The exoskeleton of Ungelaspis and Ateleaspis (Osteostraci, Agnatha) from the Lower Devonian of Severnaya Zemlya, Russia

The fragments of osteostracan exoskeleton from the Lower Devonian Severnaya Zemlya Formation of the Severnaya Zemlya archipelago, viz., Ungulaspis arctoa AFANASSIEVA & KARATAJDTE-TALIMAA, 1998 and Ateleaspis sp., were studied. The structure of the exoskeleton of non-tremataspid osteostracans was investigated by SEM for the first time. A reconstruction of the structure of the Ungulaspis arctoa exoskeleton is suggested. Because of the poorer preservation of the Ateleaspis sp., only preliminary observations on the thin structure of its exoskeleton could be made. The forms under investigation are characterized by a similar type of sculpture on the surface of the shield (small tubercles) and on the body scales (thin crests). Some similarities in the structure of their shield exoskeleton (the presence of distinctly expressed tesserae, the presence of radiating canals, the probable absence of perforated septa and pore fields) were indicated. Ungulaspis arctoa is remarkable in the possessing macromorphological features typical of both the Ateleaspis- and Scolenaspis-like osteostracans. This similarity in the structure of exoskeleton of the investigated forms provides additional evidence in support of the affinity of these groups of osteostracans and their possible origin from common ancestors.