The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian–Cambrian transition

In the most complete and first radiometrically dated Precambrian-Cambrian transition strata along the Khorbusuonka River in northern Siberia, a succession of fossilized traces of activity of the earliest infaunal animals has been identified. As supplemented by a similar record in Podolia, the Ukraine and elsewhere. This collection demonstrates that thus far all of the structurally identifiable trace fossils of infaunal life from the earliest Cambrian represent shelters of animals feeding above the sediment surface. The oldest traces of feeding within the mud are known from no earlier than the Late Tommotian. This suggests that the invention of hydraulic mechanisms for sediment penetration was forced by predation, not by trophic needs. Various ways to protect the body, by secretion of a mineral skeleton or the building of tubes with collected mineral grains, were developed by other animals at about the same time. Predation thus appears to be the triggering mechanism for the "Cambrian explosion". Subsequent increase in the depth of bioturbation resulted in a profound change of taphonomic conditions.