Scan files, 3D reconstructions, data spreadsheet and supplementary files for Heterochrony and parallel evolution of echinoderm, hemichordate and cephalochordate internal bars

Deuterostomes comprise three phyla with radically different body plans.
Phylogenetic bracketing of the living deuterostome clades suggests the
latest common ancestor of echinoderms, hemichordates and chordates was a
bilaterally symmetrical worm with pharyngeal openings, with these
characters lost in echinoderms. Early fossil echinoderms with pharyngeal
openings have been described, but their interpretation is highly
controversial. Here, we critically evaluate the evidence for pharyngeal
structures (gill bars) in the extinct stylophoran
echinoderms Lagynocystis
pyramidalis and Jaekelocarpus
oklahomensis using virtual models based on high-resolution X-ray
tomography scans of three-dimensionally preserved fossil specimens.
Multivariate analyses of the size, spacing and arrangement of the internal
bars in these fossils indicate they are substantially more similar to gill
bars in modern enteropneust hemichordates and cephalochordates than to
other internal bar-like structures in fossil blastozoan echinoderms. The
close similarity between the internal bars of the stylophorans L.
pyramidalis and J. oklahomensis and the gill
bars of extant chordates and hemichordates is strong evidence for their
homology. Differences between these internal bars and bar-like elements of
the respiratory systems in blastozoans suggest these structures might have
arisen through parallel evolution across deuterostomes, perhaps
underpinned by a common developmental genetic mechanism.