The oldest bifoliate cystoporate and two other bryozoan taxa from the Dapingian (Middle Ordovician) of north-western Russia

Abstract

AbstractBryozoans from the Dapingian (Middle Ordovician) of the Baltic paleobasin remain poorly studied and their taxonomic composition is unclear. In this paper, three bryozoan taxa, a bifoliate cystoporatePlanopora volkhovensisn. gen. n. sp., a trepostomeHemiphragma insolitumn. sp., and an esthonioporateEsthoniopora claraKoromyslova, are described from Dapingian deposits of an unusual clayey-calcareous Hecker-type mudmound on the right bank of the Volkhov River in Leningrad Oblast’, north-western Russia. Combined X-ray microtomography, scanning electron microscopy, and light microscopy of thin sections were used to characterize their morphology. Analysis of the stratigraphic distribution of early cystoporate bryozoans suggests thatPlanoporan. gen. is the oldest сystoporate bryozoan with an erect, bifoliate colony. The growth modes of these bryozoans are discussed. The colonies ofP.volkhovensisn. gen. n. sp. andE.clarahave an attachment structure, a holdfast, at their base, probably indicating their attachment to sponge spicules. The bryozoanH.insolitumn. sp. produced rod-like colonies, formed by overgrowing the problematic tubular fossilSphenothallusHall. It can be assumed that sponges with unfused siliceous spicules and individuals ofSphenothalluswere numerous on the surface of the mudmound during its formation and provided a suitable substrate for settlement of bryozoan larvae.UUID:http://zoobank.org/5715872a-8c61-446e-9413-d713fdef1a08