Title: Late Cretaceous Rhynchonellida (Brachiopoda) from Bulgaria. I. Genus Cyclothyris MʼCoy
Abstract: Brachiopods are one of the most widespread fossil groups among the Late Cretaceous macrofauna in Northern Bulgaria. This article is the first from a series of palaeontological descriptions of the Rhynchonellida representatives. The results from detailed taxonomic studies of seven species of the genus Cyclothyris M’Coy from the Cenomanian, Santonian and Campanian are presented. The literature concerning Cyclothyris is reviewed discussing in details its volume and range of variability. A wider conception of the genus volume is proposed supporting the point of view of the authors who consider Lamellaerhynchia younger synonym of Cyclothyris. The method of serial sections was applied for the first time when studying the internal morphology of Late Cretaceous rhynchonellids from Bulgaria. Having sufficient amount of material, more samples were grinded in order to establish the range of intraspecific variability. Palaeoecological data on the studied rhynchonellids and shell microstructure of the two of the taxa are presented.