Thelodonts (Agnatha) from the Lower Silurian (Wenlock) deposits of the northwest of Belarus

The paper presents data on the Lower Silurian (Wenlock) thelodonts found in the northwestern part of the Belarussian Anteclise in the Ostrovets region. Thelodont scales identified in four boreholes (Ostrovets 1, Ostrovets 7ö, Ostrovets 73 and Ostrovets 195) suggest the Silurian age of their enclosing deposits rather than the Devonian (Late Emsian-Eifelian) age as is shown in geological maps. Other groups of the fauna (conodonts, bryozoans, brachiopods, scolecodonts, etc.) also confirm a conclusion that the Devonian deposits are absent within the Ostrovets area, and the Lower Silurian rocks (Llandovery and Wenlock series) underlie the Cenozoic (Neogenic) deposits in this region. Besides, the lithological, geophysical and micropalaeontological evidences were used to distinguish the new Ostrovets Formation in the studied region. Therefore much attention was given to a comprehensive study of rocks from the Ostrovets 1 borehole, which exposes the maximum thickness of the Ostrovets Formation. In other boreholes the studied formation was not completely exposed. Data on the stratigraphic distribution of vertebrates and other fossil groups in the Lower Silurian deposits are cited. A brief taphonomic characterization of thelodont remains is presented. The external morphology and internal structure of thelodont scales were described and their micro- and SEM photographs were made. The Lower Silurian (Llandovery and Wenlock) deposits were correlated with the coeval deposits from palaeontologically best studied regions of Estonia and Sweden. New data obtained evidently suggest that the limits of distribution of both Devonian, and Silurian deposits need to be defined more exactly in geological maps of the northwestern part of the territory of Belarus.