Resolution of the earliest metazoan record: Differential taphonomy of Ediacaran and Paleozoic fossil molds and casts

Much of our knowledge of early metazoan evolution is derived from unmineralized death mask or endorelief mold and cast fossils in Ediacaran elastic sedimentary rocks. This record is often regarded as a unique 'Ediacaran taphonomic window'; however, the prevalence of soft-bodied molds and casts in Paleozoic elastic rocks has been increasing, begging an extension, or modification, to our understanding of this preservational motif. Chief amongst such fossils are eldonids, a non-biomineralized group of stem deuterostomes. Because eldonids are also preserved as compressed or flattened fossils from deposits like the Burgess Shale, Chengjiang and Kaili, they offer a comparative case study for evaluating the taphonomic fidelity of mold/cast-style preservation during this interval. EDS and Raman microspectroscopic analysis of Ordovician and Devonian eldonid molds and casts, and comparison with Burgess Shale eldonids, suggests the mold/cast taphonomic style produces significantly lower fidelity of fossil preservation. We propose that eldonid mold/cast fossils are preserved by the adsorption of reduced iron ions onto tissues composed primarily of high molecular weight (HMW) biopolymers which require enzymatic degradation prior to decay. Nucleation and growth of aluminosilicates and/or sulfides around these adsorbed ions forms a fossilizable surface veneer, preserving a death mask mold. More labile tissues could not be fossilized in this mold and cast style. Ediacaran mold and cast fossils from South Australia, the White Sea region of Russia, Namibia, and Newfoundland exhibit preservational characteristics consistent with this new proposed model. Analysis of their preservational mode suggests that the first metazoans, which would have lacked HMW biopolymeric tissues, could not have been fossilized in this particular style. Thus, understanding the origin and earliest evolution of the Metazoa requires a focus on alternative modes of fossilization.