Morphology, taphonomy, and paleoecology of terminal ediacaran vermiform fauna

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] As signposted by the fossil record, the early Cambrian Period chronicles the appearance and evolutionary diversification of most animal phyla in a geologically rapid event, traditionally termed the Cambrian Explosion. The uniqueness of this event pleads for a cause, and over the years, numerous biotic and abiotic factors have been intimated as possible triggers. However, many such explanations either fail to correspond in time or do not provide a functional mechanism to explain the evolutionary pattern of animal diversification. It is a more parsimonious scenario that a series of requisite biotic and abiotic events ushered in the Cambrian Explosion, wherein each event was necessary for the implementation of later events but did not guarantee their occurrence. The evolution of the terminal Ediacaran vermiform "wormworld fauna" was integral in building the Eltonian pyramid, a chief ecological control promoting the diversification of animals. This period of intense ecological change is prominently on display in the Gaojiashan Lagerstatte in the terminal Ediacaran Dengying Formation of southern Shaanxi Province, China. This deposit hosts numerous wormworld organisms that are exceptionally well preserved as both three-dimensionally pyritized tubes and carbonaceous compressions, including Conotubus, Gaojishania, Shaanxilithes, Sinotubulites, and the index fossil Cloudina. As the Gaojiashan Member is largely stratigraphically complete, this locality is useful for studying the budding ecosystem dynamics of early metazoan communities, herein focusing on analyses of body size dynamics of two of the most abundant wormworld taxa, Conotubus and Gaojiashania. Body size measurements were collected using image processing software and databased for four horizons of the Gaojiashan Member. Once analyses were preformed, trends in body size were immediately evident based on dominant taxa in each horizon. Abundance also yielded important information about the survivorship of each taxon relative to the competing taxon, indicating a competitive shift in dominance and a starkly contrasted pattern to earlier Ediacaran-aged complex multicellular organisms. This wormworld fauna represents an escalation of bioturbation and predation, resulting in the transition from benthic ecosystems governed primarily by competition for space and resources to those also shaped by novel pressures of ecosystem engineering and macropredation.