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Geological Society, London, Special Publications; 1986; v. 20; p. 237-245;
DOI: 10.1144/GSL.SP.1986.020.01.24
© 1986 Geological Society of London

Assemblages of Silurian graptolites in various facies of the East Baltic region

J. Paskevicius

Department of Geology and Mineralogy, University of Vitnius, 232031 Vitnius, Ciurlionio 21, Lithuania S.S.R., U.S.S.R.

All the facies beginning with black clay to carbonate mud in a complete and continuous geological section with graptolites (from the cyphus Zone to the lochkovensis Zone) can be distinguished in the Lithuanian Silurian. The assemblages of graptolites were determined, and an attempt was made to analyse dependence of the graptolites on various facies. During this work it was ascertained that the most numerous general species and individuals in the assemblages are found at the junctions of clayey and carbonate facies. These assemblages must have been developed on the continental slope of the Silurian sea. The number of species in general slightly decreases in the direction of the past open sea (black-clay facies), to say nothing of the shelf zone (the carbonate facies), where graptolites are especially scarce. Here one can trace a certain analogy between the distribution of zooplankton of the present-day seas and that of the graptolites, which composed the macrozooplankton of the seas of the geological past.

Thus the graptolites being a pelagic group of the zooplankton of the past, lived, developed and were widespread in the deeper part (probably in mezopelagic) of the open sea, mostly beginning at the continental slope.