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 rem- nants 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 Fe²⁺ ions released during the oxidation of organic matter by iron(III)- reducing bacteria, (2) inhibition by the adsorbed Fe²⁺ 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 Fe²⁺ 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 Fe²⁺ 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, Fe²⁺ ions are strongly adsorbed on chitin under experimental conditions comparable to those in pore waters of suboxic iron-rich sediments, and while data on Fe²⁺ ion adsorption on collagen and cellulose seem to be lacking, other heavy-metal ions are strongly adsorbed on these biopolymers under appropriate conditions. Second, Fe²⁺ ions bonded with functional groups of chitin, collagen, or cellulose would prevent the very specific configuration and bonding which a biopoly- mer strand has to achieve within the active-site cleft of the appropriate bacterial enzyme to make enzymatic hydrolysis possible. Close examination of two other mechanisms recently proposed for Burgess Shale-type preservation of soft tissues shows that they are implausible: preservation by inactivation of extracellular enzymes on clay minerals would require a maladaptive reliance of tissue-decomposing bacteria on free extracellular enzymes, and preserva- tion by attachment of pre-existing clay-mineral particles would require a sequence of physically improbable events. It is argued here that adsorption of Fe²⁺ ions on structural biopolymers was the first step not only in Burgess Shale-type preservation of soft-bodied and lightly armored fossils but also in the preservation of such fossils by pyritization in Beecher’s Trilobite Bed in the Upper Ordovician Frankfort Shale in upstate New York and in the Lower Devonian Hunsrück Slate in Rhineland-Palatinate and in the preservation of such fossils within siderite concretions in various localities. It was probably the first step in the preservation (obscured by alunitic weathering) of soft-bodied and lightly armored fossils in the Soom Shale of the Cape Province, South Africa, and in the preservation of Ediacaran soft-bodied fossils in the classical localities of the Ediacaran Range of Australia and Mistaken Point, Newfoundland.