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The Early Jurassic (Toarcian) actinopterygian Pachycormus is a basal taxon within Pachycormiformes, a Mesozoic marine neopterygian radiation that evolved an extreme ecomorph divergence between hyperspecialized ‘billfish-like’ macrocarnivores and gigantic suspension feeders, including the largest fish of all time. Current phylogenies place Pachycormus as an early member of the suspension-feeding lineage; however, species disparity renders character states uncertain and potential exists for considerable intraspecific variability. Given its importance for resolution of pachycormiform phylogenetic topology, we comprehensively reassessed Pachycormus fossils housed in collections across Europe and found that the proportional traits traditionally used to discriminate between species are actually consistent with an ontogenetic size morphocline. Our cladistic analyses further show that the monotypic senior synonym, Pachycormus macropterus, is a wildcard that manifests a mosaic of transitional states. This has significant implications for hypothesized Toarcian marine vertebrate provincialism because P. macropterus had a ubiquitous Boreal Tethyan distribution. Moreover, our tree-based palaeobiogeographical optimizations infer that the western Tethyan region was a pachycormiform dissemination center, with global dispersals occurring through transoceanic migration and invasion of epeiric basins. Citation for this article: Wretman, L., H. Blom, and B. P. Kear. 2016. Resolution of the Early Jurassic actinopterygian fish Pachycormus and a dispersal hypothesis for Pachycormiformes. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1206022.

International audience; Abstract Background Firm attachments binding muscles to skeleton are crucial mechanical components of the vertebrate body. These attachments (entheses) are complex three-dimensional structures, containing distinctive arrangements of cells and fibre systems embedded in the bone, which can be modified during ontogeny. Until recently it has only been possible to obtain 2D surface and thin section images of entheses, leaving their 3D histology largely unstudied except by extrapolation from 2D data. Entheses are frequently preserved in fossil bones, but sectioning is inappropriate for rare or unique fossil material. Methodology/Principal Findings Here we present the first non-destructive 3D investigation, by propagation phase contrast synchrotron microtomography (PPC-SRµCT), of enthesis histology in extant and fossil vertebrates. We are able to identify entheses in the humerus of the salamander Desmognathus from the organization of bone-cell lacunae and extrinsic fibres. Statistical analysis of the lacunae differentiates types of attachments, and the orientation of the fibres, reflect the approximate alignment of the muscle. Similar histological structures, including ontogenetically related pattern changes, are perfectly preserved in two 380 million year old fossil vertebrates, the placoderm Compagopiscis croucheri and the sarcopterygian fish Eusthenopteron foordi. Conclusions/Significance We are able to determine the position of entheses in fossil vertebrates, the approximate orientation of the attached muscles, and aspects of their ontogenetic histories, from PPC-SRµCT data. Sub-micron microtomography thus provides a powerful tool for studying the structure, development, evolution and palaeobiology of muscle attachments.