CHANGING ECOSYSTEM DYNAMICS IN THE TERMINAL EDIACARAN VERMIFORM FAUNA: AN ANALYSIS USING SEDIMENTOLOGICAL AND DIVERSITY DATA FROM THE GAOJIASHAN LAGERSTäTTE OF SOUTH CHINA

Several recent efforts have sought to explain the biotic replacement and/or extinction of the classic Ediacaran biota. While there is yet to be an established consensus, we do know that the final 10 million years of the Ediacaran Period was rife with newly established vermiform fauna. Of the best described occurrences of these ecosystems, the Gaojiashan Lagerstätte contains exceptionally well-preserved vermiform fossils, captured as ecological snapshots during obrution events. Depositional features of the enclosing sediments indicate that the fossiliferous beds depict a range of marine environments along a shelf-slope transect, although transport during event deposition may be a concern. By compiling sedimentological and taxonomic data of ~3600 individuals from four 5m thick intervals of the Gaojiashan Member (Dengying Formation), we were able to create a model for the population dynamics of the two most abundant taxa, Conotubus and Gaojiashania, along this presumed environmental transect through time. A distinct shift can be seen in the presence and relative abundance of articulated individuals of each taxon in both shallow and deep water obrution sediments, as well as along the slope. The pattern of this shift indicates competition between these two taxa, with Gaojiashania ultimately becoming more dominant in represented sections. The ecological dominance of Gaojiashania across environments occurs as bioturbation increases at the sediment-water interface, indicating tolerance to biological disturbance. This transition is additionally followed by the proliferation of one of the first biomineralizers, Cloudina—a suggested evolutionary successor to the soft-bodied Conotubus. The major transitions during this 10 my interval expanded the available ecosystem both below and above the substrate, including: 1) an ecological structural shift from specialists to generalists, coincident with the onset of bioturbation, and 2) the development of metazoan biomineralization, which allowed for stability and the development of reef-building behaviors. By the terminal Precambrian, these vermiform organisms contributed to the engineering of a complex marine ecosystem immediately preceding, and possibly setting the stage for, the Cambrian Explosion.