Lavacystis sagittalis gen. et sp. nov. from the Lower Ordovician of the Leningrad Region—the Earliest Known Pleurocystitid (Echinodermata, Rhombifera) and the First Record of the Family from the Baltic Paleobasin

Lavacystis sagittalis Rozhnov gen. et sp. nov. (family Pleurocystitidae, Rhombifera, Blastozoa), the first representative of the family found in the Baltic region, is described. It comes from the Lower Ordovician (Floian) in the Leningrad Region and is therefore the earliest known reliable representative of Pleurocystitidae. The new genus is characterized by a small theca, the absence of pectinirhombs, an unusual, wide, bilaterally symmetrical, crescent-shaped theca, and the periproct, occupying about a third of the ventral surface and located on the lateral, pointed lobe of the theca. The composition of the thecal plates, theca shape, the size of the periproct, and the absence of pectinirhombs preclude its assignment to any of the previously recognized subfamilies.