Life on land was severely impacted by the end-Devonian mass extinction, thought to be caused by extreme climatic changes and UVB radiation (Marshall et al. Reference Marshall, Lakin, Troth and Wallace-Johnson2020). Until recently the fossil record was known for an absence of tetrapod fossils in strata representing the following 25-million-year period, known as Romer's Gap. But over the last decade new discoveries in the Tournaisian have reported numerous tetrapod specimens, including those showing the earliest terrestrial capabilities (Clack Reference Clack2002; Smithson et al. Reference Smithson, Wood, Marshall and Clack2012; Anderson et al. Reference Anderson, Smithson, Mansky, Meyer and Clack2015; Clack et al. Reference Clack, Bennett, Carpenter, Davies, Fraser, Kearsey, Marshall, Millward, Otoo, Reeves, Ross, Ruta, Smithson, Smithson and Walsh2016, Reference Clack, Porro and Bennett2018, Reference Clack, Bennett, Davies, Scott, Sherwin and Smithson2019; Otoo et al. Reference Otoo, Clack, Smithson, Bennett, Kearsey and Coates2019). A major research project focused on the Tournaisian Ballagan Formation of the Scottish Borders. Here, coastal floodplains were populated by aquatic and terrestrial life: fishes (Carpenter et al. Reference Carpenter, Falcon-Lang, Benton and Henderson2014; Smithson et al. Reference Smithson, Richards and Clack2016; Richards et al. Reference Richards, Sherwin, Smithson, Bennion, Davies, Marshall and Clack2018; Challands et al. Reference Challands, Smithson, Clack, Bennett, Marshall, Wallace-Johnson and Hill2019), xiphosurans (Bicknell & Pates Reference Bicknell and Pates2019), millipedes (Ross et al. Reference Ross, Edgecombe, Clark, Bennett, Carriò, Contreras-Izquierdo and Crighton2018), bivalves (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016), ostracods (Williams et al. Reference Williams, Stephenson, Wilkinson, Leng and Miller2005, Reference Williams, Leng, Stephenson, Andrews, Wilkinson, Siveter, Horne and Vannier2006) and ichnofossil evidence of other organisms (Bennett et al. Reference Bennett, Howard, Davies, Kearsey, Millward, Brand, Browne, Reeves and Marshall2017).
The early mid-Tournaisian site of Willie's Hole in the Scottish Borders hosts the largest number of tetrapod specimens recovered from the Ballagan Formation (Smithson et al. Reference Smithson, Wood, Marshall and Clack2012; Clack et al. Reference Clack, Bennett, Carpenter, Davies, Fraser, Kearsey, Marshall, Millward, Otoo, Reeves, Ross, Ruta, Smithson, Smithson and Walsh2016; Smithson & Clack Reference Smithson and Clack2017). At the site, more than 100 tetrapod skeletons and isolated bones have been identified, along with approximately 2,000 specimens of rhizodonts, lungfish, actinopterygians, Gyracanthus spines, millipedes, shrimps, scorpions and eurypterids (Cater et al. Reference Cater, Briggs and Clarkson1989; Smithson et al. Reference Smithson, Wood, Marshall and Clack2012, Reference Smithson, Richards and Clack2016; Ross et al. Reference Ross, Edgecombe, Clark, Bennett, Carriò, Contreras-Izquierdo and Crighton2018; Clark & Ross Reference Clark and Ross2024). In common with other field sites of the Ballagan Formation, tetrapod and vertebrate material at Willie's Hole is concentrated in sandy siltstone beds (Clack et al. Reference Clack, Bennett, Carpenter, Davies, Fraser, Kearsey, Marshall, Millward, Otoo, Reeves, Ross, Ruta, Smithson, Smithson and Walsh2016).
Deposition of the Ballagan Formation took place on a coastal floodplain in a seasonally wet, tropical climate (Millward et al. Reference Millward, Davies, Brand, Browne, Bennett, Kearsey, Sherwin and Marshall2019). Deposition on the floodplain occurred during alternating dry periods and times of monsoonal high rainfall (Kearsey et al. Reference Kearsey, Bennett, Millward, Davies, Gowing, Kemp, Leng, Marshall and Browne2016), leading to flooding and the deposition of sandy siltstones in pools and lakes (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). Other habitats on the coastal floodplain were meandering rivers (Clack et al. Reference Clack, Bennett, Davies, Scott, Sherwin and Smithson2019) and waterlogged marshes with some areas of lycopsid tree cover (Kearsey et al. Reference Kearsey, Bennett, Millward, Davies, Gowing, Kemp, Leng, Marshall and Browne2016). The most common animal groups in the formation – sarcopterygian fish, bivalves and ostracods – indicate a fresh-brackish water salinity predominated (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). However, marine flooding events, via storm surges, deposited scolecodonts, Chondrites trace-makers and some marine macrofossils (Bennett et al. Reference Bennett, Howard, Davies, Kearsey, Millward, Brand, Browne, Reeves and Marshall2017). Dolostones formed as a result of marine water influx in saline to hypersaline lakes (Bennett et al. Reference Bennett, Kearsey, Davies, Leng, Millward, Smithson, Brand, Browne, Carpenter, Marshall and Dulson2021). At times these lakes dried out and gypsum evaporites precipitated in brine pans and sabkhas (Millward et al. Reference Millward, Davies, Williamson, Curtis, Kearsey, Bennett, West, Marshall and Browne2018).
This study is the first detailed and integrated investigation of the sedimentology, palaeoecology and micropalaeontology at Willie's Hole. Our analysis reveals changing environmental conditions on the floodplain over time, with widely variable salinity, and frequent periods of flooding to subaerial exposure and desiccation.
1. Methods
The Early Mississippian Ballagan Formation is exposed across the Midland Valley of Scotland and in the Scottish Borders and is Tournaisian in age (Marshall et al. Reference Marshall, Reeves, Bennett, Davies, Kearsey, Millward, Smithson and Browne2019). Willie's Hole is located on the River Whiteadder near Chirnside in the Scottish Borders (Fig. 1). It is approximately 10 miles from the key tetrapod site of Burnmouth, which is north of Berwick-upon-Tweed. The succession is thought to have been deposited approximately 3–4 million years after the Devonian–Carboniferous boundary (Clack et al. Reference Clack, Bennett, Carpenter, Davies, Fraser, Kearsey, Marshall, Millward, Otoo, Reeves, Ross, Ruta, Smithson, Smithson and Walsh2016).
Figure 1 Location map of Willie's Hole in the Scottish Borders.
In 2014 an 8 m thick succession was logged at a scale of 1 cm = 12.5 cm from the exposure at the edge of the River Whiteadder, starting below the weir, and referred to in this paper as the Main Log. Sediments on this river exposure are weathered, friable, and only accessible at low river levels.
To provide a more extensive exposed succession of the tetrapod-bearing beds within this 8 m interval, the river was temporarily dammed during an excavation organised by National Museums Scotland in July of 2015. Sections of the river bed were revealed by electro pneumatic road breaker and hand tools during the excavation, and two sedimentary logs were recorded – Detailed Logs 1 and 2 (Figs 2–4). The first log is of a 140 cm thick succession, logged at a scale of 1 cm = 5 cm. The second is of a 70 cm thick succession, logged at a scale of 1 cm = 2.5 cm. The two logs were recorded 3 m apart along the line of strike. Part of this section hosting millipedes was published in Ross et al. (Reference Ross, Edgecombe, Clark, Bennett, Carriò, Contreras-Izquierdo and Crighton2018), but the sedimentology was not described in detail.
Figure 2 Photographs of the logged field sections with the Main Log illustrated in (a–c). (a) Laminated grey siltstones, dolostone (D), conglomerate lens (C) and very fine to fine sandstone beds, Main Log, 0.5–1.3 m height. (b) Rippled siltstone, rippled sandstone and cross-bedded sandstone units of the fluvial facies association, Main Log, 1.2–2.7 m height. (c) Laminated grey siltstones, sandy siltstones and very fine sandstones, Main Log, 5.6–6.6 m height. (d) Sandy siltstones and laminated grey siltstones in the excavated section, showing the location of Detailed Log 1 and 2. Scale bars 20 cm (a–c) and 50 cm (d).
Figure 3 Main Log. The section starts at the exposure below the weir, Grid Reference [NT 87775 54741]. Fossils identified throughout the section are shown. The Ballagan Formation comprises ten facies and three facies associations: (1) fluvial facies association; (2) overbank facies association; and (3) saline–hypersaline lake facies association (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016).
The excavation yielded numerous vertebrate, invertebrate and plant fossils which are currently under study. Large samples (approximately 20 kg blocks of sediment) were excavated using an electro pneumatic road breaker and hand tools, and are housed at the National Museums Scotland collections for future study. For the research presented here, hand specimen rock samples were taken approximately every 20 cm from the main log, and every 5 cm from the two detailed sections (Appendix 1 in the supplementary material). Standard-sized polished thin sections, 30 μm thick, were made from 35 samples at the University of Leicester. They were examined using a Leica petrographic microscope to identify sedimentary facies, mineralogy and fossil components.
The fossil content of hand specimen samples was examined under a Leica binocular microscope at the University of Leicester. Four samples of siltstone and sandy siltstone facies were selected for micropalaeontological processing. Approximately 20–30 g of each sample was processed overnight in a 5 % solution of hydrogen peroxide, then wet sieved at 1,000, 425, 250, 125, 65 μm fractions and oven-dried at 40 °C. All fossil specimens present were picked from the 1,000, 425 and 250 μm fractions and total counts recorded (Appendix 2 in the supplementary material). Microfossil components are identified from literature records, or through direct comparison with macrofossil specimens from the Ballagan Formation.
2. Sedimentology
2.1. Results
Main Log: The main recorded section at Willie's Hole comprises 8 m of varied sedimentary rocks. The succession is divided into the three facies associations defined by Bennett et al. (Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). The lowermost metre comprises the first facies association, with dolostones interbedded with laminated grey siltstones. The basal dolostone bed has 1–5 cm sized gypsum nodules within a micritic dolomite matrix. In the nodules large calcite crystals have replaced gypsum crystals. This bed is equivalent to a facies 5 dolostone, dolomite with evaporite minerals, as defined by Bennett et al. (Reference Bennett, Kearsey, Davies, Leng, Millward, Smithson, Brand, Browne, Carpenter, Marshall and Dulson2021). The overlying bed is massive and comprises grey homogeneous micrite (Fig. 2a) with internal brecciation present in thin sections. This is equivalent to a facies 2 dolostone, homogeneous dolomicrite (Bennett et al. Reference Bennett, Kearsey, Davies, Leng, Millward, Smithson, Brand, Browne, Carpenter, Marshall and Dulson2021). Together these represent the saline–hypersaline lake facies association.
Dolostone nodules occur at discrete intervals throughout the succession. They are present within siltstones at 0.75, 4, 6, 7.2 and 7.6 m in the Main Log. The nodules range in size from 4 cm diameter to discontinuous nodular beds, with undulose bases. The dolostone cements siltstones or sandy siltstones. When the nodules are cut through with a rock saw, mudstone clasts and fossil bioclasts are seen internally. Dolostone nodules are equivalent to facies 1 dolostones, cemented siltstone and sandstone (Bennett et al. Reference Bennett, Kearsey, Davies, Leng, Millward, Smithson, Brand, Browne, Carpenter, Marshall and Dulson2021). The basal dolostone beds are interbedded with micaceous laminated grey siltstones with roots.
From 1 m height in the Main Log, the next 2.5 m is the second facies association, which is a fining-up succession (Fig. 3). Thin planar-laminated to cross-laminated sandstone beds, with localised conglomerate lags at their bases, comprise the base of the succession (Fig. 2a). These are overlain by a 1 m thick bed of very fine-grained sandstone with unidirectional stacked ripples, with sets of 1 cm height (Fig. 2b). This is followed by a 20 cm thick sandstone bed with cross-bedding, then 10 cm or thinner beds of laminated or rippled sandstones, interbedded with thin siltstone layers. In the top 1 m, four beds are bioturbated and the topmost bed contains roots. The succession represents the fluvial facies association.
The topmost 5 m of the Main Log represent the third facies association, which is dominated by laminated grey siltstones, sandy siltstones and very fine sandstones (Fig. 2c). Surfaces with desiccation cracks, roots or brecciated layers are common throughout the 5 m interval (Fig. 3). The thicknesses between the exposure horizons varies from 24 to 60 cm. Laminated grey siltstones are micaceous, vary in colour from light grey to dark, and most are planar-laminated. Some beds have small sets of asymmetric ripples. The ripples are straight, flat-topped, low in height and range in size from standard ripples (wavelength 2 cm) to micro-ripples (wavelength 0.5 cm).
Sandy siltstone beds have millimetre-sized clasts, are matrix supported, vary in thickness laterally, and have erosive bases. There are 22 sandy siltstone beds throughout the succession, ranging from 20 cm to 0.3 cm in thickness, with a skewed distribution towards thinner beds. The majority of sandy siltstone beds are internally structureless, but some display soft sediment deformation features (Fig. 5a) or brecciation, where the brecciated sediment is the dried-up underlying bed, infilled with sandy siltstone (Fig. 5b). Some sandy siltstone beds are weakly laminated, and they commonly grade upwards into laminated grey siltstones.
Figure 5 Sandy siltstone petrography and palaeontology. All images taken of polished thin sections in plane polarised light. (a) Grey sandy siltstone with mudstone rip-up clasts (M) which have soft sediment deformation structures, Detailed Log 2, 12 cm height. (b) Grey sandy siltstone, the sandy siltstone matrix infills brecciated cracks within the underlying siltstone (Si), Main Log, 6.37 m height. (c) Sandy siltstone dominated by grey siltstone clasts (Si), also containing plant fragments (P) and fish bone (F), Detailed Log 2, 59 cm height. (d) Sandy siltstone where green siltstone clasts dominate, Detailed Log 2, 59 cm height. (e) Black sandy siltstone with common plant fragments (P) and megaspores (Ms), Detailed Log 1, 42 cm height. (f) Structureless grey sandy siltstone with a mixture of articulated ostracods (Oa) to disarticulated ostracods (Od), Main Log, 7.57 m height. Scale bars 500 μm.
Isolated thin beds of rippled very fine sandstone occur occasionally throughout the succession. At 4.9 m a 1 cm thick bed has sinuous asymmetric ripples with a wavelength of 2.5 cm and height of 0.5 cm. A thicker rippled very fine sandstone bed occurs at 6.2 m height. It is 8 cm thick and consists of a stack of four rippled sets, with the second and third sets containing mud rip-up clasts (Fig. 6a). The ripples are asymmetric, sinuous to bifurcating, and have flat tops. Their wavelength is 3–4 cm and height 0.3–0.5 cm (Fig. 6b). Above this bed for the next 50 cm are rippled horizons of siltstone, interbedded with sandy siltstone. Here, laminated grey siltstones contain mm to cm thick very fine sandstone rippled units with asymmetric ripples in single sets up to three stacked sets. Cubic siltstone pseudomorphs after halite, 2–4 mm in size (Fig. 6c), occur within two horizons overlying the rippled sandstone bed at 6.2 m, both of which are laminated grey siltstone with micro-ripples. The topmost 5 m represents the overbank facies association.
Figure 6 Sedimentology. (a) Section through a flat-topped rippled sandstone bed comprising four sets; the middle two have mud rip-up clasts, Main Log, 6.2 m height. (b) Oblique view of the upper bedding surface of the bed shown in (a), with ripples that are asymmetric, sinuous to bifurcating, and flat topped, Main Log, 6.2 m height. (c) Halite pseudomorphs on the bedding surface of a laminated grey siltstone, Main Log, 6.3 m height. (d) Vertical section through the tetrapod bed, photographed in the Detailed Log 1, 50 cm height. Scale bars 2 cm.
Poorly developed palaeosols occur throughout the succession. The lowermost palaeosol occurs at 3.5 m height in the Main Log, at the top of the sandstone sequence, and comprises a 2–5 cm thick rooted horizon, with original sedimentary structures remaining. Roots are single tapering to branching structures 0.2 cm in diameter, preserved as carbon films. This root type is typical of Entisols, which occur throughout the Ballagan Formation (Kearsey et al. Reference Kearsey, Bennett, Millward, Davies, Gowing, Kemp, Leng, Marshall and Browne2016). Near the top of the main log is another Entisol, a red siltstone with remnant laminations, a mottled red/grey colour, and thin tapering roots of 3–5 cm depth preserved as carbon films.
Detailed Logs 1 and 2: The two logs are dominated by grey siltstones, with some lateral variations (Fig. 2d). The lower part of the succession is different for each log (Fig. 4). The base of Detailed Log 2 has a 10 cm thick green palaeosol with thin root traces throughout the bed, mottles, and 1–2 cm size dolostone nodules within the top part of the bed. This type of palaeosol is common in the Ballagan Formation and is classed as a gleyed Inceptisol (Kearsey et al. Reference Kearsey, Bennett, Millward, Davies, Gowing, Kemp, Leng, Marshall and Browne2016).
The first 40 cm of Detailed Log 1 comprises grey and black siltstones, rich in plant remains. These units correspond to the ‘plant bed’ recorded by the late Stan Wood in his field notes, and the Willie's Hole ‘shrimp bed’ reported by Cater et al. (Reference Cater, Briggs and Clarkson1989). The ‘allochthonous coal’ described from this interval was not seen in this study, but mm thick coalified laminae are occasionally present within this plant-rich succession. Detailed Log 1 has a thicker black siltstone bed at the base, while in Detailed Log 2 a grey laminated siltstone hosts sandy siltstone lenses that are 1–15 mm thick. The following units of micaceous laminated grey siltstone and then a black siltstone bed with sandy siltstone lenses, are similar in both logs and plant rich.
The next 20 cm of both logs comprises sandy siltstones containing tetrapod bones. These units correspond to the ‘amphibian bed’ recorded by the late Stan Wood, who traced the bed laterally into the field adjacent to the river. Individual beds are lenticular, laterally variable in thickness, and have erosive bases (Fig. 6d). Clasts 0.5–3 mm in length (average size 1 mm), of light or dark grey siltstone, are supported by a grey siltstone matrix. Black sandy siltstones have a black siltstone matrix containing clasts of grey siltstone (Fig. 5c), vertebrate bone, scale and plant fragments, of 0.5–5 mm size (average 1 mm). Pale green units have a matrix comprising light grey to green siltstone with clasts of green siltstone and bases that indicate erosion of underlying black sandy siltstones.
One green vertebrate-bearing sandy siltstone is present in Detailed Log 1, but two discrete beds occur in Detailed Log 2. Here the green beds have strongly erosive bases, the beds vary in thickness from 2–5 cm and have clasts composed of green siltstone (Fig. 5d), vertebrate bone, fish scale and plant fragments, of 0.5–10 mm size (average 2 mm). The two beds are separated by a black sandy siltstone bed with an internal structure of fining-upwards clasts.
Above the tetrapod-bearing beds is another succession of sandy siltstones and laminated grey siltstones, with less abundant fossils. At 90 cm height in Detailed Log 1 sandy siltstones infill cracks in a laminated grey siltstone bed. The cracks are up to 6 cm deep and 2 cm wide; they taper downwards and have jagged edges, classed as desiccation cracks. There are four other brecciated horizons in Detailed Logs 1 and 2, with cracks less than 1 cm depth, which could be desiccation cracks. The Detailed Log 1 succession ends with the rippled sandstone unit also seen in the Main Log.
2.2. Interpretation
Dolostones and evaporites of the Ballagan Formation commonly have nodular gypsum structures that indicate they were deposited in saline to hypersaline lakes, brine pans or sabkhas (Millward et al. Reference Millward, Davies, Williamson, Curtis, Kearsey, Bennett, West, Marshall and Browne2018; Bennett et al. Reference Bennett, Kearsey, Davies, Leng, Millward, Smithson, Brand, Browne, Carpenter, Marshall and Dulson2021). The saline–hypersaline lake facies association sediments, in the lowermost part of the Main Log, represent a relatively dry floodplain when evaporation was high. The Willie's Hole saline–hypersaline lakes dried out to some extent, and root horizons formed. But this drying was limited, as at no time was vegetation established for long enough for palaeosols to form.
The fluvial facies association section of the Main Log (from 1–2.5 m) represents a small river channel deposit. The fine grain size and predominance of ripple structures indicates waning of flow upwards and is typical of the upper section of fluvial point-bar deposits in the modern day (Miall Reference Miall2013) and in the Palaeozoic (Permian: Ghazi & Mountney Reference Ghazi and Mountney2009). The Willie's Hole sandstone succession has the characteristics of a meandering deposit, but Swan et al. (Reference Swan, Hartley, Owen, Howell, Ghinassi, Colombera, Mountney, Reesink and Betaman2018) concluded that it is difficult to distinguish braided from meandering systems in two-dimensional cliff section outcrops alone. The unit has some similarities with Palaeozoic crevasse splay deposits characterised by ripple lamination (Gulliford et al. Reference Gulliford, Flint and Hodgson2017), but the presence of a conglomerate lag at the base makes a smaller channel deposit more likely. Other sandstone units of the Ballagan Formation range from 1–36 m in thickness, and are interpreted as meandering to anastomosing channels (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). Cross-bedded sandstone facies recorded in Ballagan Formation boreholes are between 3–15 m thick (Millward et al. Reference Millward, Davies, Brand, Browne, Bennett, Kearsey, Sherwin and Marshall2019), which is significantly thicker than the 20 cm thick bed at Willie's Hole. This indicates that the Willie's Hole fluvial deposit was smaller, and possibly deposited more distally from the main river system. The top of the river channel becomes exposed to develop an Entisol, indicative of a period of short-lived vegetation growth (Kearsey et al. Reference Kearsey, Bennett, Millward, Davies, Gowing, Kemp, Leng, Marshall and Browne2016). The lag at the base of this sequence contains charcoal fragments indicative of fire activity.
The flat-topped rippled sandstone bed at 6.2 m on the Main Log is quite different to sandstones of the fluvial facies association. Flat-topped ripples commonly occur in the intertidal zone of tide-dominated estuaries (Desjardins et al. Reference Desjardins, Buatois, Mángano, Knaust and Bromley2012) and form due to erosion of the ripple crest (Sato et al. Reference Sato, Taniguchi, Takagawa and Masuda2011). Because these ripples are asymmetrical, they are unlikely to have been generated by wave action, but they may have formed from a storm surge event which washed marine water over the floodplain. The presence of halite pseudomorphs just above this bed indicates saline water on the floodplain which then evaporated, and halite recorded elsewhere in the Ballagan Formation has been associated with salina or sabkha environments (Millward et al. Reference Millward, Davies, Williamson, Curtis, Kearsey, Bennett, West, Marshall and Browne2018).
Siltstones and very fine sandstones occur throughout the overbank facies association sequence in the upper section of the Main Log (the top 5 m). Siltstones with horizontal laminations, fine sandstone lenses, ripples and root structures are common in the floodplain deposits of modern meandering rivers (Hagstrom et al. Reference Hagstrom, Leckie and Smith2018). Most of the Willie's Hole siltstone beds are planar-laminated and represent deposition from suspension in shallow floodplain lakes. Small asymmetric ripples or micro-ripples may have been derived from shallow sheetfloods into the lake. The concentration of plant material and the rare presence of coalified laminae in laminated grey siltstones may indicate that there were frequent flooding events on land that denuded the vegetation or the presence of long-lived wetlands which provided the anoxic conditions for plant preservation. The floodplain lakes occasionally dried out enough to become vegetated by short-lived vegetation (Entisols), or they formed the gleyed Inceptisol, interpreted as waterlogged marshy conditions (Kearsey et al. Reference Kearsey, Bennett, Millward, Davies, Gowing, Kemp, Leng, Marshall and Browne2016). However, there is no evidence of the longer-lived palaeosols observed at Burnmouth or Norham (e.g., Vertisols) suggesting that Willie's Hole represented a more active part of the floodplain (cf. Kearsey et al. Reference Kearsey, Bennett, Millward, Davies, Gowing, Kemp, Leng, Marshall and Browne2016).
Sandy siltstones were deposited as cohesive debris flows in flooding events (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). Throughout the Ballagan Formation there is a strong association between sandy siltstone beds and underlying desiccation cracks or palaeosols, representing an environmental change from drier to wetter conditions (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). The association at Willie's Hole of desiccation cracks, brecciated and rooted horizons with sandy siltstones supports this interpretation. The green tetrapod-bearing sandy siltstone beds contain green siltstone clasts that are identical to those from the gleyed Inceptisol palaeosol 40 cm below it. The interpreted source of these clasts is from the erosion of this same bed when it became a dried-out marsh, at a nearby location on the floodplain. The palaeosol would have been eroded during times of heavy rainfall and incorporated into a cohesive debris flow along with other sediment, floral and faunal material. A gleyed Inceptisol also underlies the tetrapod-bearing sandy siltstone beds of the T1 M interval at Burnmouth (Otoo et al. Reference Otoo, Clack, Smithson, Bennett, Kearsey and Coates2019).
The lateral changes to the structure and thickness of sandy siltstone beds seen at Willie's Hole are common in the Ballagan Formation. At the base of the Burnmouth succession, a black sandy siltstone changes from 15–30 cm thick laterally, with internal structure variation from weakly bedded to structureless (Challands et al. Reference Challands, Smithson, Clack, Bennett, Marshall, Wallace-Johnson and Hill2019). Sandy siltstones at Burnmouth can extend over tens of metres laterally or be localised to a few metres (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). These lateral variations in structure and thickness may be caused by differences in the underlying surface topography and changes in the speed and direction of the debris flow during deposition into water or on sub-aerial surfaces. The thickness variations of these beds could also be caused by climatic-driven changes in meteoric rainfall volume. The sandy siltstone facies are rarely reported in the geological record because it is friable and so typically poorly exposed in field outcrops that it may be mis-categorised as siltstone, monomict mud aggregate or microsites (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). The exposure of fresh sedimentary rock during the excavation, and the analysis of thin section samples, was pivotal to correctly identify this facies at Willie's Hole.
3. Palaeontology and taphonomy
The distribution of fossils present in the three logged sections is shown in Figs 3–4. Dolostones of the saline–hypersaline lake facies associations have a sparse fossil content of plant fragments and ostracods (indeterminate species). A thin section of a dolostone nodule in a siltstone bed at 4 m height in the Main Log contains a patch of five circular microfossil structures. They comprise hollow organic circular shapes 80–120 μm in diameter, with an external rim of calcite spar. No internal organic cellular structures are observed, so the specimens are attributed to Calcitarcha (calcispheres), instead of charophytes which can preserve gyrogonite cellular structures (cf. Feist et al. Reference Feist, Liu and Tafforeau2005).
Conglomerate lags, sandstone and siltstone facies of the fluvial facies association contain charcoal, plant and fish fragments. Fossil remains are sparse and comprise small broken pieces of specimens. Monocraterion trace fossils are identified in the upper beds of the sandstones from 2.5–3.1 m height in the Main Log. The traces are sub-vertical, 1 cm diameter or less, extending from 2–5 cm depth, and are filled with spreite.
Laminated grey siltstones contain tetrapods (Perittodus apsconditus, Clack et al. Reference Clack, Bennett, Carpenter, Davies, Fraser, Kearsey, Marshall, Millward, Otoo, Reeves, Ross, Ruta, Smithson, Smithson and Walsh2016), rhizodonts, actinopterygians, branchiopods, bivalves, millipedes, Spirorbis, Serpula, shrimps, arthropod cuticle (indeterminate) and ostracods (described below). Actinopterygians and rhizodonts are represented by fragmented material, with the former being more common in hand specimen. Actinopterygian material is mostly identified from isolated scales, groups of scales (cf. Turner Reference Turner1993) and isolated teeth (cf. Carpenter et al. Reference Carpenter, Falcon-Lang, Benton and Henderson2014). More complete rhizodont specimens from other sites in the Ballagan Formation have been identified as the genera Archichthys and Strepsodus (Carpenter et al. Reference Carpenter, Falcon-Lang, Benton and Henderson2014; Otoo et al. Reference Otoo, Clack, Smithson, Bennett, Kearsey and Coates2019). Branchiopods (Spinicaudata) specimens are weakly mineralised or moulds and are too poorly preserved to identify. Spirorbis is often found attached to plant stems. Shrimps are present as fragmentary carapace material. The eumalacostracan Tealliocaris and Schramocaris are present in the Ballagan Formation at Willie's Hole (Peach Reference Peach1908; Clark & Ross Reference Clark and Ross2024) and Tealliocaris is common in Tournaisian continental sites of Europe and the USA (Clark Reference Clark2013; Jones et al. Reference Jones, Feldmann, Schram, Schweitzer and Maguire2016).
Sandy siltstones contain a diverse fauna of tetrapods (Koilops herma, Clack et al. Reference Clack, Bennett, Carpenter, Davies, Fraser, Kearsey, Marshall, Millward, Otoo, Reeves, Ross, Ruta, Smithson, Smithson and Walsh2016), lungfish, rhizodonts, actinopterygians, gyracanthid spines, chondrichthyans, scorpions, eurypterids, millipedes, bivalves, ostracods, Spirorbis, Serpula and arthropod cuticle (indeterminate). Specimens of the elasmobranch chondrichthyan Ageleodus sp. are identified from teeth material (cf. Downs & Daeschler Reference Downs and Daeschler2001). Bivalves are assigned to the family Mytilidae and the genera Modiolus, and other studies have identified the species Modiolus latus (Portlock) from the Ballagan Formation (Pollard Reference Pollard1985). Seven millipede specimens belonging to six diplopod taxa were identified by Ross et al. (Reference Ross, Edgecombe, Clark, Bennett, Carriò, Contreras-Izquierdo and Crighton2018) from sandy siltstone and laminated grey siltstone beds from Willie's Hole Detailed Log 1. In general, macrofossils are well-preserved, bivalves are commonly articulated, vertebrate bones can be isolated or associated, and there is a high degree of millipede specimen articulation (Ross et al. Reference Ross, Edgecombe, Clark, Bennett, Carriò, Contreras-Izquierdo and Crighton2018). Eurypterid and scorpion cuticle is present as fragmentary material but is not complete enough to identify further.
Plant fossils are abundant in sandy siltstones and laminated grey siltstone. Large plant stems are most concentrated in black siltstones, sandy siltstones and laminated grey siltstones of the ‘plant bed’. Other plant elements present include leaves, fruiting bodies (reproductive structures), seeds, cones and megaspores (described below), with the most abundant megaspores occurring in the ‘plant bed’ (Fig. 5e). Lepidodendron and Stigmaria root fragments are identified. There is a high abundance of plant material, but the majority of specimens are broken fragments, with no in situ material identified. The classification of plant fossils is the focus of future research.
4. Micropalaeontology
The microfossil composition of each facies (total present in all size fractions) is shown in Fig. 7 and the most abundant microfossils are illustrated in Fig. 8. In all facies microfossils are well-preserved with no wear or abrasion identified. The distribution of each microfossil group is consistent across the 1,000, 425 and 250 μm processed size fractions (Appendix 2 in the supplementary material).
Figure 7 Microfossil assemblages. Percentages of total assemblage microfossil counts for a sample of each facies. Sample 1 (n = 389 specimens), Sample 2 (n = 641), Sample 3 (n = 223) and Sample 4 (n = 323). The full data table of counts for all size fractions and microfossils per gram is detailed in Appendix 2 in the supplementary material. Abbreviations: actin., actinopterygian; indet., indeterminate; rhizo., rhizodont.
Figure 8 Plate of common microfossils. (a) Rhizodont scale, exterior view, Detailed Log 1, 56 cm height. (b) Actinopterygian scale, interior view, Detailed Log 2, 12 cm height. (c) Arthropod cuticle fragment (indeterminate), Detailed Log 2, 61 cm height. (d) Seed pod, Detailed Log 2, 61 cm height. (e) Ageleodus tooth, mould in a sandy siltstone, Detailed Log 2, 61 cm height. (f) Shemonaella, adult, carapace with articulated open valves, Detailed Log 2, 28 cm height. (g) Shemonaella adult and juvenile, right valves, lateral view, Detailed Log 1, 137 cm height. (h) Paraparchites, adult, right valve, lateral view, Detailed Log 1, 86 cm height. Scale bars 500 μm.
Sample 1, a grey sandy siltstone, has a fossil abundance of 12 microfossils per gram. The assemblage is dominated by fish fragments (indeterminate), plant fragments and megaspores, with a minor component of charcoal, rhizodont and actinopterygian microfossils. Indeterminate fish material comprises pieces of scale and bone of varying texture and brown to grey colour. Plant stem fragments have a fibrous structure, while charcoal has a hollow internal structure of preserved cellular tissue. Megaspores are disc-shaped, have an ornament of small tapering spines, and are identified as Setispora pannosa. These megaspores have been found in an intact cone borne by Oxroadia conferta from Oxroad Bay, Tournaisian of Scotland (Bateman Reference Bateman1992). Rhizodont material comprises scales, which have a cream-coloured exterior surface with a fibrous structure (Fig. 8a), and interior layers with a range of structural elements including sheets of tubercules, pits or interlocking ridges and grooves. Actinopterygian material comprises scales, with a rhombic shape, smooth interior surface with keel, and a shiny exterior outer surface layer (ganoine mineralised tissue). The external ornament is typically transverse ridges and grooves of various height, with small pores.
Sample 2, an interbedded grey siltstone and sandy siltstone unit, has 24 microfossils per gram, the highest of the four samples processed in this study. Ostracods and plant stem fragments are most numerous, with a minor presence of Setispora pannosa megaspores, actinopterygian scales (Fig. 8b), indeterminate fish fragments and Spirorbis. In hand specimen it is common to see Spirorbis attached to plant stems. The ostracod assemblage comprises single valves, adults and juveniles. Paraparchites and Shemonaella are identified, along with indeterminate ostracods specimens. The hand specimen sample of this bed contains a small coprolite (7 mm length, 5 mm width, 4 mm height) filled with actinopterygian scales.
Sample 3, an interbedded grey siltstone and sandy siltstone unit, has 10 microfossils per gram. The assemblage is dominated by ostracods and plant fragments, with some indeterminate fish material. The ostracod assemblage of Paraparchites, Shemonaella and indeterminate specimens comprises single valves, adult and juvenile specimens, many of which are crushed or broken. Rare specimens include megaspores, charcoal, indeterminate arthropod cuticle, one actinopterygian scale and a Spirorbis fragment. The hand specimen has several Serpula specimens, calcified polychaete worm tubes, loosely coiled helical cylinders that are 1–2 mm in diameter. Shrimp carapace fragments are present in the hand specimen also.
Sample 4, a pale green sandy siltstone, has 14 microfossils per gram. Nearly half the assemblage comprises plant fragments, stem pieces and sheet-like small fragments. Megaspores, actinopterygians, rhizodonts, indeterminate fish fragments and indeterminate arthropod cuticle are present in fairly even proportions (Fig. 8c). One actinopterygian tooth fragment is identified by its conical shape and broken tip where the apical cap would be. One seed pod is identified (Fig. 8d). The hand specimen contains one Ageleodus tooth mould (Fig. 8e), and the thin section cuts through the base of a 1 mm wide Ageleodus tooth, perhaps originating from a juvenile chondrichthyan.
Ostracods: These bivalved crustacean microfossils are present throughout the overbank facies association at Willie's Hole, but rarely occur in the fluvial or saline–hypersaline lake facies associations. The thin sandstone bed with flat-topped ripples at 6.2 m contains abundant Shemonaella and Cavellina. In this bed all specimens are broken single valves, of a range of sizes. Laminated grey siltstones contain sparse to abundant assemblages of Shemonaella (Fig. 8f, g), Paraparchites (Fig. 8h), as well as indeterminate specimens. The carapaces and single valves of adults to juvenile instars are preserved, with valves aligned randomly on bedding planes. Some specimens have articulated open valves (Fig. 8f), but there is a high variability. Sandy siltstones have a similar ostracod assemblage, and overall have a greater number of single valves present than in laminated grey siltstones. Taphonomic character varies between and within beds. A thin section of one sandy siltstone shows articulated, semi-articulated and disarticulated specimens in close proximity (Fig. 5f).
5. Taphonomy interpretation
Dolostones and siltstones of the saline–hypersaline lake facies association contain small fragments of ostracods and fish. The relatively small exposure created by temporary damming of the river and low number of dolostone beds in the succession probably gives an under-representation of the true faunal diversity. Dolostones at Burnmouth are more extensive in field exposure and have been studied in more depth. They contain a more diverse fauna comprising non-marine to euryhaline groups (rhizodonts, dipnoans, actinopterygians, acanthodians, chondrichthyans, bivalves, ostracods, eurypterids), and marine groups (brachiopods, robust bivalve species, gastropods, Serpula, Spirorbis) and an ichnofauna dominated by marine Chondrites (Bennett et al. Reference Bennett, Kearsey, Davies, Leng, Millward, Smithson, Brand, Browne, Carpenter, Marshall and Dulson2021). Given the limited fossil material it is difficult to state whether the fossil assemblage represents an autochthonous or allochthonous assemblage.
Fluvial deposits at Willie's Hole preserve fragmentary fish fossils, charcoal and plant remains, with broken plant specimens indicating transportation during deposition (Ferguson Reference Ferguson1985). As with the saline–hypersaline lake facies association rocks, the small exposure at the locality probably restricts the range and number of fossils identified. A well-exposed 20 cm thick fossil-rich conglomerate lag from the base of a meandering fluvial channel at Burnmouth contains a more diverse and abundant fauna of tetrapods, rhizodonts, dipnoans, gyracanthids, actinopterygians and chondrichthyans (Clack et al. Reference Clack, Bennett, Davies, Scott, Sherwin and Smithson2019). In this Burnmouth bed the absence of ostracods and bivalves is attributed to taphonomic sorting during transport in the river channel, and it is possible that the same sorting processes were operating during Willie's Hole sediment deposition.
Evidence of mostly autochthonous assemblages within the laminated grey siltstone facies is the presence of weakly mineralised invertebrate specimens of shrimps, spinicaudatans, bivalves and millipedes, and well-preserved microfossils. In samples of this facies, the high incidence of ostracods with closed carapaces, a range of specimen sizes, and articulated open carapaces indicate a thanatocoenosis, or an ostracod assemblage that has been subject to minimal transport (Boomer et al. Reference Boomer, Horne and Slipper2003). However, all plant fossils preserved in this facies are broken, indicating some transport has occurred.
The flat-topped rippled sandstone bed at 6.2 m on the Main Log, a discrete unit within the overbank facies association, has a unique microfossil taphonomy. The bed hosts only ostracod single valves, showing that specimens were subject to a greater degree of transport, and the sedimentary structures and presence of halite pseudomorphs above this bed indicates deposition during a marine storm surge event. The presence of isolated Serpula specimens in laminated grey siltstone and sandy siltstone facies, instead of larger colonial structures seen in dolostones at other Ballagan Formation sites (Bennett et al. Reference Bennett, Kearsey, Davies, Leng, Millward, Smithson, Brand, Browne, Carpenter, Marshall and Dulson2021), indicates allochthonous assemblages transported from marine waters.
Sandy siltstones were deposited as cohesive debris flows, and the high fossil diversity and abundance in this facies at Willie's Hole is interpreted to be due to the concentration of fossils during the flooding and deposition process. All sandy siltstones contain an allochthonous fossil assemblage, but the presence of weakly mineralised invertebrate specimens, well-preserved microfossils, articulated ostracods and bivalves indicates a local transport. Some sandy siltstone beds have a predominance of ostracod single valves, indicating transport, with most beds containing articulated ostracod carapaces. Relatively local sediment transport is also demonstrated by the relatively small stratigraphic separation (40 cm) between the green sandy siltstone units and the underlying gleyed Inceptisol from which the matrix and clasts of the unit originated.
Most vertebrate specimens preserved in laminated grey siltstones and sandy siltstones comprise fragmentary isolated bones, scales and teeth. It is difficult to interpret taphonomy directly to depositional environment. The decay of the animal's soft tissue over time affects skeletal completeness (Syme & Salisbury Reference Syme and Salisbury2014). Carcass decay during flotation is a common phenomenon in vertebrates (Beardmore et al. Reference Beardmore, Orr, Manzocchi, Furrer and Johnson2012). In an experimental study on crocodile carcass decay in freshwater, Syme and Salisbury (Reference Syme and Salisbury2014) identified that most carcass disarticulation occurs after the ‘float and bloat’ stage, when the carcass has sunk to the sediment surface (generally from 24 days after death). While floating bones can be transported for several kilometres (Aslan & Behrensmeyer Reference Aslan and Behrensmeyer1996). When at the sediment surface, shallow sheetflood activity may have disrupted the skeletal completeness of vertebrates in shallow floodplain lakes, or desiccation cracks may have disrupted post-mortem fossil articulation. Future work to CT scan large rock specimens and elucidate the 3D preservation of vertebrate material can help shed light on these uncertainties.
6. Palaeoecology
Marine fossils in the dolostones and other facies of the Ballagan Formation have been interpreted to have washed onto the floodplain during storm surge events with the presence of marine waters enabling brief colonisation, as represented by the ichnofossils (Bennett et al. Reference Bennett, Howard, Davies, Kearsey, Millward, Brand, Browne, Reeves and Marshall2017). The rare presence of Serpula, Spirorbis and Calcitarcha in the Willie's Hole succession supports this model. Marine Serpula colonies are reported from modern times (Moore et al. Reference Moore, Saunders and Harries1998) and in the Palaeozoic (Beus Reference Beus1980; Suttner & Lukeneder Reference Suttner and Lukeneder2003). An extensive review of the palaeoenvironmental context of Spirorbis fossils by Gierlowski-Kordesch and Cassle (Reference Gierlowski-Kordesch and Cassle2015) concluded that Spirorbis has a marine origin, but is found in coastal non-marine rocks due to storm surge transportation. The origin of Palaeozoic Calcitarcha is unclear, but they are mostly associated with shallow marine environments (Versteegh et al. Reference Versteegh, Servais, Streng, Munnecke and Vachard2009), and some specimens resemble Mesozoic dinoflagellate cysts (Servais et al. Reference Servais, Munnecke and Versteegh2009).
Fluvial environments at Willie's Hole have a poor faunal record, but were probably inhabited by the same diverse aquatic fauna present in facies of the overbank facies association. Actinopterygian and rhizodont fragments are commonly found in laminated grey siltstones. Both groups are recorded at other Ballagan Formation sites, and they are interpreted to have been adapted to freshwater to brackish salinities (Carpenter et al. Reference Carpenter, Falcon-Lang, Benton and Henderson2014). The fauna identified in the Willie's Hole laminated grey siltstones is similar to that present in this facies at Burnmouth: actinopterygian fragments, rhizodont scale, ostracods, indeterminate cuticle and plant fragments (Otoo et al. Reference Otoo, Clack, Smithson, Bennett, Kearsey and Coates2019). Freshwater actinopterygians are first recorded in the Middle Devonian, and the group underwent a rise in diversity during the Tournaisian (Henderson et al. Reference Henderson, Dunne and Giles2022, Reference Henderson, Dunne, Fasey and Giles2023). Rhizodonts occupied freshwaters from the Middle Devonian, and those in the Late Devonian Catskill Formation occur in both fluvial sandstones and mudstone floodplain facies (Broussard et al. Reference Broussard, Treaster, Trop, Daeschler, Zippi, Vrazo and Rygel2020).
Invertebrates present in the laminated grey siltstone facies are Spirorbis, Serpula, shrimps, spinicaudatans, bivalves, millipedes, arthropod cuticle (indeterminate) and ostracods. Laminated grey siltstones at Willie's Hole contain shrimps which do not occur in other facies. Late Devonian–Mississippian eumalacostracans occur in freshwater-brackish (Cater et al. Reference Cater, Briggs and Clarkson1989; Clark Reference Clark2013; Gueriau et al. Reference Gueriau, Charbonnier and Clément2014a, Reference Gueriau, Charbonnier and Clément2014b) to marginal marine and marine environments (Briggs & Clarkson Reference Briggs and Clarkson1989; Clark et al. Reference Clark, Gillespie, Morris and Clayton2015, Reference Clark, Miller and Ross2018). Schramocaris is interpreted as more tolerant of marine conditions than Tealliocaris, which occurs in lower salinity environments (Clark & Ross Reference Clark and Ross2024). Shrimps, while not present in Willie's Hole sandy siltstones, do occur in other sandy siltstones from the Ballagan Formation (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016), so their absence here could be due to the limited succession studied.
Spinicaudata occur in laminated grey siltstones and sandy siltstones at Willie's Hole. They are recorded in the Tournaisian throughout the Tweed and Midland Valley of Scotland basins (Millward et al. Reference Millward, Davies, Brand, Browne, Bennett, Kearsey, Sherwin and Marshall2019). These clam shrimps are common in freshwater lakes in the Pennsylvanian where they are associated with ostracods, molluscs, aquatic and terrestrial arthropods (Vannier et al. Reference Vannier, Thiéry and Racheboeuf2003; Lerner et al. Reference Lerner, Lucas, Spielmann, Krainer, DiMichele, Chaney, Schneider, Nelson, Ivanov, Lueth, Lucas and Chamberlin2009). The Upper Famennian Evieux Formation of Strud, Belgium, hosts exceptionally preserved freshwater spinicaudatans and diverse other arthropods, in association with tetrapods and fishes (Gueriau et al. Reference Gueriau, Rabet and Hat2018; Lamsdell et al. Reference Lamsdell, Lagebro, Edgecombe, Budd and Gueriau2019). Modiolus is the most common bivalve in the Ballagan Formation and is thought to be tolerant of brackish to freshwater conditions (Williams et al. Reference Williams, Leng, Stephenson, Andrews, Wilkinson, Siveter, Horne and Vannier2006). The diplopod millipede specimens recently identified from the Ballagan Formation are all terrestrial (Ross et al. Reference Ross, Edgecombe, Clark, Bennett, Carriò, Contreras-Izquierdo and Crighton2018).
Sandy siltstones have the highest fossil diversity and microfossil abundance. Tetrapods occur in this facies and in laminated grey siltstones. Tetrapods in the Ballagan Formation had a varied ecology: some were under water for long periods, others alternated between living on the land surface and standing water (Clack et al. Reference Clack, Bennett, Carpenter, Davies, Fraser, Kearsey, Marshall, Millward, Otoo, Reeves, Ross, Ruta, Smithson, Smithson and Walsh2016; Smithson & Clack Reference Smithson and Clack2017). Because tetrapod fossils at Willie's Hole occur in sandy siltstones in association with clasts of the gleyed Inceptisol, and within laminated grey siltstones, they probably inhabited both waterlogged floodplain marshes and shallow floodplain lakes.
Lungfish, gyracanthids, chondrichthyans (Ageleodus), scorpions and eurypterids occur only in sandy siltstones at Willie's Hole, not in other facies. The facies hosts a mixed fauna of terrestrial or semi-terrestrial (tetrapods, diplopods, eurypterids, scorpions) to aquatic animals (fishes, small invertebrates). Actinopterygians, rhizodonts, plant and ostracod specimens are the most common fossils. The aquatic fauna present in the Ballagan Formation sandy siltstones is interpreted as freshwater, brackish or euryhaline (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). An analysis of over 200 sandy siltstone beds from the Norham Core and Burnmouth (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016) recorded all the faunal groups that are also present at Willie's Hole; only gyracanthids and scorpions are absent from the Willie's Hole samples. Two later studies from Burnmouth recorded gyracanthid spines from sandy siltstone beds, with a faunal assemblage similar to that of the Willie's Hole beds (Challands et al. Reference Challands, Smithson, Clack, Bennett, Marshall, Wallace-Johnson and Hill2019; Otoo et al. Reference Otoo, Clack, Smithson, Bennett, Kearsey and Coates2019). Lungfish, gyracanthids and chondrichthyans do not appear to have a unique association with overbank facies, as they have also been recorded from the Ballagan Formation in a fluvial conglomerate lag (Clack et al. Reference Clack, Bennett, Davies, Scott, Sherwin and Smithson2019).
No difference was observed in the ostracod assemblage from sandy siltstones and laminated grey siltstones. This may mean that the palaeoenvironment of the two lithologies was similar, or that the majority of the rip-up clasts and the matrix in sandy siltstones are sourced from laminated grey siltstone or palaeosols in siltstones (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). The ostracod assemblage of Shemonaella, Cavellina and Paraparchites is typical of a brackish salinity floodplain environment in the Mississippian (Williams et al. Reference Williams, Stephenson, Wilkinson, Leng and Miller2005, Reference Williams, Leng, Stephenson, Andrews, Wilkinson, Siveter, Horne and Vannier2006; Bennett Reference Bennett2008; Bennett et al. Reference Bennett, Siveter, Davies, Williams, Wilkinson, Browne and Miller2012). It is a low diversity assemblage in comparison to the assemblage recorded in an ostracod-rich palaeosol from Burnmouth (Beyrichiopsis, Cavellina, Glyptolichvinella, Paraparchites, Shemonaella, Silenites and Sulcella, Otoo et al. Reference Otoo, Clack, Smithson, Bennett, Kearsey and Coates2019). This low diversity may indicate more brackish salinities where only certain ostracod genera could thrive predominated at Willie's Hole. Williams et al. (Reference Williams, Stephenson, Wilkinson, Leng and Miller2005) hypothesised that the dominance of the genus Shemonaella in the Ballagan Formation is attributed to its ability to tolerate fluctuating salinities.
Megaspores are common in sandy siltstones and laminated grey siltstones. Their host plant of Setispora pannosa is Oxroadia conferta, an early, semi-prostrate, pseudoherbaceous rhizomorphic lycopsid that was adapted to form high-density populations of mono-specific communities, resembling extant briar patches (Bateman Reference Bateman1992). Its small size and simple anatomy were consistent with r-selection and its repeated occurrence within a sequence of volcanic ash from the nearby Tantallon Vent indicated that it was able to thrive in unstable conditions (Bateman Reference Bateman1988, Reference Bateman1992). The presence of this megaspore may indicate that land surfaces were poorly vegetated due to the high frequency of flood events, as proposed by Bennett et al. (Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). Lycopsid forests are preserved in situ in Tournaisian flood-disturbed interdistributary wetland deposits of brackish water (Falcon-Lang Reference Falcon-Lang2004). Tournaisian lycopsid megaspores are preserved in lacustrine rocks of Nova Scotia, interpreted as originating from trees living on a lowland floodplain (Glasspool & Scott Reference Glasspool and Scott2005). Lycopsid roots and plant macrofossils are common at Willie's Hole, but are understudied. Further palaeobotanical work will help to elucidate a fuller reconstruction of the Tournaisian vegetated landscape at Willie's Hole.
7. Discussion
The Willie's Hole succession represents a mosaic of coexisting palaeoenvironments on a tropical, coastal, low-lying floodplain (Fig. 9). Saline lakes, recorded at the base of the succession, formed when lakes evaporated and became hypersaline. During periods of increased precipitation, lakes became vegetated and were eroded by a small meandering river channel, which in turn dried up. Multiple river flooding events led to repeated cycles of wetting and drying creating temporary floodplain freshwater-brackish lakes. Marshes vegetated by Oxroadia conferta and desiccated sub-aerial surfaces formed at drier periods, and lycopsid trees grew in some areas of the floodplain. The lakes were recharged by meteoric flooding events, generated by monsoonal-like rainfall (cf. Kearsey et al. Reference Kearsey, Bennett, Millward, Davies, Gowing, Kemp, Leng, Marshall and Browne2016), washing in plant, animal and sedimentary material from nearby land surfaces. Sea water presence, and hence marine fauna occurrence on the floodplain, occurred via wash over from rivers and creeks at times of storm surge. Marine conditions were never fully established, and the floodplain lakes varied in salinity, from freshwater to hypersaline, but were predominantly brackish.
Figure 9 The depositional environments and palaeoecology of Willie's Hole. The general setting is a tropical, coastal, low-lying floodplain, with occasional marine input. The habitat of terrestrial to aquatic fauna is shown. T1 – Main Log lower section represents the lower 3.6 m of the Main Log. Deposition was in saline–hypersaline lakes, some of which became evaporitic, and in a meandering river channel. T2 – Main Log upper section represents the upper 5 m of the Main Log. Deposition was in freshwater-brackish floodplain lakes, some of which dried out to become desiccating lakes or vegetated marshes. Outside the fluvial system, a similar fauna inhabited all floodplain aquatic environments.
There is no single modern analogue that encompasses the range of palaeoenvironments present in the Willie's Hole succession, but comparisons can be made to certain facies associations. Modern dryland rivers in Australia have common dry/wet alternations similar to those at Willie's Hole. The overbank facies of these systems comprise pedogenic mud aggregates, weakly laminated mudstones and siltstones and desiccation cracks (Wakelin-King & Webb Reference Wakelin-King and Webb2007). The Everglades of southern Florida has a seasonal climate, freshwater to brackish lakes and marshes. Palustrine carbonates and peats are deposited and desiccation features and palaeosols are present (Platt & Wright Reference Platt and Wright1992). The area is also frequently inundated by tropical storms. Storm surge deposits washed onto modern floodplains by hurricanes and typhoons are often sand-rich beds containing marine fauna. For example, those deposited by a typhoon in the Philippines in 2013 comprise 1–8 cm thick sand beds with foraminifera, extending up to 1.7 km inland (Pilarczyk et al. Reference Pilarczyk, Horton, Soria, Switzer, Siringan, Fritz, Khan, Ildefonso, Doctor and Garcia2016). The equivalent here is the transport of small marine organisms such as Spirorbis microconchid larvae and Serpula from the coast onto the floodplain.
Geological successions with similar palaeoenvironments to that of Willie's Hole are those with a diverse mosaic of depositional settings and variable salinity. The Early Cretaceous, Leza Formation of the Cameros Basin, Spain, has carbonate and clastic coastal wetland depositional environments, including terrestrial sediments, freshwater, brackish, marginal-marine, evaporitic and tidal carbonate water bodies (Suarez-Gonzalez et al. Reference Suarez-Gonzalez, Quijada, Benito and Mas2015). The Late Devonian Catskill Formation of Pennsylvania, USA, was deposited on a wetland floodplain and has a similar faunal composition to Willie's Hole. Tetrapod fossils are preserved in the lags of meandering river channels (Broussard et al. Reference Broussard, Trop, Benowitz, Daeschler, Chamberlain and Chamberlain2018, Reference Broussard, Treaster, Trop, Daeschler, Zippi, Vrazo and Rygel2020) and in palaeosols formed in well-drained woodlands at the margins of oxbow lakes (Retallack et al. Reference Retallack, Hunt and White2009). The main difference to Willie's Hole is the predominance of red sedimentary rock in the Catskill Formation, indicating a drier climate to that of the Ballagan Formation, whose fine-grained lithologies are mostly grey in colour (Kearsey et al. Reference Kearsey, Bennett, Millward, Davies, Gowing, Kemp, Leng, Marshall and Browne2016).
Tetrapods at Willie's Hole inhabited waterlogged floodplain marshes and lakes. Vegetated floodplains were not the only environment tetrapods inhabited prior to the end-Devonian extinction. Famennian rocks with tetrapods interpreted as autochthonous fossils are reported from varied settings: open marine (Pavâri and Ketleri, Latvia: Ahlberg et al. Reference Ahlberg, Luksevics and Lebedev1994, Reference Ahlberg, Clack, Lukševičs, Blom and Zupiņš2008; Lukševičs & Zupiņš Reference Lukševičs and Zupiņš2004), shallow marine-lagoonal (Andreyevka, Russia: Lebedev & Clack Reference Lebedev and Clack1993; Alekseev et al. Reference Alekseev, Lebedev, Barskov, Barskova, Kononova and Chizhova1994; Lebedev & Coates Reference Lebedev and Coates1995), river channel in an arid climate (Strud and Becco, Belgium: Olive et al. Reference Olive, Clément, Denayer, Derycke, Dupret, Gerrienne, Gueriau, Marion, Mottequin and Prestiani2015; Denayer et al. Reference Denayer, Prestianni, Gueriau, Olive and Clement2016) and trackways from a sabkha (Easter Ross, Scotland: Rogers Reference Rogers1990). The Tournaisian tetrapod record is still poorly populated, comprising Scottish Borders and Scottish localities (Willie's Hole, Burnmouth, Auchenreoch Glen) and Nova Scotia, Canada (Blue Beach). In the Horton Bluff Formation of Blue Beach tetrapod trackways are preserved in coastal floodplain sandstones and lacustrine sandstones, and disarticulated fossils in conglomerate lags, and intertidal to fluvial sandstones and siltstones (Martel & Gibling Reference Martel and Gibling1991; Rygel et al. Reference Rygel, Calder, Gibling, Gingras and Melrose2006; Mansky & Lucas Reference Mansky, Lucas, Lucas, DiMichele, Barrick, Schneider and Speilmann2013; Anderson et al. Reference Anderson, Smithson, Mansky, Meyer and Clack2015). The coastal floodplain environment and wet climate is similar to that of Willie's Hole, but with a greater marine influence, and a more diverse ostracod fauna which indicates marine-freshwater salinity (Tibert & Scott Reference Tibert and Scott1999).
The abundance of tetrapod fossils at Willie's Hole is a factor of sedimentological and taphonomic processes. During meteoric flooding sediments, plants, terrestrial, semi-terrestrial and aquatic fauna from dried-out floodplain lakes, temporary pools and land surfaces were ripped-up, transported across the land surface and deposited into either existing or newly created floodplain lakes. Fossil preservation is enhanced in sandy siltstones because they are deposited rapidly into water, have a high clay content and are rarely bioturbated or pedogenically modified (Bennett et al. Reference Bennett, Kearsey, Davies, Millward, Clack, Smithson and Marshall2016). In the Ballagan Formation tetrapod fossils do occur in fluvial conglomerate lag deposits (Clack et al. Reference Clack, Bennett, Davies, Scott, Sherwin and Smithson2019), but are most common in sandy siltstones. This preferential preservation is similar to the taphonomic pattern of the Catskill Formation fauna, with floodplain facies hosting more articulated, less abraded vertebrate specimens than fluvial sandstones (Broussard et al. Reference Broussard, Treaster, Trop, Daeschler, Zippi, Vrazo and Rygel2020).
Does the Willie's Hole fauna show that life had fully recovered after the extinction at the Devonian–Carboniferous boundary? Clack et al. (Reference Clack, Bennett, Carpenter, Davies, Fraser, Kearsey, Marshall, Millward, Otoo, Reeves, Ross, Ruta, Smithson, Smithson and Walsh2016) reported that the large size of tetrapods from Willie's Hole meant that by the early mid-Tournaisian the group had diversified again after the extinction. After the extinction of Archaeopteris-dominated forests, the main radiation of trees did not occur until the mid–late Tournaisian (Decombeix et al. Reference Decombeix, Meyer-Berthaud and Galtier2011) or until after the Tournaisian (Marshall et al. Reference Marshall, Lakin, Troth and Wallace-Johnson2020). At Willie's Hole vegetation establishment on land after flood events began with Oxroadia thickets, and lycopsid roots show that land surfaces were present long enough for trees to grow, although probably still waterlogged, and lycopsid trees were not yet a common component of the vegetation. Vertebrate, aquatic invertebrate and terrestrial invertebrate life was diverse at Willie's Hole. The mosaic of different environments allowing for varied habitats and salinities present may have been critical for tetrapod evolution. The site reveals diverse habitats and the rich ecology on coastal floodplains shortly after the Devonian–Carboniferous extinction.
8. Conclusions
Willie's Hole is a site of exceptional fossil abundance in the Tournaisian and hosts some of the earliest terrestrial tetrapods. The three recognised facies associations of the Ballagan Formation are recorded: saline–hypersaline lakes (dolostones, evaporites), fluvial (conglomerate lags, rippled, planar-laminated, and cross-bedded sandstones and siltstones) and overbank (laminated grey siltstones, sandy siltstones, very fine sandstones and palaeosols). This represents a mosaic of depositional environments, existing on a tropical, coastal, low-lying floodplain: evaporitic saline lakes, small meandering river channels, brackish temporary floodplain lakes and marshes. Repeated flooding events of meteoric or storm surge origin are preserved in the succession and can be distinguished using the faunal assemblages. At other times the floodplain was subject to sub-aerial exposure and vegetation growth.
Diverse fauna and flora are present: tetrapods, rhizodonts, actinopterygians, gyracanthids, dipnoans, chondrichthyans (Ageleodus), molluscs (Modiolus), eumalacostracans, myriapods (diplopods), eurypterids, scorpions, branchiopods, ostracods (Cavellina, Paraparchites, Shemonaella), Spirorbis, Serpula, plant stems, arborescent lycopsids (Stigmaria, Lepidodendron) and Setispora pannosa megaspores of the creeping lycopsid Oxroadia conferta. Tetrapods inhabited waterlogged floodplain marshes, and the tetrapod-bearing and fossil-rich sandy siltstone beds were generated from erosion of dried-out floodplain marshes, pools, lakes and land surfaces when inundated by meteoric flood waters. The meteoric flooding events transported, redistributed and concentrated the eroded material relatively locally.
The site reveals the complex environmental dynamics and ecology of floodplains that thrived after the end Devonian extinction. There were changing environmental conditions over time, with widely variable salinity on a coastal floodplain that was subject to frequent periods of marine to meteoric water flooding, and subaerial exposure and desiccation. Future studies of plant fossils recovered from Willie's Hole, the taphonomic study of fossil 3D arrangement within large blocks of rock, and taphonomic component analysis will further expand our understanding of the palaeoecology and palaeoenvironments of this important site.
9. Supplementary material
Supplementary material is available online at https://doi.org/10.1017/S1755691024000112.
10. Acknowledgements
This study is a contribution to the TW:eed Project (Tetrapod World: early evolution and diversification), a major research programme investigating the building of Carboniferous ecosystems following a mass extinction at the end of the Devonian. This study was funded by NERC Consortium Grant ‘The Mid-Palaeozoic biotic crisis: setting the trajectory of tetrapod evolution’, led by the late Professor Jenny Clack (University Museum of Zoology, Cambridge, NE/J022713/1) and involving the universities of Leicester (NE/J020729/1) and Southampton (NE/J021091/1), the British Geological Survey (NE/J021067/1) and the National Museums Scotland (NE/J020621/1). T.I.K. and D.M. publish with the permission of the Executive Director, British Geological Survey (NERC). The following are thanked for fieldwork assistance: N. Fraser, M. Wood, J. Sherwin, G. Liddiard, L. Curry, K. Summers and J. Mason.
11. Competing interests
The authors declare no competing interests.
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