New Data on Pycnosteus palaeformis Preobrazhensky (Heterostraci, Psammosteiformes) from the Arukula Deposits of the Leningrad Region

New materials on the psammosteid heterostracan Pycnosteus palaeformis Preobr. characteristic of the Aruküla deposits (Middle Devonian, Eifelian) of the Main Devonian Field are presented. A pentagonal rounded, slightly convex dorsal plate unknown earlier is described. This plate bears a shallow notch at its anterior margin. A low crest that served for the attachment of the axial musculature extends medially on the visceral surface, it. The shape of the cornual plate is reconstructed upon its overlap surface over the branchial plate. This species is characterized by the unusually strongly convex keel-shaped ventral and weakly convex dorsal sides, as well as a wide spread of the supporting planes of the branchial plates. This allowed the animal to stay away from the bottom longer than other psammosteids. Morphological armor features and trunk reconstruction suggest a planktonophagous type of feeding for this animal.