Report
Teaser, summary, work performed and final results
Periodic Reporting for period 3 - TERRA (375 Million Years of the Diversification of Life on Land: Shifting the Paradigm?)
Teaser
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...
Summary
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 Previous analyses of tetrapod diversification have been based on an outdated and problematic dataset that is likely to artificially inflate apparent diversity towards the present day, and have not taken into account variation in the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 are being 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 are being used to test different hypotheses of diversification, the influence of changes in fossil record sampling, and (where possible) the relationship of diversity patterns to changes in global environments (e.g. sea level and atmospheric composition). 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
The project has achieved the majority of planned goals for the first reporting period. Four team members (one research assistant, one PDRA, two PhD students) were appointed, received training, and have all made significant and successful contributions to the project.
A key part of the work in this reporting period has been data collection and compilation. This has primarily taken two forms:
(a) We have been contributing significant volumes of new data, and improving existing data, on the stratigraphic and geographic distribution and taxonomy of terrestrial tetrapods within the Paleobiology Database (work package 1). PhD1, the PI and the research assistant have been conducting focused and intense data entry for the Palaeozoic-early Mesozoic (Carboniferous–Triassic), adding >500 new localities to the Database and editing and improving the data for many more. Palaeozoic data was previously one of the major gaps in the Database, but is now close to completion, with >960 localities and thousands of species occurrences. We have also been adding in data for some of the more poorly known intervals of the Cenozoic, and have added or edited >140 localities. Continuing to improve the Cenozoic data is a major goal for the next scientific period.
(b) We have been compiling and analysing datasets on fossil record completeness through time for major vertebrate groups (work package 2). This work has been led by PhD2, the PI and the research assistant, but has also involved undergraduate students at the University of Birmingham who have compiled and analysed data as part of their research training. Data collection is complete for saurischian dinosaurs (theropods and sauropodomorphs), several key groups of marine reptiles (including plesiosaurs and Triassic sauropterygians), pterosaurs, temnospondyl amphibians and bats. In addition, we have been compiling existing datasets for groups including anomodonts, parareptiles, ichthyosaurs and birds. This data collection has been targeted to allow us to address questions of differential fossil record completeness between the marine and terrestrial realms, between different body size and taphonomic classes, and across major extinction events. We have also been generating new data on taxonomic \'diagnosability\' of preserved fossil remains - i.e. how useful taxonomic and phylogenetic information is distributed through the vertebrate skeleton, and how preservation of this information is impacted by taphonomy.
We have carried out extensive analytical work (falling primarily within work packages 1, 2 and 5) with existing and newly compiled datasets. This has allowed us to already produce eight peer-reviewed publications, and one manuscript currently in review. The major analytical work that has been conducted is as follows:
(i) Development of improved methods for estimating diversity change in deep time, and new approaches to testing the relative performance of existing and widely used methods for estimating diversity change in deep time (i.e. SQS, TRiPS). This work has been conducted by the PDRA and the PI. We have developed a new approach that allows fossil diversity to be estimated while holding the geographic spread of sampled localities constant, thus allowing us to for the first time deal with biases that result from changes in geographic sampling through time (Close et al. 2017, Nature Communications). To compare the performance of diversity methods, we have used a combination of simulations and a novel approach that uses historical data on the accumulation of fossil record sampling to test how diversity estimates change in response to the accumulation of new fossil discoveries. Our results clearly demonstrate that diversity estimators that interpolate based on coverage of the underlying species-abundance distribution (e.g. SQS) are the best methods available for estimating relative changes in diversity through time (Close et al. 2018, Methods in Ecology & Evolution).
(ii) Analysis of
Final results
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. Between now and the end of the project, we expect to complete and publish work on alpha diversity of terrestrial tetrapods through the Phanerozoic, updated estimates of diversity change and spatial diversity patterns for the marine realm with comparisons to terrestrial diversity, estimates of changes in terrestrial tetrapod gamma diversity across the entire Phanerozoic, work on terrestrial and marine tetrapod fossil record completeness, and one or more synthetic review papers drawing together the project results and making them accessible to a diverse audience of evolutionary biologists and palaeobiologists.