On the tail of Errivaspis and the condition of the caudal fin in heterostracans

Articulated caudal fins of heterostracans are preserved in very rare cases. Their morphology and internal structure are completely unknown for the majority of species. One of the few preserved caudal fins belongs to the Early Devonian pteraspidid Errivaspis waynensis from the Welsh Borderland, UK. There are two different reconstructions of the tail: (1) strongly asymmetric, with a considerably longer ventral lobe, and a rather concave posterior margin (e. g. White 1935); and (2) almost symmetrical with an only slightly longer ventral lobe and a small notch in the posterior margin (e. g. Blieck 1984). A new reconstruction of the Errivaspis caudal fin, based on a detailed study of White's specimens is presented here. The ventral lobe is longer than the dorsal one but not so much as in White's reconstruction, and the notch in the posterior margin of the fin is less deep. The comparatively massive ventral lobe and 'en echelon' arrangement of three digitations, supporting the fin web and branching off from this lobe, indicate the hypocercal condition of the asymmetrical tail. The same internal structure is present in other heterostracans with nearly symmetrical caudal fins (e. g. Doryaspis, Nahanniaspis, Dinaspidella and Athenaegis) where all, or most, of the intermediate digitations branch off from the ventral lobe.