Trilobites from biohermal carbonate buildups of the Volkhov Regional Stage (Middle Ordovician) in North-Western Russia

Rich and special trilobite fauna was discovered in micritic limestone of large carbonate buildps located in vicinity of St. Petersburg (Volkhovian RS, Dapingian – early Darrivillian). Dimeropygids Celmus granulatus (Volb.), a group of three unusual asaphids: Porgebovites, Ottenbyaspis and Panderia and endemic raphiophorids “Globampyx” linnarssoni (Schm.) dominate in core facies of bioherms (named “Gekker-type mud-mounts”) together with accessory small trilobites. Some other species have had inhabitat in cavities or pockets within these carbonate buildups. Those are as follows: Pliomera aff. fischeri (Eichw.), Diaphanometopus volborthi (Schm.) and Ptychometopus volborthi (Schm.). Trilobite fauna of these carbonate buildps drastically differs from non-biohermal fauna. This fauna contains relicts of earliest ages such as cosmopolite distributed trilobites (at genera or family level) which lived in these refuges long time from Floian or earliest Dapingian till first half of Darriwillian surrounded by highly endemic trilobites “Baltic-type” fauna of non-biohermal bottom of the basin.