New data on the headshield of Parayunnanolepis xitunensis (Placodermi, Antiarcha), with comments on nasal capsules in antiarchs

The diversity in positions of nasal capsules has been well documented in various subgroups of placoderms or jawed stem gnathostomes. However, the condition in primitive antiarchs (i.e., the Yunnanolepidoidei) remains unclear, and the positional shift of nasal capsules in the Antiarcha has never been mentioned. Here we re-describe the headshield of Parayunnanolepis xitunensis, a yunnanolepidoid antiarch from the Lower Devonian of China, using X-ray computed tomography scanning. The study provides new anatomical details of the headshield including the sclerotic capsules, the visceral side of the rostral and pineal plates, and the dorsal profile of the endocranium. Parayunnanolepis xitunensis bears a pair of nasal capsules underlying the rostral plate between the eyes, resembling Romundina but differing from most euantiarchs, whose nasal capsules are encased in the preorbital recess in front of the eyes. A phylogenetic analysis based on a modified matrix indicates a stepwise forward migration of nasal capsules within antiarchs, accompanied by the innovation of the preorbital recess and the loss of the preorbital depression.