Periodic Reporting for period 1 - TERRA (375 Million Years of the Diversification of Life on Land: Shifting the Paradigm?)
Reporting period: 2015-07-01 to 2016-12-31
Summary of the context and overall objectives of the project
Life on land today is spectacularly diverse, representing 75–95% of all species on Earth. However, it remains unclear how this extraordinary diversity has been acquired across deep geological time. Is diversity on land today truly much higher than at any point in the geological past? This research project is addressing this major knowledge gap by reassessing the dominant paradigm of terrestrial diversification, an exponential increase in diversity over the last 375 million years, using the rich and well-studied fossil record of tetrapods (four-limbed vertebrates) as an exemplar group. Previous analyses of tetrapod diversification have been based on an outdated and problematic dataset that is likely to artificially inflate apparent diversity towards the present day, and have not taken into account variation in the sampling of the tetrapod fossil record through time. We are compiling a major new dataset, detailing the spatial and temporal distribution of terrestrial tetrapods across their entire fossil record in unprecedented detail. These data will be analysed using the latest statistical approaches in order to yield new estimates of diversity patterns for terrestrial tetrapods through deep time. We are also quantifying temporal variation in the completeness of the terrestrial tetrapod fossil record, and comparing it to the marine tetrapod and invertebrate records. Our estimates of diversity change will be compared to alternate diversification models, as well as to changes in rock record sampling, and (where possible) to changes in global environments (e.g. sea level and atmospheric composition) and marine diversity. These comparisons will allow us to address the following key questions: (i) Does terrestrial diversification follow an exponential pattern over the last 375 million years? (ii) Is the terrestrial fossil record as complete as the marine fossil record? (iii) Are long-term patterns of terrestrial diversification driven by physical changes in the Earth system such as climate change? (iv) Did marine and terrestrial biodiversity follow similar trajectories across geological time? (v) How did mass extinction events impact upon terrestrial tetrapod diversification? Our work aims to establish a new, rigorous paradigm for the long-term pattern of terrestrial diversification, and test and identify its drivers.
Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far
The project has achieved all planned goals for the first reporting period. Four highly-qualified team members (one research assistant, one PDRA, two PhD students) have been appointed, have received training, and are all making significant and successful contributions to the project. The main tasks in this first phase of the project have been data collection. This has primarily taken two forms. First, we have been contributing significant volumes of new data, and improving existing data, on the stratigraphic and geographic distribution of terrestrial tetrapods within the Paleobiology Database. This data collection in this phase has mainly focused on Palaeozoic and early Cenozoic tetrapods. Second, we have been compiling and beginning to analyse datasets on fossil record completeness through time for major vertebrate groups, including plesiosaurs, dinosaurs, pterosaurs, bats and late Mesozoic–early Cenozoic mammals. In addition to data collection, we have also been developing improved methods for estimating diversity change in deep time, specifically by allowing diversity to be estimated while holding the geographic spread of sampled localities constant, and testing the performance of existing and widely used methods for estimating diversity change in deep time (i.e. SQS, TRiPS). We have also applied some of these methods to the most mature part of the Paleobiology Database for terrestrial tetrapods, the Mesozoic–early Cenozoic, in order to make initial attempts at answering some of the key questions of the TERRA project. Initial results suggest the following: (a) Mesozoic–early Cenozoic data do not support exponential diversification at regional scales when uneven sampling is accounted for. Instead, diversity appears to be relatively static at the regional level; (b) this stasis is dramatically impacted upon by the Cretaceous-Palaeogene mass extinction, with diversity reaching a new, much higher equilibrium in the aftermath of the extinction. This suggests that mass extinctions play a key role in restructuring ecosystems and resetting limits to diversification; (c) climate may have a strong influence on changes in diversity of individual tetrapod clades through time, as demonstrated by our data for crocodylians; (d) the marine fossil record of vertebrates may be better (in terms of skeletal completeness) than that of terrestrial vertebrates. These provisional results will be expanded in phase 2 of the project by using a broader range of datasets covering a broader temporal span. Three project-related papers have already been published in leading peer-related journals, with a further four in review, and research results have been presented at seven international conferences.
Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)
The initial results and project outputs have already significantly advanced the field beyond the state of the art. Our main results from the Mesozoic–early Cenozoic data challenge prevailing hypotheses of unconstrained, exponential diversification of terrestrial biodiversity through geological time, suggesting instead that ecosystem diversity may have been constrained at regional level over long geological periods, and then fundamentally restructured by major extinction events. These results require confirmation from larger datasets and longer timescales, but they have the potential to fundamentally alter our understanding of how terrestrial biodiversity has changes through deep time. We have also developed innovative new methods that refine our ability to reliably test changes in diversity in the fossil record.
Record Number: 196472 / Last updated on: 2017-03-29