PENETRATIVE TRACE FOSSILS FROM THE LATE EDIACARAN OF MONGOLIA: EARLY ONSET OF THE AGRONOMIC REVOLUTION

The Cambrian radiation of complex animals includes a dramatic increase in the depth and intensity of bioturbation in seafloor sediment known as the agronomic revolution. This bioturbation transition was coupled with a shift in dominant trace fossil style from horizontal surficial traces in the late Precambrian to vertically penetrative trace fossils in the Cambrian. Here we show the existence of the first vertically penetrative trace fossils from the latest Ediacaran: dense occurrences of the U-shaped trace fossil Arenicolites from late Precambrian marine carbonates of the Zuun-Arts Formation of western Mongolia. Their Ediacaran age is established through stable carbon isotope chemostratigraphy and their occurrence well below the first appearance of the trace fossil Treptichnus pedum. These Arenicolites are large in diameter, penetrate down to at least 4 cm into the sediment, and were presumably formed by the activity of bilaterian animals. They are preserved as paired circular openings on 11 individual bedding planes and as U- and J-shaped tubes in vertical sections of beds. Small concave arcuate traces occur on these bedding planes as well, and they resemble the bases of U-shaped burrows from overlying beds. The horizontal trace fossil Planolites is also preserved on the bedding planes. In order to further reconstruct the 3-D morphology of burrows in one sample, it was cut perpendicular to bedding into seven parallel slabs. Scanned images of these vertical surfaces were stacked and the internal morphologies of two burrows were observed, both of which are U-shaped with irregular boundaries. Their irregular shape indicates subsequent deformation of their weak unlined walls. They are unlikely to be dolomitization structures, as XRD analysis show low levels of dolomite in these beds. XRD analysis further indicates they occur in a calcite matrix and contain calcite, dolomite, and quartz infilling. This suggests that sediment infilled the hollow tubes. The lack of insoluble residue along their outer surfaces also makes it unlikely that they are dissolution structures. Discovery of these complex penetrative trace fossils demonstrates that the agronomic revolution started earlier than previously considered and provides evidence for bilaterians in the late Precambrian.