THE MID-PALAEOZOIC BIOTIC CRISIS: SETTING THE TRAJECTORY OF TETRAPOD EVOLUTION

This project will shed light on a key stage in the evolution of life on Earth. The advent onto land of limbed vertebrates (tetrapods) was an event that shaped the future evolution of the planet, including the appearance of humans. The process began about 360 million years ago, during the late Palaeozoic, in the early part of the Carboniferous Period. Within the 20 million years that followed, limbed vertebrates evolved from their essentially aquatic and fish-like Devonian predecessors into fully terrestrial forms, radiating into a wide range of body forms that occupied diverse habitats and ecological niches. We know this because we have an adequate fossil record of the earliest limbed vertebrates from the Late Devonian, contrasting with the terrestrial forms that lived significantly later in the Early Carboniferous, about 340 million years ago. It is also clear that a mass extinction event occurred at the end of the Devonian, following which life on land and in fresh water habitats had to be re-established. Unfortunately, the formative 20 million years from the end of Devonian times has remained almost unrepresented for fossil tetrapods and their arthropod contemporaries. Thus, we know little about how tetrapods evolved adaptations for life on land, the environments in which they did so, and the timing or sequence of these events. The evolutionary relationships among these early tetrapods and how they relate to modern forms are also unclear and controversial as a result of this lack of fossil information. The entire fossil hiatus has been called 'Romer's Gap' after the American palaeontologist who first recognized it. Now, for the first time anywhere in the world, several fossil localities representing this period have been discovered in south-eastern Scotland. They have already provided a wealth of new fossils of tetrapods, fish, invertebrates and plants, and our team is the first to have the opportunity to study this material and the environmental, depositional, and climatic context in which this momentous episode took place. We have a number of major aims. The existing fossil material will form a baseline for this study, but the project will augment this by further excavating the richest of the sites so far found and subjecting it to a detailed archaeological-style analysis. We will collect from other recently recognized sites and explore for further sites with relevant potential. The fossil material will be described and analysed using a range of modern techniques to answer many questions related to the evolution of the animals and plants. Not only that, using stratigraphical, sedimentological, palynological, geochemical and isotopic data, we will establish the conditions of deposition that preserved the fossils, the environments in which the organisms lived and died, and the precise times at which they did so. We will drill a borehole that will core through the entire geological formation in which these fossils have been found. Using this, we will integrate data from our fossil sites using the signals provided by the sedimentary record to build a detailed time line showing in which horizons the fossils were found, the age of each occurrence and their sequential relationship. We will compare and correlate our data with that from contemporaneous deposits in Nova Scotia, the only other locality with information sufficiently rich to be meaningful. Our data will allow us to infer changes to the environment and the evolutionary trajectories of the animals and plants during the deposition of this formation, covering the 20 million years following the end-Devonian mass extinction. Comparison with similar data for the Late Devonian will allow us to chart the changes around the time of the mass extinction, to infer its causes and consequences, and obtain a detailed record of exactly how changes to the environment correlated with changes to the fauna and flora.

The earliest Carboniferous (Tournaisian) interval (360-340 Ma) has long formed a 'bottleneck' in studies of the evolution of terrestrial ecosystems, and particularly the earliest evolution of terrestrial tetrapods, because of the almost complete lack of fossil evidence from this key time. We are now able to populate this hiatus in fossil data as a result of recent discoveries in Scotland, and to place these specimens in a palaeoecological and stratigraphic context.
Given the current paucity of data previously available for study of the faunas and floras from this time, and the profound changes to terrestrial ecosystems that took place then, we fully expect our results to be literally and metaphorically ground breaking. The material we will discover and describe will be of international significance, and enhance the reputation of the UK as a centre of excellence for Palaeozoic tetrapod fossils and their study. We anticipate publication in high profile journals including Science and Nature. Our palaeontological results will benefit interrelated disciplines from anatomists through molecular phylogeneticists to palaeoecologists. Biomechanics of locomotion, feeding, and breathing will gain from the new insight into the basal anatomical conditions at the onset of terrestriality. Studies of the evolutionary development of skeletal systems, and the timing of key innovations in tetrapod morphological adaptations will use our findings. Molecular phylogenies will benefit from new calibration points for the origins of the tetrapod and actinopterygian crown groups. The geological aspects of the proposal will benefit those modelling ancient climates in deep time, environmental and sedimentary systems, and their influence on and relationship to key evolutionary events. The refined stratigraphical, isotopic and palynological data that our studies provide will augment the, so far, relatively poorly known picture of this key period. Our comprehensive dataset from the earliest Carboniferous of Scotland will provide a future standard of comparison for contemporary deposits in other parts of the world. The industry-standard geophysical log data will be of interest to petrophysicists in academia and their industry partners, from including civil engineers, the construction industry and the hydrocarbon industries, who are studying the characterisation and prediction of physical properties in UK rock formations. As a team, we are particular well placed for high impact in the press and other media, with two members situated in nationally and internationally renowned museums, and with close links to others. Both institutions have professional in-house officers who deal with outreach on a continuing basis. Our experience talking to media representatives, the general public and other non-academics including people local to our sites, suggests that this project is of wide public interest and appeal.