Concept: Treptichnus pedum
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 fossilArenicolitesfrom late Precambrian marine carbonates of Western Mongolia. Their Ediacaran age is established through stable carbon isotope chemostratigraphy and their occurrence stratigraphically below the first appearance of the trace fossilTreptichnus pedum. TheseArenicolitesare 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 commonly as paired circular openings on bedding planes with maximum diameters ranging up to almost 1 cm, and as U- and J-shaped tubes in vertical sections of beds. Discovery of these complex penetrative trace fossils demonstrates that the agronomic revolution started earlier than previously considered.
The beginning of the Cambrian was a time of marked biological and sedimentary changes, including the replacement of Proterozoic-style microbial matgrounds by Phanerozoic-style bioturbated mixgrounds. Here we show that Ediacaran-style matground-based ecology persisted into the earliest Cambrian. Our study in the type section of the basal Cambrian in Fortune Head, Newfoundland, Canada reveals widespread microbially induced sedimentary structures and typical Ediacaran-type matground ichnofossils. Ediacara-type body fossils are present immediately below the top of the Ediacaran but are strikingly absent from the overlying Cambrian succession, despite optimal conditions for their preservation, and instead the microbial surfaces are marked by the appearance of the first abundant arthropod scratch marks in Earth evolution. These features imply that the disappearance of the Ediacara biota represents an abrupt evolutionary event that corresponded with the appearance of novel bilaterian clades, rather than a fading away owing to the gradual elimination of conditions appropriate for Ediacaran preservation.