The Primitive Antiarch Yunnanolepis from China : A Microtomographic Study

Sammanfattning: Placoderms appeared in the Silurian period and rapidly evolved and dominated the waters during the Devonian period. Among the earliest of the placoderms are yunnanolepids, a primitive form of antiarch from the Early Devonian of China and Vietnam. Very little work has so far been devoted to this family, and previous descriptions are based on conventional preparation methods. We can now take a closer look at a yunnanolepid specimen with the help of three dimensional modelling based on a synchrotron X-ray microtomographic scan series. The well preserved anterior ventrolateral plate exhibits an opening for the pectoral fin, clearly visible structures, and outer ornamentation. A first 3D image of the mysterious hollow called Chang’s apparatus has been constructed. Chang’s apparatus has only ever been seen in Yunnanolepididae, but never described in detail, and its function remains unknown. The trunk plate has captured settling sediment and in the inwash can be found tooth plates and scales of other gnathostomes. Among these are two well preserved anterior ventrolateral plates of young antiarchs. No ornamentation can be seen on the juveniles and their postbranchial lamina had not yet developed.