EXTENSIVE COMPLEX, BED-PENETRATIVE TRACE FOSSILS IN THE EDIACARAN OF SOUTHWEST MONGOLIA

The Precambrian seafloor was characterized by hardgrounds and matgrounds, leading to a sharp geochemical gradient at the substrate-water interface. The advent of bioturbating animals - the agronomic revolution - has been hypothesized to have led to more complete sediment mixing and fluidization, fundamentally altering the geochemistry of the seafloor. Traditionally, this ecological shift is thought to have begun in the Cambrian, although it was not fully developed until the middle Paleozoic. This timing of the start of bioturbation is based on geochemical modeling, observations of only simple horizontal burrows in the Ediacaran, and the first occurrences of bed-penetrative traces in the Cambrian.

Here we document abundant and diverse trace fossils from the Ediacaran Zuun-Arts Formation in the Zavkhan basin of southwestern Mongolia. Traces from this unit include Planolites-like simple horizontal traces, Archaeonassa-like horizontal traces with levees, Bergaueria-like plug-shaped burrows and mounds, and Arenicolites-like U-shaped tubular burrows. These traces cover extensive bedding surfaces in multiple horizons throughout the Zuun-Arts Formation, suggesting an active and diverse benthic fauna during the terminal Ediacaran. At the South Bayan Gorge locality, the traces are found stratigraphically ~40 meters above the first local appearance of the cloudinomorph Zuunia from the basal Zuun-Arts Formation, 130 meters below the first local appearance of small shelly fossils (e.g., Anabarites and Protohertzina) characteristic of the Ediacaran-Cambrian boundary, and hundreds of meters below the first local occurrence of Treptichnus pedum in the overlying Bayangol Formation. These traces record both biomixing and bioirrigation behaviors that may have had substantial impact on the sediment geochemistry.

The occurrence of these fossils in Ediacaran strata may be suggestive of an asynchronous development of complex burrowing behaviors globally. The Zavkhan basin may have served as a cradle for these early trace-making animals before they radiated around the world. This work expands the record of complex, bed-penetrative trace fossils into the Ediacaran, shifting the paradigm of when these behaviors developed.