The Biological Characteristics, Ecological Role and Evolutionary Significance of Dickinsonia and Other Modular Organisms of the Ediacara Biota
The earliest fossil communities of macroscopic organisms are preserved in the Ediacara Biota. While many are difficult to classify into well-known phylogenetic groups, the exceptional preservation of these mostly soft-bodied forms, provides a plethora of material for comprehensive investigations of their biology and ecology. Exceptional deposits of the Ediacara Member from the Flinders Ranges and surrounding areas of South Australia yield abundant Ediacara Biota fossils ideal for such studies. The research presented below targets modular Ediacaran taxa, in particular the iconic and abundant fossil Dickinsonia. Patterns of growth and development in Dickinsonia are explained most parsimoniously via posterior addition of modular units and variable growth rates of units through development regulated to maintain an ovoid shape. Fossiliferous Ediacaran bedding plane surfaces, including the recently uncovered Alice’s Restaurant bed, highlight the heterogeneous distribution of taxa on the Ediacaran seafloor. Trace fossil evidence demonstrates that Dickinsonia and Yorgia were mobile, muscular organisms that fed on the organic mat through their ventral surface. Comparisons with other bilaterally symmetrical modular taxa suggests that many Ediacaran forms may have been mobile but did not engage in activities conducive to trace fossil preservation. Dickinsonia was bilaterally symmetrical, marked by distinct anterior/posterior and likely dorsal/ventral differentiation in addition to modules that met precisely at the midline. Taphonomic variants of Dickinsonia reveal that it was structurally resilient for a soft-bodied organism, highly extensible compared with most modern biopolymers, capable of elastic and plastic deformation, and composed of relatively thick, differentiated tissue. While many of these characters are consistent with bilaterians today this fossil probably represents an extinct clade within the Eumetazoa. This work establishes that the physiology and morphology of Dickinsonia were specifically suited to Ediacaran environments, and that departures from those unique conditions may have led to the ultimate disappearance of this taxa.