Mechanism for Burgess Shale-type preservation

Abstract

Exceptionally preserved fossil biotas of the Burgess Shale and a handful of other similar Cambrian deposits provide rare but critical insights into the early diversification of animals. The extraordinary preservation of labile tissues in these geographically widespread but temporally restricted soft-bodied fossil assemblages has remained enigmatic since Walcott’s initial discovery in 1909. Here, we demonstrate the mechanism of Burgess Shale-type preservation using sedimentologic and geochemical data from the Chengjiang, Burgess Shale, and five other principal Burgess Shale-type deposits. Sulfur isotope evidence from sedimentary pyrites reveals that the exquisite fossilization of organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments by means of oxidant deprivation. Low sulfate concentrations in the global ocean and low-oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability. Subsequently, rapid entombment of fossils in fine-grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments. A permeability barrier, provided by bed-capping cements that were emplaced at the seafloor, is a feature that is shared among Burgess Shale-type deposits, and resulted from the unusually high alkalinity of Cambrian oceans. Thus, Burgess Shale-type preservation of soft-bodied fossil assemblages worldwide was promoted by unique aspects of early Paleozoic seawater chemistry that strongly impacted sediment diagenesis, providing a fundamentally unique record of the immediate aftermath of the “Cambrian explosion.”

Reply to Butterfield: Low-sulfate and early cements inhibit decay and promote Burgess Shale-type preservation

“…We agree with the assertion in the work by Butterfield (1) that, even under oxidant-starved conditions, decay is proceeded by fermentation, methanogenesis, and autolytic processes, and we refer the reader to the concluding paragraph of the Fossilization section in our manuscript (3). Indeed, these compression fossils have lost >99% of their volume.…”

“…However, well-preserved skeletal fossils, such as Scenella, occur with clear margins and petrographic characteristics that are sharply distinct from the fine-grained carbonates that surround them, excluding the possibility that bioclasts were the original source of carbonate. Importantly, the cements in the Burgess Shale are entirely consistent in growth habit and distribution with the six other deposits analyzed in our study (3), including a nearby but less metamorphosed locality in the Stephen Formation, all of which clearly display early-displacive growth of carbonate at the seafloor. The assertion in the work by Butterfield (1) that seafloor precipitates are uncommon in the Cambrian is at odds with a large body of evidence (4).…”

A unifying model for Neoproterozoic–Palaeozoic exceptional fossil preservation through pyritization and carbonaceous compression

“…These preservation processes, whether pyritization or kerogenization, are governed by the same suite of facilitating taphonomic conditions, are primarily influenced by microbial decay pathways, and differ principally in the duration of interactions with the BSR and methanogenesis zones. Thus, the scientific conversation regarding contributing factors of Beecher's Trilobite-type pyritization and Burgess Shale-type carbonaceous compression should be shifted from facilitating factors such as a lack of bioturbation 49,50 and bed-capping carbonate cements 18 , to those that drive mineralization and stabilization processes and are directly responsible for fossil preservation.…”

“…By and large, kerogenization has been attributed to many of the same facilitating conditions, mostly related to rapid burial into anoxic/dysoxic palaeoenvironments 15 . While numerous other palaeoenvironmental and diagenetic considerations have been invoked for kerogenization, such as interactions with clays 6,16 or ferrous iron 17 , high alkalinity 15,18 and oxidant restriction (that is, lack of sulfate for BSR) through early diagenetic sealing 18 (though see also ref. 19), the common association of kerogenized fossils with pyrite 1,6,12,16,[20][21][22][23][24] bolsters the interrelationship of these taphonomic processes.…”

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

“…[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. Early precipitation of carbonate at the sediment surface [32] or the presence of microbial mats [33] may have promoted preservation in Burgess Shale-type deposits, for example, and microbial mats are a common feature of deposits preserving muscle tissue. [34] In other cases, a concretion may form around a carcass, preventing collapse and promoting mineralization.…”