Exceptional preservation of soft-bodied Ediacara Biota promoted by silica-rich oceans

Abstract

The Ediacara Biota, Earth’s earliest fossilized ecosystem of complex, macroscopic, multicellular organisms, occurs in terminal Ediacaran strata worldwide, yet how the fossils are preserved remains controversial. Ediacara assemblages consist of exceptionally preserved soft-bodied forms of enigmatic morphology and phylogenetic affinity. Many of these fossil assemblages are anactualistically preserved as casts and molds in sandstones (“Ediacara-style” preservation). Here we present evidence from the Ediacara Member of South Australia that Ediacara-style preservation was due to rapid, early-stage precipitation of silica cements, facilitated by the high silica saturation state of the oceans prior to the appearance of prolific silica biomineralizers. An early silicification model provides a coherent, mechanistic and empirically supported explanation for the widespread preservation of soft-bodied organisms of Ediacaran–early Paleozoic age as sandstone casts and molds. The prevalence of early silicification confirms that Ediacara-style fossil assemblages can provide an accurate window into life on the Ediacaran seafloor that can be used to reconstruct critical steps in the development and diversification of early animal ecosystems.

Soft‐Bodied Fossils Are Not Simply Rotten Carcasses – Toward a Holistic Understanding of Exceptional Fossil Preservation

“…[29,115,116] Although most common in the Ediacaran, this taphonomic window persisted until the Devonian. [29] Such fossils occur in a range of depositional environments, including deep marine basins, marginal marine settings, storm influenced shore faces, and shelf carbonates.…”

“…[29,115,116] Although most common in the Ediacaran, this taphonomic window persisted until the Devonian. [29] Such fossils occur in a range of depositional environments, including deep marine basins, marginal marine settings, storm influenced shore faces, and shelf carbonates. [116] Specimens may retain sub-millimetric details of mostly external, but sometimes internal, [117] anatomy, and are sometimes three dimensionally preserved within beds.…”

“…Candidates include rapidly forming pyrite crusts referred to as "death masks," [28,116] microbial mats, [28] clay mineral templating [120] and, most recently, early silicate cementation. [29] www.advancedsciencenews.com www.bioessays-journal.com as in concretion formation. [42] Observations of a decaying priapulid were used to interpret Burgess Shale specimens of the Cambrian priapulid Ottoia [43] but it was not until the late 1980s that experiments started to explore the impact of various controls on decay.…”

“…[26,27] Such gradients also form where a microbial mat and early diagenetic cement seal in the buried organism (Box 1): this may allow preservation in coarser sediment À even in sandstones in the case of Ediacaran assemblages. [28,29] Sediment mineralogy, particularly of clays, may also play a role in tissue stabilization. [13,26,30,31] Early cementation of the surrounding sediment promotes exceptional preservation by eliminating pore space and may create a cast of soft tissue anatomy.…”

Russia–UK Collaboration in Paleontology: Past, Present, and Future

“…Their preservation has been explained by early precipitation of pyrite -the 'Death Mask' hypothesis developed by Gehling (1999) based on specimens from the Flinders Ranges. Recent evidence from Australian examples indicates that the pyrite coating forms later in the Flinders Ranges and fossilization appears to be a result of elevated concentrations of silica in the oceans (Tarhan et al 2016) prior to the onset of silica biomineralization by sponges and radiolarians. This new model (Tarhan et al 2016) remains to be tested by investigating occurrences elsewhere in the world where pyrite is associated with Ediacara fossils, including Mistaken Point (Liu 2016) and the White Sea (Ivantsov 2016).…”