Heterostracans are pivotal in examining the evolution of feeding and the head in jawed vertebrates as the stem-gnathostomes in which the oral apparatus is best understood. However, most analyses of their feeding, especially recent studies using modern 3D imaging methods, have focused on a specific subgroup, the Pteraspidiformes. Athenaegis chattertoni from the Wenlockian of Canada is the stratigraphically oldest heterostracan to preserve an articulated feeding apparatus and is critical for reconstructing ancestral states in the lineage. Here, we use X-ray and Synchrotron microtomography to re-examine the oral apparatus of Athenaegis, revealing several differences from the original reconstruction. Most importantly, the oral region of Athenaegis consists of numerous imbricated oral plates which functioned as a single integrated unit rather than a V-shaped arrangement separated by a medial seam. We show that Athenaegis, contrary to the original description, does not possess a medial postoral plate. The oral region was not capable of opening substantially, as shown by the interdigitation of the oral plates, a close articulation with the surrounding plates, and an absence of muscle or ligament attachment points. We conclude a microphagous suspension-feeding function of the oral apparatus for Athenaegis consistent with previous hypotheses of suspension feeding in heterostracans.

All material imaged here is housed in the collections of the Laboratory for Vertebrate Paleontology, University of Alberta (UALVP), Canada. UALVP39923 and UALVP29926 were scanned using a Nikon Metrology XTH 225ST X-ray CT machine at the XTM Facility, Palaeobiology Research Group, University of Bristol, UK. The holotype, UALVP15682, was scanned using a Nikon Metrology XTH 225ST X-ray machine at the Permafrost ArChive Science (PACS) CT laboratory at the University of Alberta, Edmonton, Canada. UALVP29926 was additionally scanned using synchrotron X-ray micro-computed tomography at BM18 of the European Synchrotron Radiation Facility (ESRF) to further detail the three-dimensional oral plate morphology. 

The resulting image stacks were visualised in ImageJ 1.53o (Schneider et al. 2012) for cropping and improving contrast. Data were segmented in Avizo 2021.1 (ThermoFisher Scientific) and Mimics v.23 ((http://biomedical.materialise.com/mimics; materialize) to reconstruct the 3D morphology. Manual Avizo segmentation was completed via AI-assisted interpolation using the web-based application Biomedisa (Lösel et al. 2020). Avizo 3D models were visualised in MeshLab 2022.02 (Cignoni et al. 2008). Mimics surface meshes were exported into and imaged in Blender v. 3.4.1 (http://blender.org; Blender Foundation).

Segmented elements and fragments of Athenaegis specimen UALVP 29926 were imported as 3D models into Blender for retrodeformation (v. 3.0.0). Steps taken during retrodeformation followed processes described by Lautenschlager (2016) and Herbst et al. (2022). Further information regarding the retrodeformation process can be found in Schnetz et al. 2025. 

References

• Cignoni, P., Callieri, M., Corsini, M., Dellepiane, M., Ganovelli, F., and Ranzuglia, G. 2008. MeshLab: an Open-Source Mesh Processing Tool. ERCIM News 2008, 45–46.
• Herbst, E. C., Meade, L. E., Lautenschlager, S., Fioritti, N. and Scheyer, T. M. 2022. A toolbox for the retrodeformation and muscle reconstruction of fossil specimens in Blender. Royal Society Open Science 9, 220519.
• Lautenschlager, S. 2016. Reconstructing the past: methods and techniques for the digital restoration of fossils. Royal Society Open Science 3, 160342. 
• Lösel, P. D., van de Kamp, T., Jayme, A., Ershov, A., Faragó, T., Pichler, O., Tan Jerome, N., Aadepu, N., Bremer, S., Chilingaryan, S. A., Heethoff, M., Kopmann, A., Odar, J., Schmelzle, S., Zuber, M., Wittbrodt, J., Baumbach, T., and Heuveline, V. 2020. Introducing Biomedisa as an open-source online platform for biomedical image segmentation. Nature Communications 11, 5777. https://doi.org/10.1038/s41467-020-19303-w. 
• Schneider, C. A., Rasband, W. S., and Eliceiri, K. 2012. NIH Image to ImageJ: 25 years of image analysis. Nature Methods 9, 671–675. https://doi.org/10.1038/nmeth.2089.
• Schnetz et al. 2025. A revised interpretation of the oral anatomy of Athenaegis chattertoni Soehn & Wilson (Vertebrata, Heterostraci) from the Silurian of Northwest Territories, Canada. Palaeontology.