Resolution of the Early Jurassic actinopterygian fish <i>Pachycormus</i> and a dispersal hypothesis for Pachycormiformes

<p>The Early Jurassic (Toarcian) actinopterygian <i>Pachycormus</i> 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 <i>Pachycormus</i> 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 <i>Pachycormus</i> 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, <i>Pachycormus macropterus</i>, is a wildcard that manifests a mosaic of transitional states. This has significant implications for hypothesized Toarcian marine vertebrate provincialism because <i>P. macropterus</i> 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.</p> <p>Citation for this article: Wretman, L., H. Blom, and B. P. Kear. 2016. Resolution of the Early Jurassic actinopterygian fish <i>Pachycormus</i> and a dispersal hypothesis for Pachycormiformes. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1206022.</p

Two acanthodian tooth whorls (SEM photos with line drawings).

<p><b>A–C,</b> GIT 658-1; <b>D–F,</b> GIT 658-2. <b>A,D.</b> Oral views, labial to the right. <b>B,E.</b> Sketch interpretations of A,D. <b>C,F.</b> Lateral views, labial to the right. <b>Abbreviations: t1-t3,</b> tooth element 1–3 belonging to the linear growth serial; <b>t0?,</b> possible origination element of the linear growth serial; <b>o,</b> disorganized odontodes on the labial base.</p

The scanned scale of <i>Andreolepis hedei</i> Gross, 1968.

<p><b>A.</b> Scanned part of the scale (PMU 24786) rendered in VG Studio 2.1, crown view, red line marking the position of the slice in <b>B</b>. <b>B.</b> Longitudinal virtual thin section from the synchrotron scanning data, the red rectangle marking the region in <b>C</b>. <b>C.</b> Close-up of <b>B</b>, showing the segmentation of odontodes and the tissue composition of the crown (picture exported from VG Studio 2.1). The numbers in circles correspond to the odontodes in following <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071890#pone-0071890-g003" target="_blank">Figures 3</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071890#pone-0071890-g004" target="_blank">4</a>. <b>D</b>. A real thin section (PMU 24784) from an <i>Andreolepis</i> body scale made in the anteroposterior plane for comparison (DIC, optical microscopy); the scale is slender and comes from the posterior region of the body <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071890#pone.0071890-Chen1" target="_blank">[28]</a>. <b>Abbreviations: e</b>, enamel; <b>d</b>, dentine; <b>hc</b>, horizontal vascular canal, <b>O1-O5</b>, odontodes 1–5, <b>Shb,</b> Sharpey’s fibers.</p

The reconstructed growth pattern of odontodes in the scanned scale of <i>Andreolepis</i>, anterolateral view.

<p>For explanations see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071890#pone-0071890-g003" target="_blank">Figure 3</a>.</p

Bazzi et al_Data_Dryad_Supplementary_File_3

Tps file containing two-dimensional landmark data

The reconstructed growth pattern of odontodes in the scanned scale of <i>Andreolepis</i>, crown view.

<p><b>A–J.</b> The referred sequential addition of odontodes – in the crown of the scale. The first generation odontodes (odontodes – , see text) form a growth series, but the other younger odontodes ( – ) do not necessarily fall neatly into the same sequence even though they generally continue to get larger; the yellow horizontal vascular canal system is used as landmark to show the positions of the odontodes <b>K</b>. Crown view of the scale with buried odontodes, showing the actual surface composition of the scale. Note that the most dorsal denticles compose the enamel layers from both odontode and , odontode is partially overlapped by ; odontode is only overlapped by and posteriorly and exposed to the surface otherwise.</p

R source code

Programming commands essential for replicating our geometric morphometric and statistical analyses are here given. The main source code (Bazzi et al_Dryad_MasterRScript) is accompanied by separate R scripts (functions and analytical loops) which can be sourced directly provided the working directory has been set

Bazzi et al_Data_Dryad_Supplementary_File_1

Extended experimental procedures, results, and discussion