A progressive colonisation of marine sediments by vagile benthos in the late Ediacaran and early Cambrian led to a series of dramatic transformations in marine ecosystems. Nevertheless the Fortunian ichnoassemblages reveal unprecedented development of behavioural strategies and architectural designs. As a result of the scarcity of vertical bioturbators, only the few uppermost centimetres of sediment were affected by infaunal mixing. Multifaceted study of the trace fossil Olenichnus irregularis from the type locality (early Fortunian strata of the Olenek Uplift, northeastern Siberian Platform), originally interpreted as subhorizontal network burrows, demonstrates that these burrows represent complex three-dimensional tunnel systems, produced by infaunal bilaterians. Together with helicoidal burrows Gyrolithes, the trace fossil Olenichnus comprises one of the earliest evidence of active vertical sediment reworking by bilaterians. Taphonomy of the topotype Olenichnus and its spatial position related to other associated ichnotaxa reveal that origin of endobenthic tiering and multi-stage colonisation of bottom substrates by macroscopic bilaterians appeared at the very beginning of the Cambrian Period. Olenichnus compose the oldest mid-tier ichnoguild, associated with oxygen-depleted sediments below the redox-discontinuity surface located closely to the sediment-water interface in the Fortunian.