The Lower Palaeozoic succession of the Southern Uplands of Scotland, south of the Southern Upland Fault, is well known for its graptolite-bearing shales (e.g., Lapworth Reference Lapworth1878), including Dob’s Linn and Hartfell Score. The Southern Uplands Terrane preserves rocks relating to the closure of the Iapetus Ocean in an active tectonic setting (e.g. see Owen Reference Owen, Fortey, Harper, Ingham, Owen, Parkes, Rushton and Woodcock2000; Armstrong & Owen Reference Armstrong and Owen2001; Kelling Reference Kelling2001; Bluck Reference Bluck2013; Stone Reference Stone2014, Reference Stone, Smith and Strachan2024 and references therein for models and overviews).
Shelly Ordovician faunas from the Northern Belt of the Southern Uplands were described by Reference Peach and HornePeach & Horne in a classic Geological Survey Memoir in 1899. More recent studies, particularly from the Kirkcolm Formation at Wallace’s Cast and Kilbucho, by Euan Clarkson, Alan Owen, Howard Armstrong, David Harper and colleagues, have yielded more information, including trilobites (Clarkson et al. Reference Clarkson, Harper, Owen and Taylor1992a; Owen & Clarkson Reference Owen and Clarkson1992), brachiopods (Candela & Harper Reference Candela and Harper2009), the scleractinian coral Kilbuchophyllia (Scrutton & Clarkson Reference Scrutton and Clarkson1991; Scrutton Reference Scrutton1993, Reference Scrutton1996; Scrutton et al. Reference Scrutton, Jeram and Armstrong1997) and a demosponge Choiaella scotica (Beresi et al. Reference Beresi, Botting and Clarkson2010). Also present are gastropods, bivalves, rare polyplacophorans, cephalopods, the rostroconch Pinnocaris, crinoids and other echinoderms, sponges, conulariids, corals, algae, bryozoans and rare graptolites. Carbonate facies are rare in the Southern Uplands, although recrystallised limestone olistoliths in the Tweeddale Member of the Shinnel Formation near Peebles preserve shelly faunas. These allochthonous clasts preserve a trilobite and brachiopod fauna (Owen et al. Reference Owen, Harper and Clarkson1996) along with rarer molluscs and other fauna. Although in need of some taxonomic revision, the molluscs are briefly reviewed and considered in context with other faunal elements and palaeoenvironments from the Southern Uplands, Girvan and peri-Laurentia.
1. Stratigraphy
The largest exposures of shelly faunas occur at Wallace’s Cast and Kilbucho (situated along strike from Wallace’s Cast), part of the Kirkcolm Formation, Barr Hill Group (Fig. 1). The sediments are interpreted as mass flow deposits (Clarkson et al. Reference Clarkson, Harper, Owen and Taylor1992a, Reference Clarkson, Owen, Taylor, McAdam, Clarkson and Stone1992b), with fossils preserved in derived mudstone clasts within conglomerates. The Kirkcolm Formation, Barrhill Group (Fig. 2) is stratigraphically equivalent to the Ardwell Subgroup of Girvan (Ingham Reference Ingham, Fortey, Harper, Ingham, Owen, Parkes, Rushton and Woodcock2000) and the Bardahessiagh Formation, Pomeroy, Ireland (Harper & Parkes Reference Harper, Parkes, Fortey, Harper, Ingham, Owen, Parkes, Rushton and Woodcock2000).
Ordovician shelly fauna localities in the Northern Belt of the Southern Uplands (adapted from Clarkson et al. Reference Clarkson, Harper, Owen and Taylor1992a).
General stratigraphy of the Kirkcolm Formation, Southern Uplands, compared with that of the Barr Group and Ardmillan Group, Girvan, approximate age of the clasts in the Wrae Limestone, and the Bardahessiagh Formation, Pomeroy, Tyrone. Adapted from Clarkson et al. (Reference Clarkson, Harper, Owen and Taylor1992a), Owen et al. (Reference Owen, Harper and Clarkson1996) and information from Candela (Reference Candela2005).
Limited exposures of the Barrhill Group also occur in conglomerates at Snar Water, Sanquhar, Dumfriesshire. Fragmentary fossils are found in the conglomerates and within lithic clasts. Fossils are of the brachiopods Anisopleurella sp., Glyptorthis sp., Oepikina sp. and Sowerbyella sp. (identified by Dr Y. Candela, pers. com.), gastropods, trilobites, machaeridians (sclerites of armoured worms), sponge spicules, bryozoans, crinoid ossicles, ostracodes and a nuculoid bivalve. Many of these fossils are small and fragmentary. Brachiopod valves are cracked, similar to those from Kilbucho and Wallace’s Cast. Molluscs form a numerically small component of the fauna.
The Tweeddale Member ‘Wrae Limestone’ is composed of conglomerates containing hard recrystallised allochthonous limestone, and lithic clasts. The fossiliferous clasts contain brachiopods and trilobites (Owen & Clarkson Reference Owen, Harper and Clarkson1996), suggesting an early Sandbian (early Caradoc) age, though timing of the emplacement of the succession is uncertain.
Specimens examined in this study are held in the British Geological Survey (BGS), National Museums Scotland (NMS), Edinburgh and the Hunterian Museum (GLAHM), Glasgow.
2. Bivalves
Bivalves from Kilbucho and Wallace’s Cast were initially discussed by S. P. Tunnicliff in Clarkson et al. (Reference Clarkson, Harper, Owen and Taylor1992a). The fauna is dominated by semi-infaunal or infaunal taxa including Concavodonta, Cyrtodonta and Deceptrix. Further to his discussion, the NMS collections from Kilbucho include ?Ambonychia sp. (Fig. 3e), Cyrtodonta expansa, Orthonota sp. (Fig. 3i) and ?Vanuxemia distans (Fig. 3c). BGS collections from Wallace’s Cast are of Concavodonta, ‘Nucula’ and ?Modiolopsis sp. (Fig. 3d). A decalcified clast from the Snar Water, Sanquhar contains an internal mould of a small nuculoid bivalve. In addition, Peach & Horne (Reference Peach and Horne1899) recorded Modiolopsis sp. and Ctenodonta sp. from Duntercleuch. They recorded Ambonychia, Byssonichia, Grammysia and Modiolopsis from carbonate clasts of the Tweeddale Member at Glencotho.
Molluscs from the Southern Uplands. (a) Rostroconch Pinnocaris sp., Kilbucho, note crushing and distortion. GLAHM 131581. (b) Gastropod Pararaphistoma disciformis, note crushed shell, Kilbucho, BGS 8E 4571. (c) Bivalve ?Vanuxemia distans, NMS. G.1990.81.43, Kilbucho. (d) Bivalve ?Modiolopsis sp., Wallace’s Cast, BGS GSE 9926. (e) Bivalve Ambonychia sp. NMS.G.1990.81.40 Kilbucho. (f) Gastropod Ecculiomphalus? macromphalus, Wallace’s Cast, BGS GSE 9931. (g) ?Mestoronema scotica BGS m1521. (h) Bellerophont Pterotheca sp. NMS.G.2005.100.23 Kilbucho. (i) Bivalve Orthonota sp. NMS.G. 2005.100.41, Kilbucho, latex cast. (j) Cephalopod ?Gorbyoceras NMS.G. 20005.100.82, Kilbucho latex cast. (k) Gastropod ?Lophospira sp., Wallace’s Cast BGS m1626. (l) Polyplacophoran sclerite, Wallace’s Cast, BGS 378. All whitened with ammonium chloride sublimate. Scale bar represents 10 mm except for (g) which is 5 mm.
3. Gastropods
Gastropods from the Southern Uplands were described by Longstaff (Reference Longstaff1924) and listed from Kilbucho and Wallace’s Cast by Clarkson et al. (Reference Clarkson, Harper, Owen and Taylor1992a). They comprise mainly low-spired and planispiral forms, predominately Liospira striatula, Pararaphostoma disciformis (Fig. 3b), Ecculiomphalus macromphalus (Fig. 3f), Mesteronema scotica (Fig. 3g) and Lophospira (Fig. 3k). ‘High-spired’ gastropods are scarce. The bellerophontoid Pterotheca cf. simplex (Fig. 3h) is known from both Kilbucho and Wallace’s Cast (Clarkson et al. Reference Clarkson, Harper and Peel1995), and a few specimens of indeterminate belleropohontoids are present in the BGS collections. Peach & Horne (Reference Peach and Horne1899, p. 313) listed Maclurea (Omphileta) macromphala M’Coy from exposures at Polcraig Burn, Meikle Carco, from ’shales with sandy layers’. These specimens are likely to include the Ecculiomphalus macromphalus of Longstaff (Reference Longstaff1924), who figured a specimen (pl. 38, fig. 11.) from Kilbucho.
BGS collections from Duntercleuch, as recorded by Peach & Horne (Reference Peach and Horne1899), were of ‘Murchisonia’, ‘Raphistoma’, ‘Holopea’, ‘Turbo’, Tetranota and ‘Bellerophon’. Peach et al. (Reference Peach, Horne, Macconichie, Elliot, Laurie and Murdoch1901) list bellerophonts from Duntercleuch and the ‘Wrae Limestone’ of Glencotho. BGS collections from the Palaeosaurus database from the Tweeddale Member at Glencotho record Holopea, Loxonema and Bellerophon.
A few specimens of ?Lophospira sp. were recovered from conglomerates at Snar Water, near Sanquhar. Small trochiform? gastropods occur in a decalcified clast from this locality. Also from near Sanquhar are three small planispiral gastropods from Stoodfold Burn, Bail Hill. They are found in concretions within micaceous mudstone that contains the graptolite Dicellograptus sp., suggesting they were transported downslope in mudballs.
4. Other molluscs
Other molluscs known from the Kirkcolm Formation include rare cephalopods with one specimen held in the NMS collections from Kilbucho, possibly of ?Gorbyoceras (Fig. 3j). Two orthoconic nautoloids from Wallace’s Cast and one specimen from Kilbucho are held in the BGS collections. Rare polyplacophoran sclerites from Wallace’s Cast consist of a sclerite of Chelodes sp. and an indeterminate sclerite held in the BGS collections (Fig. 3l). A crushed specimen of the rostroconch Pinnocaris sp. (Fig. 3a) from Kilbucho was collected by E. N. K. Clarkson and A. W. Owen in 1986.
5. Network and statistical analyses
Network analysis (Newman Reference Newman2006) using the open-source programme Gephi v0.10.1 (Bastian et al. Reference Bastian, Heyman and Jacomy2009) was used to compare the similarity of the Southern Uplands faunas and other similar aged Ordovician faunas. Network analysis examines the connections and interrelationships of large volumes of data in networks and is shown visually in graphs. ‘Nodes’ (e.g., locality) are connected to ‘edges’ (e.g., genera present). Attributes such as stratigraphy, terrane and so on can be included. Various parameters and layouts can be applied to the data to produce different graphs. This produced a similar result to Detrended Correspondence Analysis in the statistical package PAST v5 (Hammer et al. Reference Hammer, Harper and Ryan2001) but has the advantage that data points (nodes) can be clearly viewed without them overlapping as is often the case in PAST.
Presence/absence data of Darriwilian–Katian brachiopods, chitinozoans, conodonts, graptolites, molluscs and trilobites were used to compare peri-Laurentian faunas (see database for peri-Laurentian faunas in the supplementary information available at https://doi.org/10.1017/S1755691025100972) from Girvan, Southern Uplands, Pomeroy and Central Belt graptolite-bearing facies of Ireland. The ‘exotic’ Grangegeeth terrane was included with peri-Laurentia as it contains predominately Laurentian taxa (Owen et al. Reference Owen, Haper and Romano1992).
Other localities from the late Darriwilian–Katian from Northern England, Wales, the Welsh Borderlands, Anglesey, Leinster, Bellewstown, were also used in a larger dataset with the peri-Laurentian terranes (see database for all faunas in the supplementary information). Data for genera were compiled from primary literature sources (see reference list for Gephi and PAST database in the supplementary information), fieldwork, museum collections and the BGS Palaeosaurus database.
Using many different taxa from one locality will give a more complete sample of environments than using only one. Taxonomic detail available, particularly in the literature, was variable. Some taxa are simply listed as, for example, ‘gastropod’. Taxonomy and stratigraphy were updated where possible, though inaccuracies and omissions will occur. Some taxa, such as trilobites and brachiopods, are considered to be more useful for biogeography and environmental preferences (sediment type, water depth, etc.). Mollusca, however, do have sediment/water depth preferences (e.g., see Novack-Gottshall & Miller Reference Novack-Gottshall and Miller2003a, Reference Novack-Gottshall and Miller2003b for an overview). This also provides a more complete sample of faunal elements at the localities.
Figure 4 shows the graph produced by Gephi of Darriwilian–Katian peri-Laurentian faunas from Girvan, the Southern Uplands, Pomeroy and Grangegeeth. Water depth can be estimated as the Barr Group, Girvan is predominately shallow water carbonates with the deepest water in the graptolite-bearing shales and mudstones of the Southern Uplands and Central Belt, Ireland. The Southern Uplands shelly faunas (Kirkcolm Formation) plot close to those of Pomeroy (Bardahessiagh Formation). Despite numerous shared taxa, the Girvan Sandbian–Katian faunas, including the Craighead Limestone Formation, do not tend to plot close to those of Pomeroy or the Kirkcolm Formation. Localities in the Ardwell Subgroup, Girvan tend to be deeper water environments, often with graptolite-bearing facies (e.g., Laggan, Pinmore and Pinmery). The Knockerk Formation, Grangegeeth shares similarities with Girvan, indicating increasing provinciality of taxa in the Sandbian–Katian. Graptolite-bearing localities of the Southern Uplands and the Central Belt, Ireland plot far from the shallow(er) siliciclastic facies.
Network analysis graph of peri-Laurentian faunas. Increasing water depth and increasing siliciclastic facies can be approximated from top right, with the Darriwilian Barr Group (GB D 89–99), Girvan reflecting shallow, mainly carbonate facies with hemipelagic graptolite shales of the Southern Uplands (SU) and Central Belt, Ireland (CN D) at the bottom. The Southern Uplands Kirkcolm Formation (6–11 SUK S) is placed close to Pomeroy (15–19 Ty S). Similar aged Girvan faunas (24–30 GA S, 38-45 GA S) are further away and tend to be from deeper water environments. The Craighead Limestone Formation (23 GCH S) is not in close proximity to the Kirkcolm and Bardahessiagh formations. (Gephi with Modularity, ForceAtlas 2, Threads 2, Scaling 15, Adobe Photoshop.) For simplicity only the localities (nodes) are shown and the connecting lines removed.
Analysis (Fig 5) with a selection of localities from Wales, Anglesey, the Lake District, and Leinster, Ireland along with the peri-Laurentian faunas still places Pomeroy close to the Southern Uplands shelly faunas. Similar aged faunas from the Ardwell Subgroup of Girvan plot further away towards deeper more graptolite-bearing facies. Some Sandbian faunas from Girvan plot close to those of Leinster and Wales, indicating a breakdown in provinciality and mixing of taxa.
Gephi analysis of peri-Laurentian faunas (as above) and Darriwilian–Katian faunas from Anglesey and Wales, Leinster Terrane, Ireland and Bellewstown, Ireland and Lake District. The faunas of Pomeroy (54–58 TYS) and the Kirkcolm Formation (33–38 SUS) are placed close together. The similarly aged Girvan faunas (Craighead Formation (65–67 GS), Ardwell Farm Formation (68–74 GS) and Balclatchie Formation (81–83 GS) tend to plot further away and towards deeper water environments. (Gephi with Modularity, ForceAtlas 2, Threads 2, Scaling 15, Adobe Photoshop.) For simplicity only the localities (nodes) are shown and the connecting lines removed.
6. Comparisons with faunas from Girvan and Pomeroy
The Ordovician faunas from the Southern Uplands share similarities with, but also have distinct differences from, those of Girvan and Pomeroy. The diversity and abundance of molluscs in the Kirkcolm Formation are smaller than those of Girvan and decrease along strike from east (Wallace’s Cast and Kilbucho) to west (Sanquhar area), though this could well be due to limited exposure, small sample sizes and collection failure.
Discoidal and low-spired gastropods are the most abundant gastropods found in the Southern Uplands. Lenticular, open or loosely coiled gastropods are considered to be largely sedentary ciliate feeders (Peel Reference Peel, Basset and Lawson1984) in low energy environments. A low diversity fauna similar to the Kirkcolm Formation is found in the mudstones of the Sandbian (lower Caradoc) Balclatchie Formation at Dow Hill, Girvan (Stewart Reference Stewart2011). Here the gastropod fauna is dominated by Pararaphistoma disciformis, Ecculomphalus sp. and univalved molluscs including Archinacella, bellerophontids and monoplacophorans. The trilobite fauna of Dow Hill (Bluck & Ingham Reference Bluck, Ingham, Lawson and Weedon1992) reflects a moderately deep water environment with species of Lonchodomas, Reedolithus and Teratorynchus.
No specimens of the distinctive hypertrophic gastropods Maclurites and Maclurina appear to be present in the collections of Southern Uplands shelly faunas. Macluritid gastropods are thought to be algal grazers while juvenile, but sedentary suspension feeders when adult (Novack-Gotshall & Burton Reference Novack-Gotshall and Burton2014). They are found worldwide, but particularly in shallow warm water carbonate facies in Laurentia, Baltica and Siberia (Rohr Reference Rohr, Gray and Boucot1979; Rohr & Gubanov Reference Rohr, Gubanov, Cooper, Droser and Finney1995). Macluritid gastropods are not recorded in the limestone clasts from the Tweeddale Member at Wrae Hill or Glencotho. Although corals and the alga Mastopora are recorded from the Kirkcolm Formation conglomerates, the cyanophyte Girvanella is not. Girvanella is common in the lower part of the Stinchar Limestone Formation, Barr Group of Girvan. The only record of Girvanella from the Southern Uplands appears to be Lamont (Reference Lamont, Craig and Duff1975), who mentions Girvanella in allochthonous cherts in a quarry (with pillow structures) from near Noble House, near Lamancha. Red cherts from here contain conodonts indicating a mid-Ordovician age. The lack of Girvanella and scarcity of algal grazing gastropods in the Sandbian Kirkcolm Formation suggests that the shelly faunas were not derived from very shallow waters.
The Bardahessiagh Formation gastropod fauna is less well known (Portlock Reference Portlock1843; Reed Reference Reed1952; Tunnicliff Reference Tunnicliff1980) but is similar to that of the Ardwell Subgroup of Girvan. Genera recorded by these authors include Clathrospira, Subulites, Cyclonema, Lophospira, ‘Pleurotomaria’, Trochonema, Archinacella, Sinuites and Tremanotus Maclurea and Maclurites are not recorded.
Candela (Reference Candela, Brunton, Cocks and Loy2001, Reference Candela2005) noted that the brachiopod faunas from parts of the Bardahessiagh Formation reflect rapidly deepening sea levels. Candela & Harper (Reference Candela and Harper2009) also suggested that the brachiopod faunas of the Southern Uplands reflect a deeper water environment than was originally suggested by Clarkson et al. (Reference Clarkson, Harper, Owen and Taylor1992a). This is supported by the gastropod fauna, as a muddy substrate, particularly in deeper water, would preclude the presence, or at least abundance, of Maclurites and other algal grazing gastropods.
Peach & Horne (Reference Peach and Horne1899) and Peach et al. (Reference Peach, Horne, Macconichie, Elliot, Laurie and Murdoch1901) list bivalves from the Southern Uplands shelly faunas from Geological Survey collections. However, Hind (Reference Hind1910), in his paper on Girvan bivalves, makes no reference to Southern Upland bivalves, only comparing the Girvan species with those of Pomeroy and North America. Tunnicliff (in Clarkson et al. Reference Clarkson, Harper, Owen and Taylor1992a) considered the bivalve faunas of the Kirkcolm Formation, particularly those from Kilbucho, to be very similar to those of the Bardahessiagh Formation, Pomeroy. The bivalves of Bardahessiagh are dominated by semi-infaunal and infaunal cyrtodontids and epibyssate ambonychiids (Tunnicliff Reference Tunnicliff1982).
Bivalves are rare in Girvan in the Darriwilian Superstes Mudstones, Barr Group and the Albany Group and only become more numerous and diverse in the Sandbian and Katian Balclatchie Ardwell Subgroup. Epibyssate ambonychiid bivalves occur in the ‘solid limestone and nodules’ of the Tweeddale Member, Glencotho (Peach & Horne Reference Peach and Horne1899, p. 258). The brachiopod and trilobite faunas are similar to the lower Caradoc taxa of Girvan (Owen et al. Reference Owen, Harper and Clarkson1996) but share no distinctive geographic or environmental provenance. Leggett (Reference Leggett1980) suggested that the Tweeddale Member represented a slump deposit from a subducted seamount. Conodont faunas also suggest the presence of ‘cryptic’ tracts or arcs in the Iapetus. Bergström & Ferretti (Reference Bergström and Ferretti2018) recorded a specific assemblage of conodonts termed the Peridon-Pygodus Recurrent Species Association from various Ordovician carbonate olistostomes, including the Tweeddale Member and Cobbs Head, Newfoundland. A similar conodont assemblage is also found in olistostomes in the Garn Formation, Anglesey, but not in Wales. Conodonts from the Barr Group Stinchar Limestone, Girvan (Bergström Reference Bergström1990) share similarities with those of the Wrae Limestone.
Other conodont-bearing allochthonous Ordovician limestone clasts occur in Silurian and Devonian conglomerate rocks in several places in the Southern Uplands (e.g., Armstrong et al. Reference Armstrong, Owen and Clarkson2000; Armstrong & Owen Reference Armstrong, Owen, Friend and Williams2000). These clasts are dated as Darriwilian–Sandbian (upper Llanvirn–lower Caradoc).
Scrutton et al. (Reference Scrutton, Jeram and Armstrong1997) considered that the absence of the distinctive scleractiniomorph coral Kilbuchophyllia in Girvan, but present in Pomeroy and the Southern Uplands, indicated that these two areas were probably in closer proximity to each other than to Girvan during the Late Ordovician. Sinistral strike-slip motion was considered to be important in the current position of the faunas. It is possible that faulting and sedimentary slumping has cut out part of the Girvan succession, removing possible Kilbuchophyllia-bearing facies. The lack of Kilbuchophyllia in the Craighead Limestone Formation (itself lying unconformably on older Ballantrae Complex), which does contain corals, including early tabulate corals (White & Shengwu Reference White and Shengwu2004), might suggest a genuine absence.
The provenance of the derived fossils of the Kirkcolm and Bardahessiagh formations is unknown. They may have been displaced from somewhere on the Laurentian shelf or from a now ‘cryptic’ terrane or arc. These areas must have been in close proximity with each other to allow faunal mixing, but with enough environmental barriers between them and Girvan to prevent migration by certain taxa. The Girvan Sandbian–Katian succession reflects a transgression, with graptolite facies and deep water taxa such as cyclopygid trilobites occurring in the Albany Group and Ardwell Subgroup. As the brachiopod and trilobite faunas of Bardahessiagh Formation also have a deeper water component, deepening (and/or colder) water may have been a significant factor in the migration of taxa, particularly of shallow or warm water benthic species.
Supplementary material
Supplementary material is available online at https://doi.org/10.1017/S1755691025100972.
Acknowledgements
This study originally developed as part of a PhD funded by the University of Glasgow scholarship under the supervision of Alan Owen and Martin Lee. Neil Clark of the Hunterian Museum and Mark Dean and Mike Howe at the BGS permitted access to specimens in their care. Field work for NMS to Kilbucho in 2005 was conducted by Lyall Anderson, Bill Baird, Yves Candela, Euan Clarkson, Sarah Stewart and Cecilia Taylor. NMS specimens were photographed by NMS volunteer Bill Crighton. Field work at Snar Water was with Alan Owen and Craig Dooris. The reviewers are thanked for comments and suggestions which have greatly improved the manuscript.
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