The regions of South Siberia provide a huge amount of important
palaeontological data to improve the existing interregional
stratigraphical and palaeobiogeographical constraints. This paper is
addressed to the late Silurian brachiopods from the West Sajan. In
total, 36 species (31 genera, 21 families) have been identified. The
finds of brachiopods in the Shishtykskaya Formation have been
examined. These localities have been investigated: 1) the Usinskaya
Depression, middle part of the Usa River; 2) middle part of the
Bolshie Uri River; 3) left bank of the Ak-Khem River, and 4) the
Karatosh River. Brachiopods help to establish precise correspondences
between the formations of the West Sajan and some adjacent regions
like Tuva and Gornyj Altaj. The presence of the both Ludlowian and
Pridolian is confirmed within the study region. Very interesting is an
attempted analysis of brachiopod communities and a
palaeobiogeographical interpretation of its results (see Figs. 6,
7). The West Sajan belong to the Uralian- Cordilleran Region. The
northeastern part of the former was dominated by the Subrianid Fauna,
whereas its southwestern part was dominated by the Tuvaella Fauna. The
latter is considered as a representative of the Central Asian
Province. The regional evolution and palaeobiogeographic
differentiation of the late Silurian brachiopods within the West Sajan
were governed by the water depth, salinity, and regional
transgressions/regressions. An ecological boundary of the Central
Asian Province is suggested. These taxa are described and illustrated
in Brachiopoda /Pelmatozoa 935 the reviewing paper: Leptaena sp.,
Mesodouvillina sp., Mclearnites prosperus KULKOV, 1985,
Mesoleptostrophia filosa (SOWERBY, 1839), Prochonetes cf. striatellus
(DALMAN, 1828), Morinorhynchus williamsi (KULKOV, 1967), Dolerorthis
sp., Isorthis initalensis (TCHENYSHEV, 1937), Schizophoria cf.
paraprima JOHNSON, BOUCOT & MURPHY, 1973, Conchidium biloculare
(HISINGER, 1799), Lamelliconchidium tchergense KULKOV, 1968,
Rhynchotreta cuneata (DALMAN, 1828), Hemitoechia daphne (BARRANDE,
1879), H. ubsuensis (TCHERNYSHEV, 1937), H. nuculaeformis (KULKOV,
1974), Stegerhynchus angaciensis (TCHERNYSHEV, 1937), S. borealis (VON
BUCH, 1834), S. dichotomus sp. nov., Machaeraria nymphaeformis
(NIKIFOROVA, 1937), Atrypina cf. erugata AMSDEN, 1968, Plectatrypa
imbricata (SOWERBY, 1839), Atrypa reticularis (LINNAEUS, 1758),
Atrypoidea operosa (KULKOV, 1967), Meristoidea sajanica gen. et
sp. nov., Merista sp., Didymothyris didyma (DALMAN, 1828).,
Nucleospira pisum (SOWERBY, 1839), Cyrtia cf. tuvaensis E. IVANOVA,
1962, Eospirifer sp., Howellella nucula (BARRANDE, 1879),
H. angustiplicata (KOZLOWSKI, 1929), Altajella sajanica sp. nov.,
Tannuspirifer cf. pedaschenkoi (TCHERNYSHEV, 1937), T. posterus
KULKOV, 1967, Sajanospirifer uriensis gen. et sp. nov., and Spirinella
sp. Two remarks should accompany the present review. First, the late
Silurian is an informal time unit, which comprises such formal units
as the Ludlow and the Pridoli. The latters were only adopted by the
International Commission on Stratigraphy (GRADSTEIN et
al. 2004). Second, this study deals with formations, whereas it is
incorrect to call the mentioned lithostratigraphic units as
suites. The term “suite” is recommended neither to sedimentary nor to
metamorphic successions (SALVADOR 1994). Undoubtedly, this paper
presents many interesting information, and, therefore, it will be very
useful for all Silurian brachiopodologists and specialists in the
mid-Paleozoic stratigraphy of South Siberia and Central Asia.
References GRADSTEIN, F. M., OGG, J. B. & SMITH, A. G. (2004): A
geologic time scale 2004. – XIX + 589 pp., 112 figs., 86 tables., 1
encl.; Cambridge (Cambridge University Press). SALVADOR, A. (ED.)
(1994): International stratigraphic guide. A guide to stratigraphic
classification, terminology, and procedure. – 214 pp.; Boulder,
Colorado (Geological Society of America; International Subcommission
on Stratigraphic Classification).
D. A. RUBAN
Zentralblatt Geo. Pal. T. II Jg. 2007 H. 5/6