Revision of the concept of the orthide brachiopod Cyrtonotella in the Middle Ordovician of the East Baltic

摘要

The two lowermost Darriwilian (Middle Ordovician) brachiopod species that were previously included in the genus Cyrtonotella are re-studied and compared with the type species Cyrtonotella semicircularis (Eichwald) from the St Petersburg Region. Based on the new data the classification of the important group of orthids, widely distributed in the Sandbian Kukruse to Keila regional stages in the East Baltic, is improved. The lower Darriwilian specimens, previously considered to be Cyrtonotella semicircularis, are ascribed to the genus Leoniorthis as the new species Leoniorthis rubeli, which occurs in the St Petersburg Region and in northern Estonia. The other species Cyrtonotella pakriensis Rubel is included in the new genus Rogorthis, which is restricted to the sandy limestones of the Pakri Formation (Kunda Stage) in northern Estonia. The type species of the genus Cyrtonotella, C. semicircularis (Eichwald), probably occurs in the upper Darriwilian Aseri Regional Stage in the St Petersburg Region, and is the first appearance of that genus.