Determining the dynamics of the colonization of the pelagic realm and evolution of the early Palaeozoic biological pump

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The biological pump has been critical for supporting biodiverse life in the oceans since the first animals evolved. This pump transports nutrients fixed by primary producers near the ocean surface to diverse bottom dwelling animal communities, and facilitates the oxygenation of the deep sea. The coevolution of animals with the biological pump established more efficient nutrient cycling, and was a key factor for the diversification of animals during the early Palaeozoic (c. 520 million years ago, mya). Throughout animal history, the biological pump has controlled the composition and ecological structure of the marine biosphere, and its breakdown is associated with mass extinction events.Animals that live in the water column (pelagic) are crucial for the vertical transport of nutrients in the modern biological pump, with arthropods a dominant component for the last 500 million years. Arthropods (a group that includes modern millipedes, shrimp and insects) dominate marine biomass, consuming phytoplankton and each other. They contribute carcasses and faecal pellets to sinking aggregates and move vertically in the water column, fuelling deep water ecosystems. The first colonization of the pelagic realm by arthropods in the early Palaeozoic was likely critical to strengthening the biological pump and driving the diversification of early animals. The fossil record provides evidence that arthropods were critical to establishing an animal-dominated biological pump in the Cambrian (520 mya), and its strengthening in the Ordovician (490 mya). However, our understanding of the tempo and dynamics of the colonization of the pelagic realm by arthropods is incomplete. This is because determining whether extinct animals were pelagic or benthic relies on combining multiple sources of data relating to the morphology of the animal, as well as geological context, not all of which are available for all species.I will apply a Bayesian network approach to quantitatively assess the ecology of extinct arthropods and determine their changing contribution to the pelagic realm through the early Palaeozoic. This approach combines fossil data (e.g., geological context, morphology) and ecology (e.g., pelagic/benthic, feeding mode), objectively assigning a life mode for extinct animals. The network will be 'trained' using data from modern arthropods (e.g. lophogastrids, phyllocarids, ostracods), and applied to morphologically-similar Palaeozoic forms (e.g., bivalved arthropods, bradoriids, phosphatocopines, ostracods). As this approach can handle missing data, and thus utilize less well known and more poorly preserved taxa, I will construct a high temporal resolution record of the life mode of large and small extinct arthropods, thus determining the tempo and dynamics of the colonization of the pelagic realm by this ecologically important phylum. Thereby I will provide the first quantitative assessment of the diversity of arthropods in the pelagic realm over broad timescales, covering the Cambrian explosion and Great Ordovician Biodiversification Event, as well as the recovery periods after major mass extinctions (end Ordovician and Permian-Triassic).Thus, I will investigate the changing contribution of arthropods to the pelagic realm and evolution of the first biological pump, determine how quickly it recovered after mass extinctions, and its impact on the diversity of animals living close to the seafloor. This has implications not just for understanding the coevolution of animals and the first animal-dominated biological pump, but can also be used to make predictions about the impact of humans on the oceans and the knock-on effects of ocean acidification and overfishing on biodiversity.