On the edge of exceptional preservation: insights into the role of redox state in Burgess Shale-type taphonomic windows from the Mural Formation, Alberta, Canada

Abstract

Animals originated in the Neoproterozoic and ‘exploded’ into the fossil record in the Cambrian. The Cambrian also represents a high point in the animal fossil record for the preservation of soft tissues that are normally degraded. Specifically, fossils from Burgess Shale-type (BST) preservational windows give paleontologists an unparalleled view into early animal evolution. Why this time interval hosts such exceptional preservation, and why this preservational window declines in the early Paleozoic, have been long-standing questions. Anoxic conditions have been hypothesized to play a role in BST preservation, but recent geochemical investigations of these deposits have reached contradictory results with respect to the redox state of overlying bottom waters. Here, we report a multi-proxy geochemical study of the Lower Cambrian Mural Formation, Alberta, Canada. At the type section, the Mural Formation preserves rare recalcitrant organic tissues in shales that were deposited near storm wave base (a Tier 3 deposit; the worst level of soft-tissue preservation). The geochemical signature of this section shows little to no evidence of anoxic conditions, in contrast with published multi-proxy studies of more celebrated Tier 1 and 2 deposits. These data help confirm that ‘decay-limited’ BST biotas were deposited in more oxygenated conditions, and support a role for anoxic conditions in BST preservation. Finally, we discuss the role of iron reduction in BST preservation, including the formation of iron-rich clays and inducement of sealing seafloor carbonate cements. As oceans and sediment columns became more oxygenated and more sulfidic through the early Paleozoic, these geochemical changes may have helped close the BST taphonomic window.

Small shelly fossils and carbon isotopes from the early Cambrian (Stages 3–4) Mural Formation of western Laurentia

“…The Mural Formation was sampled at three different locations: the type Mumm Peak section in Jasper National Park in western Alberta (sample prefix MP; see Balthasar 2004 and Sperling et al . 2018 for locality information); a new section in a glacial valley to the north‐west of Mumm Peak in eastern British Columbia (the informally named Rocky Lake camp, sample prefix RL); and the Dezaiko Range further to the north in British Columbia (sample prefix DR). At Mumm Peak the basal limestone and middle shale units were sampled for SSF while carbon isotope samples were collected from a measured stratigraphic section of the entire formation, including the upper carbonate unit that was heavily dolomitized.…”

“…Additional data on the geology, sedimentology, and palaeontology of the Mural Formation can be found in Fritz & Mountjoy (1975), Balthasar (2004) and Sperling et al . (2018). The Waucoban Series is traditionally divided into a series of trilobite zones: the Fallotaspis , Nevadella and Bonnia – Olenellus Zones in ascending order.…”

“…However, Burgess Shale‐type exceptional preservation in the Mural Formation is at best marginal (a Tier 3 deposit), and there is little geochemical evidence for anoxic conditions that might result in more spectacular preservation (Sperling et al . 2018). The bulk of the known fauna has been derived from mudstones of the middle part of the Mural Formation.…”

“…The discovery of exceptionally preserved fossils such as soft‐shelled brachiopods (Balthasar & Butterfield 2009), anomalocaridids and other non‐mineralized biota (Sperling et al . 2018) has also sparked interest in the formation. However, Burgess Shale‐type exceptional preservation in the Mural Formation is at best marginal (a Tier 3 deposit), and there is little geochemical evidence for anoxic conditions that might result in more spectacular preservation (Sperling et al .…”

Deciphering trophic interactions in a mid-Cambrian assemblage

“…This resulted in exceptional preservation of organisms and provided a unique fingerprint of the Cambrian. Interpretation of the details of the entombment and preservation processes differ among studies (Gaines et al, 2012;Schiffbauer et al, 2014;Sperling et al, 2018).…”