Systematic position, distribution, and shell structure of the Devonian linguloid brachiopod Bicarinatina bicarinata (Kutorga, 1837)

The type species of the brachiopod genus Bicarinatina Batrukova was first described by Kutorga in 1837 (Zweiter Beitrag zur Geognosie und Paläaontologie Dorpats und seiner nächsten Umbegebungen. St.-Petersbourg, 51 pp.) as Lingula bicarinata from the Middle Devonian sandstone beds now assigned to the Aruküla Regional Stage in Tartu, Estonia. Gravitis (1981, in Devon i karbon pribaltiki (Brangulis et al., eds), pp. 455–462. Zinatne, Riga) described eight new species of the genus Bicarinatina. Revision of the collections in St Petersburg and Riga showed that four of these species names should be considered as subjective junior synonyms of the type species Bicarinatina bicarinata (Kutorga) and that the type specimens of the remaining four new species were either juvenile or too poorly preserved for species-level identifications. Therefore, B. bicarinata should be considered the only valid species of the genus Bicarinatina occurring in the Middle Devonian on the Baltic plate, known from Estonia, Latvia, and northwestern Russia. The shell structure of B. bicarinata is baculate symmetrical, with the finest baculate sets in the innermost parts of the valves and thicker and aggregated baculi in outer parts, due to stronger taphonomic changes in outer parts of the shell. In cross section, the valves are 70–100 µm thick.