Reconstruction of Charniodiscus yorgensis (Macrobiota from the Vendian of the White Sea)

Reconstruction and new description of the rare species Charniodiscus yorgensis Borchvardt et Nessov, 1999, which has previously been represented by a single, presently lost specimen, is provided. New material collected in the type locality and closely situated outcrops of Zimnii Bereg of the White Sea allows reconstruction of the structure of this fossil form in more detail. Like other species of the genus Charniodiscus Ford, 1958, the specimen of C. yorgensis, replaced by pyrite initially nonmineralized skeleton of an organism, consisted of a bilobate frond with the rachis passing downwards into the basal disk. It is shown that each lobe of the frond of C. yorgensis was an entire membrane arranged in transverse folds, as an accordion. Plicate lateral lobes, with their free margins directed upwards, adjoin sharp ridges of folds on either side of the membrane. Semipouches formed of bends of lateral lobes were probably the bases of tubular or seedlike elements of branches of the second order, recognized in C. oppositus Jenkins et Gehling, 1978 and C. arboreus (Glaessner, 1959) from Australian localities. Unique records of C. yorgensis buried in an upstanding position corroborate the well-known hypothesis of the vertical lifetime orientation of Charniodiscus.