Silurian retiolitids of the East European Platform

Late Llandovery to late Ludlow isolated retiolitid assemblages from deep boreholes drilled in the Polish part of the East European Platform are used to restore phylogeny of the group. The process of reduction of the network skeleton (reticulum and clathria), as well as reduction of the transverse rods and change of virgula position started in the Wenlock. All Ludlow retiolitids have a central virgula and reduced rhabdosomes. Colonies of the Gothograptus lineage with finite growth, related to reduction in the number of thecae, first appeared in the Late Wenlock and continued in the Ludlow. Their evolution leads either to development of finite rhabdosomes with few thecae (Holoretiolites) or to almost complete reduction of clathria and reticulum (Plectodinemagraptus gen. n. of the Plectograptus lineage). The latter homeomorph of the Ordovician abrograptid Dinemagraptus is the last known retiolitid of late Ludlow age (Cucullograptus hemiaversus / C. aversus Zone). The family Retiolitidae consists of five separate lineages. Sokolovograptus polonicus sp. n., Neogothogroptus gen. n., Holoretiolites atrabecularis sp. n., Semiplectograptus urbaneki gen. et sp. n., Plectodinemagraptus gracilis gen. et sp. n. are proposed. S. polonicus from the Cyrtograptus murchisoni Zone has a strongly reduced rhabdosome.