Arthropoda (e.g. insects, crustaceans, spiders and centipedes) comprises the majority of animal biodiversity and includes model organisms like the fruitfly Drosophila melanogaster. A rich fossil record, abundant genomic information, high morphological disparity, and unparallelled diversity, have made arthropods a key model system for macroevolution. Yet, our understanding of arthropod evolution remains incomplete. In part, this is because alternative interpretations of their morphology support different evolutionary hypotheses. For example, the phylogeny of total-group Chelicerata (i.e. the living chelicerates – e.g. sea spiders, horseshoe crabs, spiders and scorpions – and all fossils more closely related to these than to any other arthropod) depends on alternative interpretations for the origin of the chelicerae. These pincer-like first head appendages are found in all living chelicerates, but it is unclear whether they are primitive, derived, or converging characters. Central to this debate are the sea spiders (Pycnogonida), a poorly-understood, marine, chelicerae-bearing lineage, dissimilar in many respects to other chelicerates. It has been suggested that pycnogonids might not be chelicerates and that chelicerae might be convergent or a primitive trait for Arthropoda, which would necessitate a reassessment of early arthropod evolution. Here, I will use a cutting edge computed tomography approach to provide the first detailed study of fossil pycnogonid morphology, and to generate a new morphological dataset spanning all key arthropod taxa. Bayesian and Maximum Likelihood-based total evidence approaches will then be used to combine the new morphological dataset with genomic data and test hypotheses of (1) chelicerate and arthropod relationships, (2) the evolution of the arthropod body plan, (3) the origin and diversification rate of the highly divergent Pycnogonida, and (4) to estimate an evolutionary timescale for Pycnogonida and Chelicerata.
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