Search

The known diversity of tetrapods of the Devonian period has increased markedly in recent decades, but their fossil record consists mostly of tantalizing fragments1–15. The framework for interpreting the morphology and palaeobiology of Devonian tetrapods is dominated by the near complete fossils of Ichthyostega and Acanthostega; the less complete, but partly reconstructable, Ventastega and Tulerpeton have supporting roles2,4,16–34. All four of these genera date to the late Famennian age (about 365–359 million years ago)—they are 10 million years younger than the earliest known tetrapod fragments5,10, and nearly 30 million years younger than the oldest known tetrapod footprints35. Here we describe Parmastega aelidae gen. et sp. nov., a tetrapod from Russia dated to the earliest Famennian age (about 372 million years ago), represented by three-dimensional material that enables the reconstruction of the skull and shoulder girdle. The raised orbits, lateral line canals and weakly ossified postcranial skeleton of P. aelidae suggest a largely aquatic, surface-cruising animal. In Bayesian and parsimony-based phylogenetic analyses, the majority of trees place Parmastega as a sister group to all other tetrapods. Three-dimensionally preserved fossils of Parmastega aelidae, a newly described tetrapod from the earliest Famennian (Late Devonian) of Russia, provide detailed insights into the morphology and palaeobiology of the earliest tetrapods.

The enigmatic tetrapod Acherontiscus caledoniae from the Pendleian stage of the Early Carboniferous shows heterodontous and durophagous teeth, representing the earliest known examples of significant adaptations in tetrapod dental morphology. Tetrapods of the Late Devonian and Early Carboniferous (Mississippian), now known in some depth, are generally conservative in their dentition and body morphologies. Their teeth are simple and uniform, being cone-like and sometimes recurved at the tip. Modifications such as keels occur for the first time in Early Carboniferous Tournaisian tetrapods. Acherontiscus , dated as from the Pendleian stage, is notable for being very small with a skull length of about 15 mm, having an elongate vertebral column and being limbless. Cladistic analysis places it close to the Early Carboniferous adelospondyls, aïstopods and colosteids and supports the hypothesis of ‘lepospondyl’ polyphyly. Heterodonty is associated with a varied diet in tetrapods, while durophagy suggests a diet that includes hard tissue such as chitin or shells. The mid-Carboniferous saw a significant increase in morphological innovation among tetrapods, with an expanded diversity of body forms, skull shapes and dentitions appearing for the first time.

Stan Wood had a gift for finding exceptional Early Carboniferous fossils. Among them are 32 type specimens. His discoveries significantly changed our understanding of the history of life on Earth. Many of the fossils he collected are on display in museums across the UK and the localities he discovered continue to yield important new material. Here we briefly review some of Stan Wood's key achievements and describe the legacy he left.

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.

Chondrichthyan 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.

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.

The early tetrapod Crassigyrinus scoticus was a large aquatic predator from the mid Carboniferous (late Viséan or early Serpukovian) of Scotland, around 330 My in age. There are five main specimens with cranial remains: an articulated skeleton; two incomplete skulls; and two lower jaws. Crassigyrinus retains several apparently primitive features of the palatal dentition and lower jaw, and its phylogenetic position is disputed. A partial lower jaw resembling that of Crassigyrinus was discovered at Burnmouth in the Borders region of Scotland. The horizon in which it was found is dated as late Tournaisian, CM palynozone, around 350 My in age. Though it lacks dentition, the jaw preserves much of the postsplenial, angular and surangular, whose appearance externally and internally is almost identical to that of C. scoticus. Internally, the jaw shows a similarly limited extent of the suturing between the splenial series and the prearticular, a primitive condition. Externally, the type and distribution of dermal ornamentation closely matches that of C. scoticus, as does the deeply excavated and marginally positioned lateral line groove. As well as external and internal features, all specimens of C. scoticus are of similar skull size, though the Burnmouth jaw is somewhat smaller. If correctly attributable to Crassigyrinus, this specimen extends the existence of the genus by approximately 20 million years towards the base of the Carboniferous.

The end-Devonian to mid-Mississippian time interval has long been known for its depauperate palaeontological record, especially for tetrapods. This interval encapsulates the time of increasing terrestriality among tetrapods, but only two Tournaisian localities previously produced tetrapod fossils. Here we describe five new Tournaisian tetrapods ($\textit{Perittodus apsconditus}$, $\textit{Koilops herma}$, $\textit{Ossirarus kierani}$, $\textit{Diploradus austiumensis}$ and $\textit{Aytonerpeton microps}$) from two localities in their environmental context. A phylogenetic analysis retrieved three taxa as stem tetrapods, interspersed among Devonian and Carboniferous forms, and two as stem amphibians, suggesting a deep split among crown tetrapods. We also illustrate new tetrapod specimens from these and additional localities in the Scottish Borders region. The new taxa and specimens suggest that tetrapod diversification was well established by the Tournaisian. Sedimentary evidence indicates that the tetrapod fossils are usually associated with sandy siltstones overlying wetland palaeosols. Tetrapods were probably living on vegetated surfaces that were subsequently flooded. We show that atmospheric oxygen levels were stable across the Devonian/Carboniferous boundary, and did not inhibit the evolution of terrestriality. This wealth of tetrapods from Tournaisian localities highlights the potential for discoveries elsewhere.

Romer’s Gap, the interval following the end-Devonian extinction event, has been described as a post extinction trough for vertebrates. It is a time roughly equivalent to the Tournaisian stage of the early Carboniferous, and has been characterized by a lull in diversity of survivors. Lungfish typified this description. One species was known from one locality. Recently, a diverse collection of lungfish tooth plates, representing seven new forms, was recovered from new Tournaisian vertebrate localities in northern Britain. They display a range of previously unknown morphologies, with tooth shape and wear patterns not seen in other post-Devonian forms. A comparison of tooth ridge number and tooth ridge angle in lungfishes from the Famennian, Tournaisian and Visean reveals marked differences between Late Devonian and early Carboniferous taxa. The most common tooth plate shape in the Famennian is absent from our sample of Tournaisian taxa. Two completely new shapes have evolved, one with a relatively low tooth ridge angle, no greater than 40^o, and in which most of the tooth ridges are essentially parallel, and the other with a much higher tooth ridge angle of up to 180^o where the tooth ridges are highly divergent. This high level of morphological diversity over a narrow time period suggests that, following the end-Devonian extinction, gaps in ecospace left by the extinction of major groups of fishes were exploited by a previously unrecorded radiation of lungfishes. Whilst taxonomic diversity of lungfishes declined following the end-Devonian extinction, recovery and diversification among tooth-plated forms was rapid, and morphological disparity amongst these forms subsequently increased. Contrary to previous assumptions morphological disparity among lungfish did not decline until much later in the Carboniferous.

The cranial anatomy of the iconic early tetrapod Eusthenopteron foordi is probably the best understood of all fossil fishes. In contrast, the anatomy of the lower jaw - crucial for both phylogenetics and biomechanical analyses - has been only superficially described. Computed tomography data of three Eusthenopteron skulls were segmented using visualization software to digitally separate bone from matrix and individual bones from each other. Here, we present a new description of the lower jaw of Eusthenopteron based on microcomputed tomography data, including the following: detailed description of sutural morphology and the mandibular symphysis; confirmed occurrence of pre- and intercoronoid fossae on the dorsal aspect of the lower jaw; and the arrangement of the submandibular bones. Furthermore, we identify a novel dermal ossification, the postsymphysial, present on the anteromedial aspect of the lower jaw in Eusthenopteron and describe its distribution in other stem tetrapod taxa. Sutural morphology is used to infer load regimes and, along with overall skull and lower jaw morphology, suggests that Eusthenopteron may have used biting along with suction feeding to capture and consume large prey. Finally, visualization software was used to repair and reconstruct the lower jaw, resulting in a three-dimensional digital reconstruction.