The Agronomic Revolution implies a gradual sophistication of organism-substrate interactions and progressive colonization of shelf environments by vagile endobenthos in the late Ediacaran and Cambrian periods. These changes are impressively mirrored in a palaeontological record of ichnoassemblages in marine siliciclastic facies. In contrast, endobenthic communities in carbonate successions remained comparatively poor until a fundamental transformation during the Great Ordovician Biodiversification Event. Herein we report complex open burrows (Thalassinoides and Olenichnus) from the Sukharikha Formation exposed near the Plakhinskii Island (Yenisei River, Igarka Uplift). Lithological and chemostratigraphical correlation with the stratotype suggest the Terreneuvian age of the fossil-bearing strata accumulated in storm-agitated mid- to distal inner-ramp settings. Each of the ichnotaxa occurs in a specific deposit in the studied section: Olenichnus is localised exclusively in the background-sedimentation deposits, whereas Thalassinoides occurs in the storm-event beds. Our data support early reports suggesting that colonization of marine carbonate sediments by burrowing metazoans was initiated already in the Terreneuvian and became a solid basis for the following development of deep-tier bioturbation in carbonates in the Ordovician Period.
