Abstract
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.
Highlights
Wysogórski (1900) described the evolutionary trends of the Orthis calligramma group and differentiated the concavo-convex brachiopods as a separate linage with the species Orthis moneta Eichwald, 1861, O. moneta mut. Wysogórski, 1900, O. actonia Sowerby 1839, typ. and O. actonia var. Wysogórski, 1900
The new species Leoniorthis rubeli differs from Cyrtonotella semicircularis and from all other Upper Ordovician species assigned to the genus Cyrtonotella in its essentially smaller size, fascicostellate radial ornament and the features of dorsal interior
The results of this work and revision of previous data on the stratigraphical range of the genus Cyrtonotella can be summarized as follows: the first appearance of the genus Cyrtonotella does not fall into the early Darriwilian Volkhov Stage
Summary
Wysogórski (1900) described the evolutionary trends of the Orthis calligramma group and differentiated the concavo-convex brachiopods as a separate linage with the species Orthis moneta Eichwald, 1861, O. moneta mut. Wysogórski, 1900, O. actonia Sowerby 1839, typ. and O. actonia var. Wysogórski, 1900. Wysogórski, 1900 These species were later included in the genus Nicolella (Öpik 1930; Alikhova 1953; Williams in Whittington & Williams 1955; Oraspõld 1959). This linage has a lateral branch with two species Orthis semicircularis (Eichwald, 1829) and an informally identified transitional species O. semicircularis mut. Both species differ from O. moneta in their fascicostellate ornamentation. Wysogórski are much smaller than the specimens of O. semicircularis typ. (Terebratula semicircularis of Eichwald 1829), which occurs stratigraphically at a somewhat higher level
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