Ballagan Formation Research Articles

Caridoid crustaceans from the Ballagan Formation (Tournaisian, Lower Carboniferous) of Willie's Hole, Chirnside, Scottish Borders, UK

ABSTRACT The descriptions of two co-occurring cardioid crustaceans from the Ballagan Formation (Tournaisian, Lower Carboniferous) of Chirnside, Scottish Borders, help to resolve the taxonomy of the genus Tealliocaris. Tealliocaris robusta Peach, 1908 is assigned to Schramocaris to form S. robusta (Peach, 1908) comb. nov. on the basis of morphological characters such as the rugosity and position of the branchial carinae as well as the nature of the pleon, and becomes the earliest representative of this genus in Scotland. A new species of Tealliocaris is also recognised from this locality and is described as T. briggsi sp. nov., based on the smooth carapace, lack of pleonic grooves and the number of spines on the scaphocerite and lateral margin of the anterior carapace. The systematic position of the Pendleian specimens identified by Peach (1908) as ‘Tealliocaris robusta var.’ is finally resolved and described as T. weegie sp. nov.

The occurrence of elevated δ34S in dissolved sulfate in a multi-level coal mine water system, Glasgow, UK

Stable isotopic compositions of δ18O, δ2H and dissolved sulfate δ34S in water from abandoned and flooded coal mines are used to interpret the water's and solutes' origin and interactive history. These isotopic ratios have been determined in mine water from a shallow (<100 m) series of overlapping coal mine workings at the UK Geoenergy Observatories Glasgow Geothermal Energy Research Field Site (UKGEOS GGERFS). Comparison has been made between dissolved sulfate δ34S of water in mined Carboniferous coal-bearing strata, that of water which had interacted with equivalent unmined strata, and δ34S in sulfide-bearing minerals (pyrite) in the host sedimentary rocks and local evaporite (gypsum) of the Lower Carboniferous Ballagan Formation. δ18O and δ2H confirm a meteoric origin for coal mine waters. The δ34S arithmetic mean and standard deviation of the pyrite (+5.0 ± 15.5‰) and water from unmined strata (+0.3 ± 2.1‰) were found to be similar, whereas the mine water exhibited elevated isotopic values (+20.3 ± 1.1‰), plotting closer to that of modern-day seawater (+21.2‰) and Ballagan Formation gypsum (+18.9 ± 0.5‰). Whilst the origin of dissolved sulfate in the mine water remains unclear, it is unlikely to be wholly due to simple pyrite oxidation. Influence of evaporite dissolution, fractionation associated with microbial sulfate reduction, and mixing with saline formation waters of marine, evaporitic or of another origin, cannot be ruled out.

Historic palaeobotanical collection reveals in situ microspores and pollen from Early Carboniferous (Tournaisian) ovules from the Ballagan Formation of Scotland

Ovules from the Albert Long Collection were investigated for in situ microspores and pollen. The ovule Genomosperma kidstoni contains Prolycospora claytonii a trilete microspore previously attributed to the lycopods. Already reported associations between Colatisporites decorus and Stamnostoma huttonense, Colatisporites denticulatus and Lyrasperma scotica and Remysporites magnificus and Protopitys scotica were verified. This demonstrates the importance of historic collections and particularly ovules for determining the palaeobotanical affinities of dispersed miospores and pollen.

Palaeoecology and palaeoenvironment of Mississippian coastal lakes and marshes during the early terrestrialisation of tetrapods

The Ballagan Formation of northern Britain provides an exceptional record of Early Mississippian ecosystems that developed as tetrapods emerged onto land. In this paper, we study two 500-metre sections of the formation near Berwick-upon-Tweed, which are characterised by abundant ferroan dolostone beds. Five lithofacies are identified: cemented siltstone and sandstone, homogeneous dolomicrite, mixed dolomite and siltstone, mixed calcite and dolomite, and dolomite with evaporite minerals. Cemented sediments have non-planar to planar subhedral dolomite crystals, up to 40 μm in size, whereas other facies predominantly comprise dolomicrite or planar euhedral dolomite rhombs 15 μm in size, with patches of larger rhombs indicating partial recrystallisation. The macro- and microfossil content of the dolostones is dominated by sarcopterygian (rhizodont) and actinopterygian fish, bivalves, Serpula, ostracods and Chondrites trace fossils; with rarer Spirorbis, chondrichthyans (Ageleodus, hybodonts and ?ctenacanths, xenacanths), non-gyracanth acanthodians, gastropods, eurypterids, brachiopods, plant debris, wood, lycopsid roots, charcoal, megaspores, phycosiphoniform burrows, Zoophycos? and Rhizocorallium. The oxygen and carbon isotope composition of dolomites range from –3.6‰ to –1.7‰ (for δ18O) and –2.6‰ to +1.6‰ (for δ13C) respectively indicating dolomite growth in mixed salinity waters. Frequent marine storm-surge events transported marine waters and animals into floodplain lakes, where evaporation, interstitial sulphate-reducing bacteria, iron reduction and methanogenesis allowed dolomite growth in the shallow sub-surface. Secondary pedogenic modification (by roots, brecciation, desiccation, and soil forming processes) is common and represents lake evaporation with, in some cases, saline marsh development. The dolostone facies are part of a complex environmental mosaic of sub-aerial dry floodplain, wet marshy floodplains, rivers, and lakes ranging in salinity from freshwater to hypersaline. Marine influence is strongest at the base of the formation and decreases over time, as the floodplain became drier, and forested areas became more established. Coastal lakes were an important habitat for animals recovering from the end-Devonian Hangenberg Crisis and may have acted as a pathway for euryhaline fishes, molluscs and arthropods to access freshwater environments.

The South Kintyre Basin: its role in the stratigraphical and structural evolution of the Firth of Clyde region during the Devonian–Carboniferous transition

Late Devonian–Early Carboniferous rocks at the southern end of the Kintyre Peninsula closely resemble those of the Kinnesswood and Clyde Sandstone formations in more easterly portions of the Firth of Clyde. For example, a previously unrecognized thick marlstone with pedogenic calcretes is present in the Kinnesswood Formation at the south tip of the peninsula and, on the west coast, south of Machrihanish, a striking cliffed exposure includes massive phreatic calcretes developed from cross-bedded sandstones and red mudstones closely resembling those of the Clyde Sandstone on Great Cumbrae. A similar phreatic calcrete unit is present in the lower part of the Ballagan Formation in south Bute. The presence of vadose and phreatic calcrete provides valuable information concerning palaeoclimatic conditions in southwestern Scotland during the Devonian–Carboniferous transition. Overlying thick volcanic rocks are correlative with the Clyde Plateau Volcanic Formation. The sediments accumulated in the South Kintyre Basin on the west side of the Highland Boundary Fault (HBF). Formation of this basin, and the North East Arran and Cumbraes basins in the northeastern part of the Firth of Clyde, is interpreted as a response to development of a ‘locked zone’ in the HBF during an episode of sinistral faulting.

A lungfish survivor of the end-Devonian extinction and an Early Carboniferous dipnoan radiation

Until recently the immediate aftermath of the Hangenberg event of the Famennian Stage (Upper Devonian) was considered to have decimated sarcopterygian groups, including lungfish, with only two taxa, Occludus romeri and Sagenodus spp., being unequivocally recorded from rocks of Tournaisian age (Mississippian, Early Carboniferous). Recent discoveries of numerous morphologically diverse lungfish tooth plates from southern Scotland and northern England indicate that at least 10 dipnoan taxa existed during the earliest Carboniferous. Of these taxa, only two, Xylognathus and Ballgadus, preserve cranial and postcranial skeletal elements, which have yet to be described. Here we present a description of the skull of a new genus and species of lungfish, Limanichthys fraseri gen. et sp. nov., which hails from the very earliest Tournaisian in the Ballagan Formation of Burnmouth, southern Scotland. The new specimen represents the earliest definitive Tournaisian lungfish skull material, thus providing an invaluable insight into the response of this group – and, indeed, Sarcopterygii as a whole – immediately following the latest Devonian Hangenberg event. Phylogenetic analysis places Limanichthys fraseri within the Devonian ‘phaneropleurid-fleurantiid’ grade of lungfish and shows that the Carboniferous lungfish represent forms that have their origins deep in the Middle and Late Devonian as well as those from a unique Carboniferous radiation. http://zoobank.org/urn:lsid:zoobank.org:pub:101FFE43-42D5-4950-9169-A900113B6F1D

A Tournaisian (earliest Carboniferous) conglomerate-preserved non-marine faunal assemblage and its environmental and sedimentological context.

A conglomerate bed from the Tournaisian Ballagan Formation of Scotland preserves a rich array of vertebrate and other nonmarine fossils providing an insight into the wider ecosystem and paleoenvironment that existed during this pivotal stage of Earth history. It challenges hypotheses of a long-lasting post-extinction trough following the end-Devonian extinction event. The fauna recovered includes a wide size range of tetrapods, rhizodonts, and dipnoans, from tiny juveniles or small-bodied taxa up to large adults, and more than one taxon of each group is likely. Some fauna, such as actinopterygians and chondrichthyans, are rare as macrofauna but are better represented in the microfossil assemblage. The fauna provides evidence of the largest Carboniferous lungfish ever found. The specimens are preserved in a localized, poorly-sorted conglomerate which was deposited in the deepest part of a river channel, the youngest of a group of channels. In addition to the fossils (micro- and macro-), the conglomerate includes locally-derived clasts of paleosols and other distinctive elements of the surrounding floodplains. Charcoal fragments represent small woody axes and possible larger trunk tissue from arborescent pteridosperms. Preservation of the fossils indicates some aerial exposure prior to transport, with abrasion from rolling. The findings presented here contrast with other published trends in vertebrate size that are used to interpret a reduction in maximum sizes during the Tournaisian. The richness of the fauna runs counter to the assumption of a depauperate nonmarine fauna following the end-Devonian Hangenberg event, and charcoal content highlights the occurrence of fire, with the requisite levels of atmospheric oxygen during that stage.

A fish and tetrapod fauna from Romer's Gap preserved in Scottish Tournaisian floodplain deposits

AbstractThe end‐Devonian mass extinction has been framed as a turning point in vertebrate evolution, enabling the radiation of tetrapods, chondrichthyans and actinopterygians. Until very recently ‘Romer's Gap’ rendered the Early Carboniferous a black box standing between the Devonian and the later Carboniferous, but now new Tournaisian localities are filling this interval. Recent work has recovered unexpected tetrapod and lungfish diversity. However, the composition of Tournaisian faunas remains poorly understood. Here we report on a Tournaisian vertebrate fauna from a well‐characterized, narrow stratigraphic interval from the Ballagan Formation exposed at Burnmouth, Scotland. Microfossils suggest brackish conditions and the sedimentology indicates a low‐energy debris flow on a vegetated floodplain. A range of vertebrate bone sizes are preserved. Rhizodonts are represented by the most material, which can be assigned to two taxa. Lungfish are represented by several species, almost all of which are currently endemic to the Ballagan Formation. There are two named tetrapods, Aytonerpeton and Diploradus, with at least two others also represented. Gyracanths, holocephalans, and actinopterygian fishes are represented by rarer fossils. This material compares well with vertebrate fossils from other Ballagan deposits. Faunal similarity analysis using an updated dataset of Devonian–Carboniferous (Givetian–Serpukhovian) sites corroborates a persistent Devonian/Carboniferous split. Separation of the data into marine and non‐marine partitions indicates more Devonian–Carboniferous faunal continuity in non‐marine settings compared to marine settings. These results agree with the latest fossil discoveries and suggest that the Devonian–Carboniferous transition proceeded differently in different environments and among different taxonomic groups.

A new tetrapod from Romer's Gap reveals an early adaptation for walking

A new early tetrapod, Mesanerpeton woodi gen. et sp. nov., collected by Stan Wood from the Ballagan Formation, Tournaisian CM palynozone, at Willie's Hole, Scottish Borders, is described. It includes vertebrae like those of Crassigyrinus, with poorly developed neural arches, a well ossified ulna with a large olecranon, and a humerus that is structurally intermediate between the pleisiomorphic condition of Devonian taxa and that of all later forms. A comparative analysis of this new material and other tetrapodomorph humeri revealed how an increase in humeral torsion transformed the course of the brachial artery and median nerve through the bone, from an entirely ventral path to one in which the blood vessel and nerve passed through the entepicondyle from the dorsal to the ventral surface. Increasing humeral torsion is suggested to improve walking in early tetrapods by potentially contributing to an increase in stride length, and is one of a number of changes to limb morphology during the Early Carboniferous that led to the development of terrestrial locomotion.

Reinterpreting the age of the uppermost ‘Old Red Sandstone' and Early Carboniferous in Scotland

ABSTRACTIn Scotland, the base of the Ballagan Formation has traditionally been placed at the first grey mudstone within a contiguous Late Devonian to Carboniferous succession. This convention places the Devonian–Carboniferous boundary within the Old Red Sandstone (ORS) Kinnesswood Formation. The consequences of this placement are that tetrapods from the Ballagan Formation were dated as late Tournaisian in age and that the ranges of typically Devonian fish found in the Kinnesswood Formation continued into the Carboniferous. The Pease Bay specimen of the fishRemigolepisis from the Kinnesswood Formation. Comparisons with its range in Greenland, calibrated against spores, show it was Famennian in age. Detailed palynological sampling at Burnmouth from the base of the Ballagan Formation proves that the early Tournaisian spore zones (VI and HD plus Cl 1) are present. TheSchopfitesspecies that occurs through most of the succession isSchopfites delicatusrather thanSchopfites claviger. The latter species defines the late Tournaisian CM spore zone. The first spore assemblage that has been found in Upper ‘ORS' strata underlying the Ballagan Formation (Preston, Whiteadder Water), containsRetispora lepidophytaand is from the early latest Famennian LL spore zone. The spore samples are interbedded with volcaniclastic debris, which shows that the Kelso Volcanic Formation is, in part, early latest Famennian in age. These findings demonstrate that the Ballagan Formation includes most of the Tournaisian with the Devonian–Carboniferous boundary positioned close to the top of the Kinnesswood Formation. The Stage 6 calcrete at Pease Bay can be correlated to the equivalent section at Carham, showing that it represents a time gap equivalent to the latest Famennian glaciation(s). Importantly, some of the recently described Ballagan Formation tetrapods are older than previously dated and now fill the key early part of Romer's Gap.

Palaeogeography of tropical seasonal coastal wetlands in northern Britain during the early Mississippian Romer's Gap

ABSTRACTThe lower Mississippian Ballagan Formation of northern Britain is one of only two successions worldwide to yield the earliest known tetrapods with terrestrial capability following the end-Devonian mass extinction event. Studies of the sedimentary environments and habitats in which these beasts lived have been an integral part of a major research project into how, why and under what circumstances this profound step in the evolution of life on Earth occurred. Here, a new palaeogeographic map is constructed from outcrop data integrated with new and archived borehole material. The map shows the extent of a very low-relief coastal wetland developed along the tropical southern continental margin of Laurussia. Coastal floodplains in the Midland Valley and Tweed basins were separated from the marginal marine seaway of the Northumberland–Solway Basin to the south by an archipelago of more elevated areas. A complex mosaic of sedimentary environments was juxtaposed, and included fresh and brackish to saline and hypersaline lakes, a diverse suite of floodplain palaeosols and a persistent fluvial system in the east of the region. The strongly seasonal climate led to the formation of evaporite deposits alternating with flooding events, both meteoric and marine. Storm surges drove marine floods from the SW into both the western Midland Valley and Northumberland–Solway Basin; marine water also flooded into the Tweed Basin and Tayside in the east. The Ballagan Formation is a rare example in the geological record of a tropical, seasonal coastal wetland that contains abundant, small-scale evaporite deposits. The diverse sedimentary environments and palaeosol types indicate a network of different terrestrial and aquatic habitats in which the tetrapods lived.

A new tetrapod from Romer's Gap reveals an early adaptation for walking

ABSTRACTA new early tetrapod,Mesanerpeton woodi gen. et sp. nov., collected by Stan Wood from the Ballagan Formation, Tournaisian CM palynozone, at Willie's Hole, Scottish Borders, is described. It includes vertebrae like those ofCrassigyrinus,with poorly developed neural arches, a well ossified ulna with a large olecranon, and a humerus that is structurally intermediate between the pleisiomorphic condition of Devonian taxa and that of all later forms. A comparative analysis of this new material and other tetrapodomorph humeri revealed how an increase in humeral torsion transformed the course of the brachial artery and median nerve through the bone, from an entirely ventral path to one in which the blood vessel and nerve passed through the entepicondyle from the dorsal to the ventral surface. Increasing humeral torsion is suggested to improve walking in early tetrapods by potentially contributing to an increase in stride length, and is one of a number of changes to limb morphology during the Early Carboniferous that led to the development of terrestrial locomotion.

A new terrestrial millipede fauna of earliest Carboniferous (Tournaisian) age from southeastern Scotland helps fill ‘Romer's Gap'

ABSTRACTA diverse millipede (diplopod) fauna has been recovered from the earliest Carboniferous (Tournaisian) Ballagan Formation of the Scottish Borders, discovered by the late Stan Wood. The material is generally fragmentary; however, six different taxa are present based on seven specimens. Only one displays enough characters for formal description and is namedWoodesmus sheariRoss, Edgecombe &amp; Clark gen. &amp; sp. nov. The absence of paranota justifies the erection of Woodesmidae fam. nov. within the Archipolypoda. The diverse fauna supports the theory that an apparent lack of terrestrial animal fossils from ‘Romer's Gap' was due to a lack of collecting and suitable deposits, rather than to low oxygen levels as previously suggested.

Diverse and durophagous: Early Carboniferous chondrichthyans from the Scottish Borders

ABSTRACTChondrichthyan teeth from a new locality in the Scottish Borders supply additional evidence of Early Carboniferous chondrichthyans in the UK. The interbedded dolostones and siltstones of the Ballagan Formation exposed along Whitrope Burn are interpreted as representing a restricted lagoonal environment that received significant amounts of land-derived sediment. This site is palynologically dated to the latest Tournaisian–early Viséan. The diverse dental fauna documented here is dominated by large crushing holocephalan toothplates, with very few, small non-crushing chondrichthyan teeth. Two new taxa are named and described. Our samples are consistent with worldwide evidence that chondrichthyan crushing faunas are common following the Hangenberg extinction event. The lagoonal habitat represented by Whitrope Burn may represent a temporary refugium that was host to a near-relict fauna dominated by large holocephalan chondrichthyans with crushing dentitions. Many of these had already become scarce in other localities by the Viséan and become extinct later in the Carboniferous. This fauna provides evidence of early endemism or niche separation within European chondrichthyan faunas at this time. This evidence points to a complex picture in which the diversity of durophagous chondrichthyans is controlled by narrow spatial shifts in niche availability over time.

Ichnofauna record cryptic marine incursions onto a coastal floodplain at a key Lower Mississippian tetrapod site

Ichnofossils from a Lower Mississippian (Tournaisian) coastal plain succession with a rich vertebrate fauna, including early tetrapods, record repeated, short-lived marine interactions that influenced floodplain lake development and ecosystems. The ichnofauna contrasts with the more typical Scoyenia Ichnofacies of other Devonian-Carboniferous tetrapod sites. In the Tournaisian Ballagan Formation of the Scottish Borders, Chondrites is identified in 128 horizons within a 500-m succession and is associated with lesser occurrences of phycosiphoniform burrows, Diplocraterion, Rhizocorallium and Zoophycos?. Chondrites and phycosiphoniform burrows commonly occur within dolostones, interpreted as part of a saline-hypersaline lake facies association, containing a non-marine to marginal marine macrofauna of bivalves, ostracods, Spirorbis, Serpula and sarcopterygian fish. Marine scolecodonts are reported from 18 horizons of diverse lithology, four of which co-occur with Chondrites. The single-colonisation, single-tier, high ichnofabric index and thin beds (mean 10cm thick) that characterise Chondrites horizons indicate: 1) adverse environmental conditions; 2) rapid colonisation of the sediment; and 3) short-lived bioturbation episodes. Other ichnotaxa identified are Monocraterion and Asterosoma?, which occur exclusively within the overbank or fluvial facies associations, and are here interpreted to have lived in freshwater conditions. The Chondrites trace-makers are interpreted as having originated in shallow-marine waters and were transported into floodplain lakes by storm surge events, although a few examples suggest more significant transgressions. Ichnofossils are much more common than marine body fossils or scolecodonts, and thus record cryptic marine incursions that might otherwise remain unidentified. This study contributes to our understanding of the complex mosaic of environments present when terrestrial tetrapods first appeared.

The terrestrial landscapes of tetrapod evolution in earliest Carboniferous seasonal wetlands of SE Scotland

The Lower Mississippian (Tournaisian) Ballagan Formation in SE Scotland yields tetrapod fossils that provide fresh insights into the critical period when these animals first moved onto land. The key to understanding the palaeoenvironments where they lived is a detailed analysis of the sedimentary architecture of this formation, one of the thickest and most completely documented examples of a coastal floodplain and marginal marine succession from this important transitional time anywhere in the world. Palaeosols are abundant, providing a unique insight into the early Carboniferous habitats and climate.More than 200 separate palaeosols are described from three sections through the formation. The palaeosols range in thickness from 0.02 to 1.85m and are diverse: most are Entisols and Inceptisols (63%), indicating relatively brief periods of soil development. Gleyed Inseptisols and Vertisols are less common (37%). Vertisols are the thickest palaeosols (up to 185cm) in the Ballagan Formation and have common vertic cracks. Roots are abundant through all the palaeosols, from shallow mats and thin hair-like traces to sporadic thicker root traces typical of arborescent lycopods.Geochemical, isotope and clay mineralogical analyses of the palaeosols indicate a range in soil alkalinity and amount of water logging. Estimates of mean annual rainfall from palaeosol compositions are 1000–1500mm per year. The high mean annual rainfall and variable soil alkalinities contrast markedly with dry periods that developed deep penetrating cracks and evaporite deposits. It is concluded that during the early Carboniferous, this region experienced a sharply contrasting seasonal climate and that the floodplain hosted a mosaic of closely juxtaposed but distinct habitats in which the tetrapods lived. The diversification of coastal floodplain environments identified here may link to the evolution and movement of tetrapods into the terrestrial realm.

Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes

AbstractFlood‐generated sandy siltstones are under‐recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix‐supported siltstones with millimetre‐sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil‐bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non‐marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems.

Carboniferous (Tournaisian) fish assemblages from the Isle of Bute, Scotland: systematics and palaeoecology

We describe fish assemblages from the Carboniferous (mid- to late Tournaisian) Ballagan Formation at two localities, Hawk's Nib and Mill Hole, on the Isle of Bute, Scotland. Fossil material occurs in thin, locally reworked dolomitic limestone beds, interpreted as the deposits of very shallow lakes or lagoons, developed on, or adjacent to, a seasonally dry coastal plain. The mostly disarticulated fossils comprise isolated teeth, mandibles, scales, tesserae, dermal bones, lepidotrichia and vertebrae. The fauna includes rhizodonts (cf. Archichthys portlocki, cf. Strepsodus sauroides), lungfish (Sagenodus sp.), other sarcopterygians (Megalichthys sp.), one shark (Ageleodus pectinatus), climatiiform acanthodians and indeterminate actinopterygians. The Mill Hole assemblage is especially noteworthy because it includes some putative juvenile forms (Archichthys and Sagenodus). A critical review of the habitat preferences of the documented taxa suggests that most were either euryhaline or, in the case of Archichthys, probably endemic to brackish or freshwater settings. The Bute fish beds fall within a crucial evolutionary period during which many fish and other animal groups were infiltrating nonmarine environments, either passively or actively. It may be that lakes and lagoons may have functioned as protected nurseries for juveniles during this wave of colonization.

Early Carboniferous stratigraphy in the Firth of Clyde area: new information from the Isle of Bute

Synopsis On the Isle of Bute, the Carboniferous Period is represented mainly by volcanic rocks of the Clyde Plateau Volcanic Formation (Viséan, mid- to late Holkerian). Although previously considered to lie on rocks of the Upper Old Red Sandstone (Bute Conglomerate, mainly Famennian) or Kinnesswood Formation of the Inverclyde Group (mainly Tournaisian), exposed contacts in south Bute show the volcanic rocks to lie disconformably on the lower part of the Clyde Sandstone Formation (higher in that same group). Detailed mapping and measurement of stratigraphic sections at several locations in south Bute reveals the Courceyan to Chadian (Inverclyde) sedimentary succession to comprise the Kinnesswood, Ballagan, and the lower part of the Clyde Sandstone formations. In the Ascog area in the east-central part of Bute, the Birgidale Formation (Viséan; Holkerian) overlies these formations and is conformably overlain by volcanic rocks of the Clyde Plateau Volcanic Formation. Newly discovered exposures of the Ballagan Formation at Dunagoil Bay on the west side of south Bute extend the known distribution of the formation on the island. The relatively thin development of Inverclyde formations in south Bute probably reflects the palaeogeographical position of the area on the northeastern shoulder of the now-submerged North-East Arran Trough. The Inverclyde Group rocks are mostly of terrestrial origin but the Ballagan Formation denotes a short-lived shallow marine incursion. Overall, these rocks record a transition from semi-arid to humid climatic conditions, consonant with the suggestion, based on palaeomagnetic investigations, that at the time of deposition the area migrated northwards through arid to tropical palaeolatitudes.

Evidence that Early Carboniferous ostracods colonised coastal flood plain brackish water environments

A study of the stable isotope composition ( δ 18O, δ 13C) of biogenic (ostracod, mollusc) and authigenic carbonates in the Ballagan Formation, Lower Carboniferous of Scotland, coupled with evidence from sedimentology and associated fossil fauna and flora, supports the argument that this formation was deposited in a coastal flood plain setting, in brackish (0.5 < 30‰ NaCl) and hypersaline (> 40‰ NaCl) waters, but in the absence of persistent normal marine conditions. The oxygen isotope data from the Ballagan Formation divide into three clusters: a diagenetic field defined by low δ 18O (< − 11‰ VPDB); an intermediary field ( δ 18O − 11‰ to − 9‰) composed of a mixture of known primary and secondary (diagenetic) carbonates; and samples within the range of − 9‰ to − 4‰ which, as far as we can ascertain, are largely unaltered. No samples give typical Early Carboniferous δ 18O marine values. Average marine carbonates from Europe have δ 18O between − 4‰ to − 3‰. The Ballagan Formation carbonates were probably deposited in evaporated freshwater and/or brackish water. This conclusion is supported by the presence of evaporites (gypsum, anhydrite, halite pseudomorphs) and common desiccation-cracked mudstone surfaces throughout the Ballagan Formation, suggesting conditions of fluctuating salinity in ephemeral bodies of water. The stable isotope data support the notion that the ostracod assemblages of the Ballagan Formation were colonising brackish water and hypersaline ecologies on a coastal flood plain during the Early Carboniferous, a stage of development that may have encouraged their colonisation of fully non-marine (limnetic) environments during the later Carboniferous. The ostracods include cytherellacean and kloedenellacean species known from marginal marine sites elsewhere, but probably tolerant of brackish water, podocopid species such as ‘ Bythocypris’ aequalis that may have been adapted for brackish water settings on coastal flood plains (ephemeral lakes and lagoons), and paraparchitacean-dominated assemblages that may signal harsh (hypersaline or desiccating) environments.