Boreskov

PP‐53FROM OFFSHORE TO ONSHORE:A NEW RANGEOMORPH SPECIES FROM CENTRAL URALKolesnikov A.V.Novosibirsk State University, Novosibirsk, RussiaAn onshore to offshore evolutionary pattern is evident in benthic marine invertebrateclades of the Paleozoic and post‐Paleozoic age. Ordinal rank taxa are observed to originateonshore, diversify offshore, and eventually relinquish nearshore habitat (Jablonski et al.,1983). An important exception to this pattern is found in rangeomorphs, a group of LateProterozoic foliate organisms that have a body divided by numerous commissures intochambers resembling ramified tubules. It is the oldest group of Vendian macroorganisms, assuggested by the fossils found in the Drook Formation of the Conception Group ofNewfoundland with U–Pb‐zircon age 578.8 ± 0.5 Ma (Van Kranendonk et al., 2008).Theoretical growth modeling of rangeomorph structural elements has demonstrated thatthe growth strategies used by these organisms allowed them to maintain high surface areato volume ratios despite their macroscopic size, which is consistent with morphologicalexpectations for osmotrophs (Laflamme et al., 2009). Rangeomorphs originated offshore indistal low‐energy shelf where they dominated sessile macrocommunities until their suddenmysterious disappearance 560 My ago (with a notable exception of Charnia masoni). Thereis growing evidence, however, that 560 My ago rangeomorphs migrated from distal lowenergyshelf to higher energy proximal habitats. For example, the rangeomorphs Rangeaschneiderhoehni and Bomakellia kelleri from the time interval of 560–545 Ma are confined todistributary channels of prodelta. In 2003 fossils of a new rangeomorph species (see figure)was found in the Krutikha Member of the Chernokamen Formation cropping out along thebanks of the Usva River in the Central Urals. Preliminary study allows the followingconclusions to be drawn: (1) The new rangeomorph species is characterized by unusuallylarge chambers and relatively simple arrangement of commissures; hence, it would beincapable of attaining the surface area to volume ratios necessary for strict osmotrophy. (2)The Krutikha Member is correlated on the basis of sequence stratigraphy with the ZiganFormation of the South Urals; volcanic tuffs within Zigan Formation in a section near Ust‐Katav gave a U–Pb‐zircon age of 548.2 ± 7.6 Ma which makes the new species one of theyoungest members of the group. (3) The Krutikha Member is characterized by chanellizedsandstones with multistory cross‐bedding, casts of desiccation cracks, salt crystalpseudomorphs, and weathered surface paleohorizons; the new species, therefore, wasadapted to extremely shallow water habitats with variable salinity and periodic exposure.Discovery of the new rangeomorph species in the Central Urals is the first demonstration of227