Mechanisms of fossilization of the soft-bodied and lightly armored faunas of the Burgess Shale and of some other classical localities

Abstract

The splendid preservation of the Middle Cambrian Burgess Shale fauna, a fauna of exceptional importance for our understanding of the evolution of life, has not been adequately explained. Preservation of diagenetically altered remnants of the original organic tissues and formation of chlorite/illite coatings and cuticle replacements, both documented in the Burgess Shale fossils though not necessarily occurring together, can be understood as products of the same mechanism of fossilization of soft tissues. It is argued here that this mechanism consists of the following steps: (1) adsorption on structural biopolymers such as chitin, cellulose, and collagens of Fe 2؉ ions released during the oxidation of organic matter by iron(III)-reducing bacteria, (2) inhibition by the adsorbed Fe 2؉ ions of further bacterial decomposition of these biopolymers, which enables them to persist and later become kerogens; (3) in some microenvironments, nucleation of crystals of an iron(II)-rich clay mineral, a berthierine or a ferroan saponite, on the Fe 2؉ ions adsorbed on the preserved biopolymers and growth of such clay-mineral crystals to form a coating on the organic remains and/or to replace parts of the organism. The critical factors in the Burgess Shale-type preservation of Early and Middle Cambrian soft-bodied and lightly armored animals were probably: (1) rapid transport of live or freshly killed organisms into suboxic water, (2) extensive suboxic diagenesis in a sediment of high iron(III)/ (organic carbon) ratio, and (3) curtailment of the supply of sulfate ions shortly after the onset of pyritization. The proposed model of early diagenesis that results in Burgess Shale-type fossil preservation critically depends on the availability of steady suboxic depositional environments in open oceanic settings at depths of the order of 100 m in which iron(III)-rich fine-grained sediments, rapidly deposited with the entrained animals by turbidity currents, could accumulate without being disturbed by storm waves and deep currents. Evidence discussed in the present paper suggests that such conditions were common in the Early and Middle Cambrian.Adsorption of Fe 2؉ ions on structural biopolymers as a means of protecting organic fossil remains from decomposition by bacterial enzymes is a novel suggestion and needs to be demonstrated by direct experimentation. It is based on the following considerations. First, Fe 2؉ ions are strongly adsorbed on chitin under experimental conditions comparable to those in pore waters of suboxic iron-rich sediments, and while data on Fe 2؉ ion adsorption on collagen and cellulose seem to be lacking, other heavy-metal ions are strongly adsorbed on these biopolymers under appropriate conditions. Second, Fe 2؉ ions bonded with functional groups of chitin, collagen, or cellulose would prevent the very specific configuration and bonding which a biopolymer strand has to achieve within the active-site cleft of the appropriate bacterial enzyme to make enzymatic hydrolysis possible.Close examination of two other mecha...

Exceptionally preserved fossil assemblages through geologic time and space

“…The first explanation attributes the abundance of exceptionally preserved fossil assemblages in the upper Neoproterozoic-lower Paleozoic to seawater chemistry, which given its unique make-up in that interval, supported a multitude of preservational processes and opened all taphonomic windows. High silica allowed for silicification (Knoll, 1985;Maliva et al, 1989); influx of phosphate promoted phosphatization ; and aspects that inhibited degradative chemistry resulted in organic preservation (Butterfield, 1990;Gaines et al, 2012aGaines et al, , 2012bOrr et al, 1998;Petrovich, 2001). Yet, as our conceptual model shows, the major taphonomic processes do not occur under entirely unique environmental conditions.…”

Sediment Effects on the Preservation of Burgess Shale-Type Compression Fossils

“…Despite reports of accelerated carcass decay associated with some sediments (e.g., Plotnick 1986;Allison 1988;Briggs and Kear 1993), most models for BST preservation invoke mineralspecific diagenesis as an essential factor, either by suppressing normal enzyme-microbial-based decay processes (e.g., Butterfield 1990Butterfield , 1995Gaines et al 2005Gaines et al , 2012 or secondarily by enhancing the recalcitrance of relatively labile substrates (e.g., Orr et al 1998;Petrovich 2001). Surprisingly, there has been little attempt to test these various models, though Martin et al (2004) have shown that sedimentary particles will adhere to lobster eggs in the presence of bacteria, and Naimark et al (2013) have documented substantially enhanced preservation of Artemia when they are buried in kaolinite.…”

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

“…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.…”