The fragments of osteostracan exoskeleton from the Lover Devonian Severnaya Zemlya Formation of the Severnaya Zemlya archipelago, viz., Ungulaspis arctoa AFANASSIEVA & KARATAJUTE-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 shied (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.
New osteostracans (Agnatha, Osteostraci) Zenaspis dzieduszyckii sp. nov. and Wladysagitta janvieri gen. et sp. nov., as well as new form of Diademaspis are described from the Lower Devonian of Podolia (Ukraine). Among them Zenaspis dzieduszyckiisp. nov. differs from all other representatives of the genus by its smaller overall size and relatively larger orbits. Wladysagitta janvieri is remarkable by having a rostral process. This new species together with “Cephalaspis” acutirostris Stensiö, 1932 from Old Red deposits of Shropshire (Great Britain) are proposed to be united under common generic name Wladysagitta. Since Diademaspis sp. is completely unknown in outline of its shield, it could not be named as new species, although it should be a really new form. New diagnostic features, such as relative width of the orbits, relative width of the dorsal field, and the position of the posterior end of the lateral fields relatively to the margins of the headshield, are proposed to differentiate genera within the Zenaspididae. Probably, the rostral process of osteostracans should be considered as a multifunctional device having at least a sensory function for food−search, possibly hydrodynamic functions, or may play a role in scaring predators.
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