The three-dimensionally articulated oral apparatus of a Devonian heterostracan sheds light on feeding in Palaeozoic jawless fishes

Attempts to explain the origin and diversification of vertebrates have commonly invoked the evolution of feeding ecology, contrasting the passive suspension feeding of invertebrate chordates and larval lampreys with active predation in living jawed vertebrates. Of the extinct jawless vertebrates that phylogenetically intercalate these living groups, the feeding apparatus is well-preserved only in the early diverging stem-gnathostome heterostracans. However its anatomy remains poorly understood. Here we use X-ray microtomography to characterise the feeding apparatus of the pteraspid heterostracan Rhinopteraspis dunensis (Roemer, 1855). The apparatus is composed of thirteen plates arranged approximately bilaterally, most of which articulate from the postoral plate. Our reconstruction shows that the oral plates were capable of rotating around the transverse axis, but likely with limited movement. It also suggests the nasohypophyseal organs opened internally, into the pharynx. The functional morphology of the apparatus in Rhinopteraspis precludes all proposed interpretations of feeding except for suspension/deposit feeding and we interpret the apparatus as having served primarily to moderate the oral gape. This is consistent with evidence that at least some early jawless gnathostomes were suspension feeders and runs contrary to macroecological scenarios that envisage early vertebrate evolution as characterised by a directional trend towards increasingly active food acquisition.

Methods
Rhinopteraspis dunensis NHMUK PV P 73217 was scanned using a Nikon Metrology XTH 225ST X-ray tomography instrument based in Bristol Palaeobiology, University of Bristol. Two scans were undertaken, each composed of two stacked scans. The first scan included the whole headshield at a voltage of 223 kV and a current of 193 μA, with 3141 projections, and with a 1 mm Sn filter, obtaining a dataset with 63.41 μm voxel resolution. The second scan targeted the oral region specifically, including the posterior third of the rostrum, the orbital, oral, and pineal parts of the headshield, and the anterior quarter of the dorsal and ventral discs, at a voltage of 180 kV and a current of 178 μA, with 3141 projections of 8 frames and 708 ms each, and with a 0.5 mm Cu filter, achieving a voxel resolution of 22.91 μm.The resulting tomographic datasets were segmented in Mimics v.25 (materialise) to create 3D models.