Boreskov

PP‐56EVOLUTION OF ORGANISM–SEDIMENT INTERACTION IN TRANSITION TOPHANEROZOIC BIOSPHEREMarusin V.V.Trofimuk Institute of Petroleum Geology and Geophysics, Novosibirsk, RussiaThat infaunal habit evolved concurrent with origin of Bilateria has long beenappreciated, but the triggering mechanism remained unclear. J. Dzik (2005) argues that tracefossils from the Vendian–Cambrian transition represent shelters of infaunal animals feedingfrom the sediment surface, and that infaunal habit evolved as a protective measure againstpredators. We studied trace fossil record through Vendian–Cambrian transition in theKhorbusuonka and Kessyusa Groups in the Olenek Uplift of Arctic Siberia and came to thefollowing conclusions. 1. The world’s oldest bioturbation is in the Khatyspyt Fm ofKhorbusuonka Gr and represented by meniscate backfilled burrows Nenoxites. Severalfeatures of the Nenoxites ichnofabric (absence of fecal material, avoidance of earlier selfmadetrails) suggest that the organism actively burrowed by peristalsis without processingsediment through the gut. The most likely purpose of borrowing was search for food. 2.Lower Cambrian is characterized by a diversity of meniscate burrow systems that representsediment processing behavior in search for food. These are U‐shaped burrows with spreitestructuresformed as a result of intensive horizontal or vertical sediment reworking (tracefossils Rhizocorallium and Diplocraterion), meniscate backfilled burrows radiating from acentral vertical mine (Zoophycos), J‐shaped burrows with protrusive or retrusive spreitestructures(Syringomorpha). 3. Trace fossil Treptichnus is generally regarded as permanentlyopen burrows used as shelters by a surface detritus feeders (Jensen, 1977). Importantly,ichnofabric in the uppermost Vendian Syharghalakh Fm of the Kessyusa Gr consists ofcompact descending spirals typical for sediment‐processing behavior. These trace fossilswere discovered and described by M.A. Fedonkin (1985), albeit as two separate ichnospeciesPlanispiralichnus grandis and Protospiralichnus circularis; however, the spirals consist ofseries of sediment‐probing elements that are diagnostic of treptichnid trace fossils and aretherefore reinterpreted as ichnospecies Treptichnus circularis (see figure). Trace fossil recordof the Khorbusuonka and Kessyusa groups reveals the true meaning of the Vendian–Cambrian Agronomic Revolution: organism‐sediment interaction started as exploration ofnew food resources, and not as sheltering from evolved predators. This study was supportedby RFBR grants no. 09‐05‐00520 and 10‐05‐00953, RAS Program “Biosphere Origin andEvolution” and National Geographic Society.232