The primitive antiarch Yunnanolepis from China : a microtomographic study

• Placoderms first appeared during the Silurian, after which they diversified and spread rapidly to dominate the Devonian seas. One of the stratigraphically earliest placoderms is Yunnanolepis, a primitive antiarch from the Early Devonian of China and Vietnam. To date, little specific research has been devoted to yunnanolepids, and previous assessments have utilised conventional descriptive methods. In contrast, this study constructed exceptionally detailed 3D-models based on a synchrotron X-ray microtomographic scan series of Yunnanolepis material from the Muséum National d'Histoire Naturelle, Paris. Our novel data set revealed a well-preserved anterior ventrolateral plate (AVL) with the opening for the pectoral fin, and other hitherto unknown structures such as the transverse crista, postbranchial lamina, and external ornamentation. The first 3D image of the mysterious ’Chang's apparatus’ was also generated. ‘Chang's apparatus’ in known only in Yunnanolepididae, and its function remains unknown. Disarticulated tooth plates and scales of other gnathostomes were also found with the specimen, and include very small AVL plates of young antiarchs. These lack ornamentation and their postbranchial laminae are weakly developed compared to osteologically more mature individuals.

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