Brachiopods of the Order Spiriferida at the Devonian–Carboniferous Boundary of Transcaucasia

Abstract

A distinct change in the taxonomic and ecological structure of assemblages of brachiopods of the order Spiriferida is observed at the Devonian and Carboniferous boundary of Transcaucasia. Representatives of the superfamily Cyrtospiriferoidea dominated in the assemblages in the Late Devonian, and became extinct by the end of the Devonian, while the Early Carboniferous is dominated by the superfamily Spiriferoidea. Cyrtospiriferoids represented a complicated subtype of the anchored ecological type ethologically associated with hard substrates, while the spiriferoids represented the main subtype of the anchor ecological type and the weighted subtype of the free-lying ecological type, replacing each other in ontogeny of brachiopods inhabiting mostly soft substrates. Although the Devonian-Carboniferous boundary in Transcaucasia lies within a continuous succession of lithologically uniform carbonate and carbonate-terrigenous rocks, a change in the diversity and ecological type of spiriferids indicates a change in the density of the substrate. This corresponds to a general global shift from the Devonian calcite sea to an aragonite Carboniferous sea, which affected the density of marine substrates. Thus, the stratigraphic procedure in this case can include the diversity analysis of both the taxonomic and ecological compositions of brachiopods.