Decoding the fossil record of embryology at the dawn of animal evolution

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Embryology, the study of development, is pivotal to unravelling the evolutionary history of animals, and how changes to patterns of development have produced the branching events in the Tree of Life. Palaeontology can provide the only direct tests of such hypotheses but, traditionally, it has been silent in such debates because of a dearth of embryological data preserved in the fossil record. The outlook for evolutionary biology and for palaeontology was changed at once with the discovery of rich and diverse assemblages of fossil embryos in rocks deposited at the time that major animal groups were first emerging. These fossils have a huge potential for understanding developmental evolution at this critical episode of evolutionary history, but it is largely unrealised because of difficulties in distinguishing preserved biological structure from later geological mineralization, and because it is difficult to understand the 3D anatomy of an individual fossil embryo let alone the changes that took place during development from one embryological stage to another. These challenges have led to wildly differing interpretations of which animal group the fossil embryos belong to - a debate that must be concluded before evolutionary conclusions can be made. In fact the situation is so bad that although the fossil embryos are often beautifully preserved, including cellular and sub-cellular details, some scientists even doubt they belong to animals, preferring instead to interpret them as giant bacteria. Our interpretation of the fossils will be aided by analysis using a high resolution version of a medical CT scanner, which is powered by a particle accelerator and allows us to analyse the 3D anatomy of a fossil embryo, including its internal structure, based on a virtual computer model. This allows us to analyse details of an embryo's anatomy that are less than a millionth of a millimetre, revealing details of cell movements and even sub-cellular structure that are necessary to determine the embryology, affinity and, ultimately, the evolutionary importance of these half billion year old fossils. Our CT scans also reveal differences in the chemical composition of preserved biological and geological structures, aiding the interpretation of the fossils. We will determine the basis of these chemical differences and use the results to judge between conflicting interpretations of controversial fossil embryos and adults from the same geological layers. These experiments will be conducted to better understand known fossil embryos, but also to uncover the anatomy of fossil embryos and larvae that are completely new to science, in the collections of our international team which includes almost every leading scientist in the field of fossil embryos. We will also undertake rotting experiments to understand the decay of (i) giant bacteria and, (ii) the subcellular structures in animal embryos. These data will be used to guide our interpretation of the biological structure in the fossils, allowing us to decide whether the bacteria or animal-embryo model of interpretation fits best. Ultimately, our aim is to uncover the embryology of fossil embryos as a way of working out their closest living relatives, and whether the pattern of embryology has evolved between the fossil embryos and their living relatives. Collectively, these answers will reveal the role of embryological evolution in one of the most fundamental and most-debated episodes in animal evolutionary history: the establishment of the modern animal groups that we see about us today.