Tremadocian monoplacophoran molluscs from the Pechora Basin of Russia
Alexander P. Gubanov1 and Olga K. Bogolepova1
1 Department of Earth Sciences, Uppsala University, Norbyvägen 22, SE-752 36, Uppsala, Sweden. E–mail: firstname.lastname@example.org
Key words: Monoplacophoran molluscs. Tremadocian. Pechora Basin. Baltica. Russia.
In the Pechora Basin of northeastern Baltica, the Bol’shepul’skaya–1 drill–hole penetrated coarse-grained sandstones unconformably overlying Riphean metamorphic rocks at a depth of 1587 m. About 50 metres above these unfossiliferous sandstones, grey mudstones yield the abundant well–preserved monoplacophoran mollusc Kirengella cf. kultavasaensis Doguzhaeva. Higher in the drill-hole (interval of 1246-1252 m) are siltstones intercalated with limestones with rare acrotretid brachiopods and conodonts which suggest a late Tremadocian age for these strata. Kirengella kultavasaensis was originally described from the Lower Ordovician Kidryasovo Formation of the southern Urals.
Kirengella cf. kultavasaensis is very similar in morphology to Kirengella ayaktchica Rozov 1968, which has been described for the first time from the Upper Cambrian–Lower Ordovician transitional beds in the southern (the Kirenga River) and northwestern (the Kulyumbe River) parts of the Siberian platform, though the Pechora molluscs have more laterally compressed shell. Kirengella is also known from the uppermost Cambrian Minaret Formation of Antarctica and the Lower Ordovician strata of the Ozark Uplift of Missouri. Kirengella was reported from Severnaya Zemlya as well, but our studies do not support this assignment, as molluscs have much wider, coiled shell that is charcteristic of Helcionella rather than Kirengella.
In general, large–size monoplacophoran molluscs became one of the dominant group of shallow-water fauna during the Late Cambrian–Early Ordovician and had a world-wide distribution. Such faunas are known from Australia, Antarctica, North China, North America, Baltica and Siberia. Due to rare presence of non-molluscan fossils in these shallow–water sediments, monoplacophorans are of considerable value for biostratigraphic correlation and palaeobiogeographic reconstruction.
Received: February 15, 2003
Accepted: June 15, 2003