwilian Stage in Iran (Ghavidel-syooki et al. 2014).

Laufeldochitina protolardeuxi, only recorded from the lowest sample of Assemblage 3b, was originally described from the middle Arenig of Morocco (Soufiane & Achab, 1993), and Rhabdochitina magna was reported as an associated species of the lower–middle Arenig Eremochitina baculata Biozone (Fl1–Fl2; Fig. 7) of Gondwana (Paris, 1990). Other records of R. magna, however, are from the Dapingian (Liang et al. 2018), Darriwilian (Jenkins, 1967; Grahn et al. 1996; Rickards et al. 2010; Tammekand et al. 2010; Wang et al. 2018; Nõlvak et al. 2019) and higher stages (Paris, 1990; Oulebsir & Paris, 1995; Vandenbroucke, 2008 a, b). Similar forms have been reported as Rhabdochitina cf. magna from the middle Floian of Argentina (de la Puente & Rubinstein, 2013) and the Tremadocian of Morocco (Nowak et al. 2016).

Other species from Assemblage 3b are long-ranging, but Belonechitina micracantha corroborates the evidence of Desmochitina ovulum to indicate that this part of the succession is not older than late Floian. Belonechitina micracantha is only present in the lowest sample from Assemblage 3b but reoccurs higher in the South Wales succession. Elsewhere, its lowest occurrence is in the lower Azygograptus suecicus graptolite Biozone of the upper Floian Stage (upper Fl3) in South China (Wang, X. et al. 2005; Liang et al. 2018; Fig. 7). As with Assemblage 3a, there is nothing to limit the assemblage to the late Floian, but the base of the Dapingian Stage is placed at a higher level in the Whitland area. Assemblage 3b is correlated accordingly with the Fl3 Stage Slice (Figs 6, 7).

Assemblage 3c comprises chitinozoans from the Castelldraenog Member of the Colomendy Formation, the Cwmfelin Boeth Formation and the basal Pontyfenni Formation in the Whitland area, and therefore spans the Whitlandian–Fennian stage boundary (Fig. 6). The lowest sample, from the Castelldraenog Member, yielded only three species, Conochitina cucumis (Fig. 10k), Conochitina decipiens and Lagenochitina obeligis, and the base of the assemblage is marked by the lowest occurrence of C. cucumis.

Conochitina cucumis is the eponymous species of the cucumis Biozone of Baltoscandia (Nõlvak & Grahn, 1993). The base of the biozone was originally placed in the upper Volkhov Stage of Baltoscandia, at a level that correlates approximately with the basal Darriwilian Stage (Nõlvak & Grahn, 1993; Webby et al. 2004). Nõlvak et al. (2019), however, repositioned its base to a lower level, within the Dapingian Stage. Furthermore, a form from the lower unit of the Dawan Formation in the Huanghuachang section of South China, designated as Conochitina cf. cucumis by Chen et al. (2009), has its lowest occurrence in the upper Floian, Stage Slice Fl3, in the lower Azygograptus suecicus graptolite Biozone and upper Oepikodus evae conodont Biozone (cf. sample positions in Wang et al. 2005, fig. 8; 2009, fig. 5).

Assemblage 3c is a low-diversity assemblage. Conochitina decipiens occurs in all samples but was not recorded from any of the overlying assemblages. Belonechitina micracantha, Desmochitina ovulum, Rhabdochitina magna and Lagenochitina obeligis range through Assemblage 3c, occurring in one or more samples, but the most diverse microfauna with all five of these species plus Bursachitina laminaris, relatively common Conochitina cucumis and Desmochitina minor is from the Cwmfelin Boeth Formation (sample CA 13-051). The lowest occurrence of Bursachitina laminaris (Fig. 9p, q) is at this level.

Bursachitina laminaris was described by Tang et al. (2007) from South China and is shown as occurring in only one sample from the lower Darriwilian Stage on their range charts (austrodentatus Biozone, Dianbatou section: Tang et al. 2007, fig. 5). It was reported in the text of their paper, however, as occurring in the ‘3rd Stage’ (i.e. Dapingian) and lower Darriwilian Stage, and as ranging from the Azygograptus suecicus graptolite Biozone to the Undulograptus austrodentatus graptolite Biozone. As the suecicus Biozone spans the Floian–Dapingian boundary in South China (Zhang et al.2007, 2010), it follows that the lowest occurrence of Bursachitina laminaris there is likely to be close to the base of the Dapingian Stage (Fig. 7).

Based on their previous records, the occurrences of Bursachitina laminaris and Conochitina cucumis in Assemblage 3c are taken here to indicate a level close to the Floian–Dapingian stage boundary. Possible positions for the base of the Dapingian Stage are at (1) the lowest occurrence of C. cucumis and therefore correlating with a level in the Whitlandian Stage (Fig. 6); (2) the lowest occurrence of B. laminaris and therefore within the lowest Fennian Stage; or (3) within the assemblage, somewhat arbitrarily at a level that coincides with the base of the Fennian Stage. There is no conclusive evidence to favour one of these options over the others, but the lowest occurrence of C. cucumis and the base of Assemblage 3c is adopted here. Assemblage 3c is thus provisionally interpreted as being early Dapingian in age (Figs 6, 7). This position also suggests correlation of Assemblage 3c, at least in part, with the Gondwanan Desmochitina ornensis Biozone (Fig. 7), following the correlations depicted by Webby et al. (2004) and Cooper and Sadler (2012), although the index species is not present at this level in South Wales.

There is a significant change in the upper part of the succession that distinguishes assemblages 4–6 from assemblages 2 to 3. It coincides with division of the dendrogram into two high-level clusters (Fig. 6), the change taking place within the Pontyfenni Formation. Chitinozoan faunas from assemblages 4–6 contain Gondwanan index species, and these are accompanied by graptolites to aid correlation. The dating and the correlation of assemblages 4–6 are consequently more secure.

Assemblage 4 corresponds to the chitinozoan fauna from the Pontyfenni Formation at Pont-y-Fenni. Its base is marked by the lowest occurrence of Cyathochitina cf. cycnea (Fig. 8e) Vandenbroucke et al. (2013), which ranges through but is restricted to the assemblage. It also contains Desmochitina ornensis? (Fig. 9l) and Belonechitina henryi (Fig. 10c, d), both of which are Gondwanan index species, and is subdivided into assemblages 4a and 4b at the lowest occurrence of the latter. Cyathochitina cycnea is a replacement name in Vandenbroucke et al. (2013) for Cyathochitina giraffa Hennissen et al. (2010), described from the Darriwilian–Sandbian section at Dawangou in the Tarim Basin of NW China, but a junior homonym of Cyathochitina giraffa Grahn and Nõlvak (2010) from the Upper Ordovician (Sandbian) of Sweden.

Assemblage 4a is based on the two samples considered to be the lowest from the section at Pont-y-Fenni (CA 13-036, CA 13-037). Sample CA 13-036 yielded only Bursachitina laminaris, Cyathochitina cf. cycnea and Desmochitina ovulum (Fig. 9d). The second sample also has Bursachitina laminaris, accompanied by Conochitina cf. havliceki (Fig. 10i, j), Desmochitina erinacea? (Fig. 9k), D. ornensis? (Fig. 9l), relatively common D. aff. cocca (Fig. 9n) and Rhabdochitina magna.

The characteristic rugged, scaly wall of D. ornensis was rarely observed on specimens from South Wales, hence the question over the identification. Nevertheless, the specimens from South Wales are similar to D. ornensis in size and morphology, and the difference in wall ornamentation could be the effect of alteration. Despite the uncertain identification, the occurrence of D. ornensis? suggests correlation of Assemblage 4a with the Gondwanan Desmochitina ornensis Biozone (Paris, 1990; Fig. 7). This biozone is correlated in turn with the highest Floian and lower Dapingian stages in some schemes (Webby et al. 2004); Cooper & Sadler, 2012), although Videt et al. (2010) showed a higher correlation with stage slices Dp2 and Dp3. Interpretation of the age of Assemblage 3c as early Dapingian places Assemblage 4a within the Dapingian Stage, and in Stage Slice Dp1 following the correlations of Cooper & Sadler (2012) and Webby et al. (2004), or possibly Dp2 after Videt et al. (2010).

Associated species from the ornensis Biozone of Gondwana are Conochitina pseudocarinata, Lagenochitina obeligis, Sagenachitina oblonga, Tanuchitina achabae and Velatachitina veligera. None of these occur in Assemblage 4a, although C. pseudocarinata, L. obeligis and T. achabae are all present at other levels in South Wales.

Of the other chitinozoan species present in Assemblage 4a, Bursachitina laminaris and Rhabdochitina magna range through. Conochitina havliceki was described by Paris & Mergl (1984) from the upper Arenig Tetragraptus cf. pseudobigsbyi graptolite Biozone of the Prague Basin, Bohemia. Specimens from South Wales do not have a foveolated surface, as originally described by Paris and Mergl (1984) and are here designated C. cf. havliceki. Specimens similarly designated as Conochitina cf. havliceki from South America, but not necessarily the same as those from South Wales, range from the lower Floian Tetragraptus phyllograptoides graptolite Biozone (Heuse et al. 1999) into the lower–middle Floian Conochitina decipiens Interval Zone of Grahn (2006) and possibly into the lower Darriwilian dentatus graptolite Biozone (Grahn, 2006, fig. 2). Desmochitina erinacea has Darriwilian and Sandbian records from Baltoscandia (Nõlvak & Grahn, 1993; Tammekand et al. 2010; Nõlvak et al. 2019) and South China (Tang et al. 2007). The other species of Desmochitina are long-ranging, from at least the Floian Stage to the Darriwilian Stage, and higher in the case of D. cocca (Paris et al. 2007; Rickards et al. 2010; Tammekand et al. 2010; Liang et al. 2018).

Assemblage 4b is defined by the total range of Belonechitina henryi and is the most diverse chitinozoan assemblage recorded from the Arenig Series of South Wales. In South China, the base of the Dapingian Stage was correlated with the base of the Belonechitina cf. henryi Biozone by Wang et al. (2009), but the FAD of the species is usually taken to be above the base of the stage. The Belonechitina henryi chitinozoan Biozone of Gondwana (Paris, 1990) thus succeeds the ornensis Biozone in the Dapingian Stage (Fig. 7), but there are differences of correlation between different schemes. The base of the henryi Biozone was placed in Stage Slice Dp1 by Cooper and Sadler (2012) and its top in Stage Slice Dp3. In contrast, Videt et al. (2010) placed the base of the henryi Biozone in Stage Slice Dp3 and its top in the lower Darriwilian Dw1 Stage Slice.

Other species restricted to Assemblage 4b include Conochitina redouanei (Fig. 10a, b), Cyathochitina aff. calix (Fig. 8o), Cy. touggourtensis (Fig. 10e), Lagenochitina lata, L. maxima (Fig. 10m), L. pirum (Fig. 8c) and Tanuchitina aff. granbyensis (Fig. 10n, r). Cyathochitina calix and Lagenochitina pirum have lowest occurrences in the Dapingian Stage elsewhere. Cyathochitina calix, for example, has lowest occurrences in the Dapingian Stage of Baltoscandia and South China (Grahn, 1984; Liang et al. 2018; Nõlvak et al. 2019) and has been recorded from the Darriwilian Stage of Baltoscandia, Avalonia and South China (Jenkins, 1967; Grahn, 1984; Nõlvak & Grahn, 1993; Grahn & Nõlvak, 2007; Tammekand et al. 2010; Herbosch & Verniers, 2014; Liang et al. 2018; Nõlvak et al. 2019). It is the index species for the early Darriwilian Cyathochitina calix Biozone (Paris, 1990; Webby et al. 2004; Cooper & Sadler, 2012). Lagenochitina pirum has its lowest occurrence in the upper Dapingian Stage of South China (Brocke et al. 2000; Chen et al. 2009), and has been recorded from the Darriwilian Stage of South China (Chen et al. 2009), Qaidam (Wang et al. 2018, including forms recorded as ‘cf.’), Australia (Quintavalle & Playford, 2006) and eastern Laurentia (Achab, 1989). In the last of these, Lagenochitina pirum is the index species of the Darriwilian C. pirum Biozone (Achab, 1989). L. maxima is very similar to Conochitina ulsti sp. nov. described by Nõlvak et al. (2022) from the Kunda Stage (lower Darriwillian) in central Latvia.

The other forms restricted to Assemblage 4b have so far only been recorded lower or higher in the Ordovician succession. Conochitina redouanei and Cyathochitina touggourtensis were both described by Oulebsir and Paris (1993) from the Eremochitina brevis Biozone (upper Floian) of Algeria; Lagenochitina lata has been recorded from the Tremadocian of Baltoscandia (Liang et al. 2017; Nõlvak et al. 2019), and the Tremadocian and Floian of South China (Wang et al. 2005; Chen et al. 2009; Liang et al. 2018); and Tanuchitina granbyensis has been described and recorded from the Darriwilian Stage of Sweden (Grahn et al. 1996) and Latvia (Nõlvak et al. 2021). There are no complete specimens of T. granbyensis in the Welsh assemblage, but the largest specimen measures more than 600 µm and the specimens have the typical ovoid apex with a carina that corresponds to the description given by Grahn et al. (1996). Conochitina cucumis and Tanuchitina granbyensis highlight increasing affinities between the Welsh and Baltoscandian assemblages during the Dapingian and early Darriwilian, in addition to Cyathochitina calix, although the latter was not found in association with the others in central Latvia (Nõlvak et al. 2021).

Belonechitina micracantha, Desmochitina aff. cocca, D. minor, D. ovulum, Lagenochitina obeligis, Rhabdochitina gracilis and R. magna all range through the assemblage. Bursachitina laminaris, Conochitina cf. havliceki, Cyathochitina cf. cycnea, Desmochitina erinacea? and D. ornensis? range into and have their local highest occurrences in Assemblage 4b.

The relatively common occurrence of Belonechitina henryi throughout the Pont-y-Fenni section is taken to indicate correlation of Assemblage 4b with the henryi Biozone and therefore with a level that is probably in the upper part of the Dapingian Stage, perhaps Dp2–Dp3, or the basal Darriwilian Stage (Dw1). This correlation is corroborated and perhaps further restricted by graptolites from the Pontyfenni Formation at Pont-y-Fenni, which are reported to indicate the Undulograptus sinicus Subzone of the basal Darriwilian U. austrodentatus Biozone (Owens, 1999).

Assemblage 5 straddles the Pontyfenni–Llanfallteg formation boundary in the lower part of the Llanfallteg section and comprises samples CA 13-014 to CA 13-019 (Fig. 6). Its base is marked by the lowest occurrence of Desmochitina aff. bulla (Fig. 9h, i), which is present in all samples and relatively common in three (CA 13-016 to CA 13-018). The lowest occurrences of Cyathochitina protocalix? (Fig. 10f, q, s) and Tanuchitina achabae? (Fig. 10o, p) in sample CA 13-017, midway through Assemblage 5 and low in the Llanfallteg Formation, are used to subdivide the assemblage into 5a and 5b.

Assemblage 5a is a low-diversity assemblage in the lower three samples (CA 13-014–CA 13-016). It consists mainly of Desmochitina spp., with D. aff. cocca, D. minor and D. ovulum in addition to D. aff. bulla. Belonechitina micracantha is the only other species present. All occur in all three samples.

Desmochitina bulla is the index species of the bulla Biozone (Paris, 1990), which succeeds the henryi Biozone in the Gondwanan Arenig biozonal scheme and is placed in the lower Darriwilian Stage Slice Dw1 (Videt et al. 2010; Cooper & Sadler, 2012; Fig. 7). Desmochitina aff. bulla differs from D. bulla in having a wider opening and a less sub-spherical shape. Nevertheless, correlation of Assemblage 5a with Stage Slice Dw1 is consistent with previous correlations of the highest part of the Anglo-Welsh Arenig Series with the lower Darriwilian Stage.

Cyathochitina protocalix was described by Paris (1981) from the Armorican Massif, where it was shown as ranging across the Arenig–Llanvirn series boundary and was described as a good marker for the boundary interval between the two series. It is the marker species for the protocalix Biozone of Paris (1990), which was regarded as the lowest Llanvirn biozone. More recently published schemes, however, have placed the protocalix Biozone in the upper Arenig (Webby et al. 2004) or have shown it spanning the Arenig–Llanvirn series boundary (Videt et al. 2010; Cooper & Sadler, 2012). As the base of the Llanvirn Series is placed higher in the succession, a Dw1 age is indicated here for Assemblage 5b.

Cyathochitina protocalix was not positively identified in Assemblage 5b due to preservation, but specimens referred to as Cyathochitina protocalix? nevertheless display the restriction just above the base of the vesicle that is typical of the species (Fig. 10f, q, s). Tang et al. (2007) described a similar form from the lower Darriwilian (Dw1) of South China, but as there were only three poorly preserved specimens and they were significantly smaller than those from the Armorican Massif (France), they were kept in open nomenclature as Hyalochitina cf. protocalix.

Tanuchitina achabae was described by Paris (1981) from the base of the Pissot Formation in the Armorican Massif as an accessory species of the lower Dapingian Desmochitina ornensis Biozone, to which it was restricted (Paris, 1990, fig. 3). Higher in the Pissot Formation, however, Paris (1981) described a second form as Tanuchitina sp. aff. achabae, with a longer carina and ranging from the Belonechitina henryi Biozone into the Cyathochitina protocalix Biozone. As no entire specimen was found in the Welsh material, we cannot determine which form is the closest.

Assemblage 6 was identified as one of the significant clusters on the dendrogram (Fig. 6), but its base is not marked by any lowest occurrences. It comprises ten samples in the upper part of the Llanfallteg Formation, from CA 13-024 at the base to CA 13-005 at the top. Most species range up from underlying assemblages. Belonechitina micracantha, Cyathochitina protocalix?, Desmochitina aff. cocca, Rhabdochitina gracilis and R. magna range through all or most of the assemblage; Conochitina primitiva (Fig. 9u), Cyathochitina campanulaeformis, Desmochitina aff. bulla and Tanuchitina achabae? have highest occurrences in the lower part of the assemblage; and Lagenochitina obeligis returns in sample CA 13-001, above its absence from Assemblage 5. The only lowest occurrence within Assemblage 6 is that of Tanuchitina domfrontensis (Fig. 10t, u), which is present in samples CA 13-033 and CA 13-035 in the lower half of the assemblage and again in the top two samples (CA 13-004, CA 13-005) where it is relatively common.

Tanuchitina domfrontensis was described by Paris (1981) from the Pissot Formation of the Armorican Massif, where its lowest occurrence is in the protocalix Biozone at a level that coincides with the lowest occurrence of pendant graptoloids (Paris, 1981, fig. 7). This led Paris (1981) to observe that the FAD of Tanuchitina domfrontensis might provide a useful criterion for the position of the Arenig–Llanvirn boundary in other successions, notably in the Cacemes Group of Portugal, from which it was also recorded. The successive lowest occurrences of Cyathochitina protocalix and Tanuchitina domfrontensis in the Pissot Formation further led Paris (1981) to propose a subdivision of the Cyathochitina protocalix Biozone into a lower subzone of Rhabdochitina ?gracilis – Cyathochitina protocalix and an upper subzone of Cyathochitina protocalix – Tanuchitina domfrontensis.

Tanuchitina domfrontensis is more bulbous than T. achabae and smaller. Very few complete specimens were recovered from Wales, but based on measurement and morphology, both T. achabae? and T. domfrontensis are present in Assemblage 6, albeit not continuously and their ranges do not overlap. Tanuchitina achabae? occurs in one sample at the base of Assemblage 6 (Fig. 6). Tanuchitina domfrontensis occurs higher in the Llanfallteg Formation, from a few metres below the Arenig–Llanvirn boundary as defined by Fortey and Owens (1987) on graptolite evidence. Consequently, either the base of the Llanvirn Series is slightly lower than previously located in the section, or the range of T. domfrontensis straddles the Arenig–Llanvirn boundary rather than its FAD pinpointing the boundary. Either way, Assemblage 6 spans the boundary between the Arenig and Llanvirn series and between stage slices Dw1 and Dw2 (Figs 6, 7).

Sample CA 13-054 was collected north of Castell Draenog [c. SN 208 217] at a locality mapped as the Abergwilli Formation of Abereiddian (early Llanvirn) age (British Geological Survey, 2010; Burt et al. 2012). It yielded Bursachitina laminaris, Conochitina decipiens, Desmochitina ornensis?, D. aff. cocca, D. minor and Lagenochitina obeligis. Other samples in which Bursachitina laminaris and Desmochitina ornensis? occur together are from the top of Assemblage 4a and the base of Assemblage 4b in the Pontyfenni Formation (Fig. 6). Most of the other species range through that interval. If the beds at the locality are from the Llanvirn Series, it implies a recurrence at a higher stratigraphic level of a chitinozoan assemblage associated with the Pontyfenni Formation. An alternative explanation is that the sample is from an outcrop of the Pontyfenni Formation north of and overlying or in faulted contact with the Whitland Shale Member (see Fortey & Owens, 1987, Fig. 2).

Three species were identified from the two productive samples of the Henllan Ash Member: Conochitina decipiens, Lagenochitina obeligis (Fig. 9s) and Rhabdochitina magna from sample TVDB 12-052, and L. obeligis and R. magna from sample TVDB 12-054. All three species range through long intervals in the South Wales succession, but their ranges overlap in assemblages 3b and 3c (Fig. 6). Applying the same ranges to the sections around Arenig Fawr would indicate a late Moridunian to early Fennian age and correlation with the late Floian Stage Slice Fl3 or early Dapingian Stage Slice Dp1. This is consistent with the late Moridunian age shown for the Henllan Ash by Fortey et al. (2000), or with the Whitlandian age supported by the graptolite fauna (Zalasiewicz, 1984, 1986; Zalasiewicz et al. 2009). But although they support previous interpretations of the age of the Henllan Ash, all three species have longer stratigraphic ranges elsewhere so their evidence must be considered permissive rather than conclusive.

The chitinozoans recorded from Arenig sections in the Carmarthen and Whitland areas of South Wales indicate correlation with stage slices from the upper Tremadocian through the Floian and Dapingian stages to the middle Darriwilian Stage, but with a degree of certainty that varies through the succession. Assemblage 1 is certainly close to the Tremadocian–Floian boundary and probably late Tremadocian (Stage Slice Tr3). The age and correlation of Assemblage 2a has the greatest uncertainty, but it is suggested to correlate with the middle to upper Floian Stage (Fl2–Fl3). There is more certainty over correlation of assemblages 2b, 3a, 3b and 3c. The occurrences of Conochitina gueddichensis, Conochitina pseudocarinata and Desmochitina ovulum in Assemblage 2b all suggest that the assemblage is not older than the late Floian Fl3 Stage Slice. Correlation of the overlying assemblages is constrained by their observed or, in the case of Assemblage 3c, inferred superpositional relationships and by the occurrence of species that support a late Floian or younger age. The certainty of correlation increases upwards, with that of assemblages 4, 5 and 6 and their sub-assemblages supported by the presence of index species, or related forms, and by the occurrence of graptolites to corroborate the chitinozoan evidence, particularly in the Pontyfenni and Llanfallteg formations.

Chitinozoan zonal schemes covering the same stratigraphic interval (Fig. 7) have been developed for South Gondwana (Paris, 1990), based on successions in North Africa, SW and Central Europe, with a separate scheme for the upper Tremadocian of Morocco published by Nowak et al. (2016); South America (Grahn, 2006), with a separate scheme for NW Argentina (de la Puente & Rubinstein, 2013); Baltoscandia (Nõlvak & Grahn, 1993; Grahn et al. 1996), Estonia (Nõlvak et al. 2019) and Sweden (Grahn & Nõlvak, 2007); South China (X Wang et al.2005, 2009; Chen et al. 2009; W Wang et al. 2013; Liang et al.2017); and Laurentia (Achab, 1989). Each of these regions and zonal schemes has species in common with South Wales, but the regions with the highest numbers in common are South China, South Gondwana and Baltica. South China has the most in common overall. For assemblages 2 and 3, 16 of the 19 named species from South Wales (84 %) are also known from South China, 10 (53 %) are known from South Gondwana, and another 10 are known from Baltica. For assemblages 4–6, 18 of the 24 named species (75 %) are known from South China, 18 from South Gondwana and 13 (54 %) from Baltica.

The lack of Gondwanan index species in assemblages 2 and 3 is notable. The absence of Conochitina symmetrica and Eremochitina baculata, the index fossils of the lowest two biozones in the Floian Stage of Gondwana, might be explained by an absence of strata or a lack of samples (Figs 6, 7), but this does not explain the absence of Eremochitina brevis. This species is the eponymous index of Paris’s (1990) upper Floian brevis Biozone with which assemblages 2 and 3 are mainly correlated (Fig. 7). Furthermore, Eremochitina brevis has been widely reported from North Africa (Paris, 1990; Oulebsir & Paris, 1995; Videt et al.2010; Nowak et al. 2016), South China (Liang et al.2017, 2018) and NW Argentina (de la Puente & Rubinstein, 2013). Avalonia rifted away from Gondwana at around the beginning of the Ordovician Period (Domeier, 2016), but there is no indication of increased faunal provincialism following rifting to explain the absence of the species from South Wales. Moreover, Avalonia’s trajectory only served to increase the distance from Gondwana during the Fennian Stage, whereas Gondwanan index species are present at that level.

The Eremochitina brevis Biozone is itself in need of revision. It was originally defined as a total range biozone (Paris, 1990), but Nowak et al. (2016) recorded Eremochitina brevis from the upper Tremadocian of Morocco and furthermore introduced an Eremochitina brevis Biozone (Fig. 7) at a much lower level than the brevis Biozone of other schemes (Paris, 1990; Grahn, 2006; de la Puente & Rubinstein, 2013). The occurrence of Eremochitina brevis in Morocco is accompanied by graptolites of the Araneograptus murrayi Biozone, acritarchs of the messaoudensis–trifidum microflora, and other chitinozoans that are consistent with a late Tremadocian age such as Lagenochitina cf. destombesi and the Euconochitina paschaensis–symmetrica group. Nor is this Tremadocian record of E. brevis the only one to come to light recently, as Liang et al. (2017, 2018) recorded the species from the upper Tremadocian Tungtzu Formation of South China. This indicates a downward extension of the species range in both areas and necessitates a redefinition of Paris’s (1990) brevis Biozone from a total range to a partial range biozone. It is nevertheless unfortunate that Nowak et al. (2016) chose to use the same species as the index of a Tremadocian biozone, bringing with it the potential for confusion.

The absence of Gondwanan index species from the Moridunian succession in South Wales perhaps has a parallel with graptolite faunas in South Wales. In their synoptic history of the Arenig Series in South Wales, Fortey and Owens (1987) noted that ongoing transgression in the early Arenig resulted in soft, muddy sea-floor conditions represented by the Pibwr Member, but while oceanic connections were sufficiently developed for the appearance of a few graptolites at one level, a fully oceanic suite of species is not known. Fortey and Owens (1987) postulated that the subsequent development of a probable barrier to circulation further west, during deposition of the Cwmffrⓦd and Cwm yr Abbey members, then produced a stagnant basin with restricted circulation in the Carmarthen region. This interpretation was augmented by Traynor’s (1988) investigation of sedimentary processes during deposition of the Arenig Series in South Wales, which concluded that the Arenig deposits across South Wales were ponded in small interconnected marine sub-basins with facies and facies distributions controlled by intra-Arenig tectonic activity during an overall sea-level rise. Graptolites first become locally numerous in the Afon Ffinnant, Blaencediw and Colomendy formations, with species of Azygograptus and Expansograptus, and together with cyclopygid trilobites bring the first indications of oceanic conditions. Graptolites from the Pontyfenni and Llanfallteg formations are yet more cosmopolitan and oceanic, the Pontyfenni Formation perhaps representing the most oceanic conditions (Fortey & Owens, 1987, p. 104). It is very likely that the change from a restricted marine basin to the more oceanic setting documented by Fortey and Owens (1987) affected chitinozoans as much as the graptolites, and offers an explanation for the presence of Gondwanan index species in the higher parts of the succession, in contrast to their absence from the lower part.

The results from this study have implications for the extent of the regional Whitlandian Stage. The chitinozoan assemblages from the Carmarthen and Afon Ffinnant formations suggest correlation of much of the Moridunian Stage succession in the Carmarthen area with the Fl3 Stage Slice (Figs 6, 7). As the base of the regional Fennian Stage is placed at a level low in the Dapingian Stage, based on recognition of the ornensis and henryi biozones higher in the succession, the interval available for correlation with the Whitlandian Stage is restricted to the upper part of the Fl3 Stage Slice and the lower part of the Dp1 Stage Slice (Figs 6, 7).

A consequence of re-correlating the Whitlandian Stage with the upper part of the Fl3 Stage Slice and the lower part of Dp1 is that Whitlandian strata in South Wales would have been deposited in a relatively short period of time. A cumulative thickness curve (Fig. 11) shows trends from the Tremadocian Stage to the mid Darriwilian Stage that take account of the revised correlation. Although the curve is not corrected for compaction or tectonic thickening, it steepens in the upper Floian and lowest Dapingian stages, represented by the 500 m minimum thickness of the Colomendy Formation (Table 1). Similarly, cumulative thickness curves published by Verniers et al. (2002, fig. 3) and Linnemann et al. (2012, fig. 6) for the Lower Palaeozoic of the Brabant Massif, also part of Avalonia, show increases in cumulative thickness around 470 Ma. The Brabant formation associated with this, however, is the Tribotte Formation (Linnemann et al.2012, fig. 6), which is dated biostratigraphically as early Darriwilian (Herbosch & Verniers, 2014).

An assessment of global chitinozoan distribution patterns in the Lower and Middle Ordovician is not an aim of this paper, but it has been shown that such patterns respond to large-scale environmental changes related to palaeoclimate (Vandenbroucke et al. 2010 a, b). Several studies using proxy data, sequence stratigraphy and Global Circulation Models have suggested important global cooling during the Early–Middle Ordovician (Trotter et al. 2008), with ice caps present from the Darriwilian if not earlier (Turner et al.2011, 2012; Dabard et al. 2015; Pohl et al. 2016 a, b; Rasmussen et al. 2016). The data presented here has the potential to contribute to a better understand of chitinozoan distribution patterns and migrations during this period in the development of Earth’s climate system, as well as augmenting stratigraphic correlations.

This work is part of a PhD study financed by the Ecole Doctorale 104 Sciences de la Matière, du Rayonnement et de l’Environnement (SMRE) of Lille University. TRAV and CEAA acknowledge financial support from the CNRS (INSU, action SYSTER), the French ‘Agence Nationale de la Recherche’ through [grant ANR-12-BS06-0014] ‘SeqStrat-Ice’, and the SMRE doctoral school of the University of Lille. SGM publishes by courtesy of the Executive Director, British Geological Survey, NERC, UK. We thank Richard Ramsey and Catherine Russell for their assistance with the collection of field samples, Laurence Debeauvais for laboratory preparations and Philippe Recourt for SEM imaging.

None.