Burgess Shale-type Preservation Research Articles

Experimental attachment of sediment particles to invertebrate eggs and the preservation of soft-bodied fossils

Clay minerals can be an important agent in the fossilization of soft tissues, notably in the Ordovician Soom Shale of South Africa and the Cambrian Burgess Shale of Canada. The replication of morphology has been attributed to adsorption of pre-existing clay minerals, or direct precipitation of authigenic clays onto tissues. Attachment of quartz and kaolinite to the surface of lobster eggs demonstrates experimentally for the first time that soft tissues could fossilize in pre-existing minerals. However, the eggs became coated only in the presence of metabolizing bacteria. This experimental approach could be used to explore why Burgess Shale-type preservation declined after the mid-Cambrian.

Geochemical evidence for oxygenated bottom waters during deposition of fossiliferous strata of the Burgess Shale Formation

Deciphering the oxygenation potential of bottom waters during deposition of the fossiliferous strata of the Burgess Shale is key to understanding the paleoecology of the organisms preserved as fossils and the processes involved in their preservation. Methods of paleo-redox determination that are based upon trace fossils, organic carbon content, or size analysis of pyrite framboids are problematic when applied to the Burgess Shale. Fortunately, patterns in redox-sensitive trace elements hold great potential for determining paleo-redox conditions for these greenschist-facies rocks because they are independent of both age and metamorphic grade. Four Burgess Shale-Type (BST) fossil-bearing sections were studied; three lie within the Burgess Shale Formation, whereas one lies within the slightly younger Duchesnay unit of the Middle Chancellor Formation. The geochemical proxies of seawater redox conditions that were used are Mo, U/Th, V/Cr, V/(V+Ni), Ni/Co and V/Sc. Each of these redox-sensitive indices suggests that oxygenated conditions prevailed in the overlying seawater during deposition of beds within which BST preservation developed. Non-fossiliferous beds at the base of the Mt. Stephen Trilobite Beds yield three geochemical indices (V/[V+Ni], V/Sc, Ni/Co) that indicate deposition under dysoxic to anoxic conditions. Thus the only beds to have been deposited under anoxic conditions are barren of fossils of soft-bodied taxa, further contradicting the notion that bottom-water anoxia is a factor in BST preservation. The laminated nature of the Burgess Shale and evidence for subsurface anoxia, coupled with evidence for oxygenated bottom waters, presence of bacterial mats, and surficial trace fossils suggest that the Burgess Shale was deposited in the exaerobic zone (oxic–anoxic boundary at the water–sediment interface). Implications of this reinterpretation of the depositional environment of the Burgess Shale include: (1) taphonomic models for BST preservation must account for the presence of oxygen in bottom waters; (2) paleoecological interpretations of the Burgess Shale need to account for a probable autochthonous benthic component in what is a complex assemblage of fossils that includes elements derived from the platform; and (3) the possibility of Burgess Shale communities based upon chemosynthesis and/or bacteria grazing must be considered.

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

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.

New fossil worms from the Lower Cambrian of the Kinzers Formation, Pennsylvania, with some comments on Burgess Shale-type preservation

Two new fossil worms are described from the Lower Cambrian Kinzers Formation of southeast Pennsylvania. Both are unique specimens.Kinzeria crinitanew genus and species has a body divided into three regions. The head bears a prominent set of elongate tentacles, presumably employed for feeding. The elongate trunk tapers slightly in a posterior direction. It contains a prominent intestinal tract, the contents of which indicate a deposit feeding habit. The tail is an expanded structure, with either a spatulate or sagittate outline. The mode of life ofK. crinitais uncertain, but the animal may have been semi-sedentary.Atalotaenia adelanew genus and species is more poorly known, lacking the anterior. The preserved body is vermiform, with external annulations and a rounded posterior. There is a prominent internal strand, consisting of a probable intestine and an associated fibrous unit, possibly representing muscles. This worm may have been infaunal. These discoveries extend further our understanding of the ancient diversity of Laurentian Burgess Shale-type faunas. In common with the Burgess Shale itself and the Sirius Passet fauna (Peary Land, Greenland) the location of fossil-Lagerstätte [see Shields (1998) for a discussion of the terminology of Lagerstätten] of the Kinzers Formation adjacent to a prominent escarpment reinforces earlier evidence of the paleotopography exerting an important control on the distribution of Burgess Shale-type faunas. Whether this is a result of localized faunal abundances or taphonomic control is, however, uncertain.

Burgess Shale-type preservation of both non-mineralizing and ‘shelly’ Cambrian organisms from the Mackenzie Mountains, northwestern Canada

Lower to Middle Cambrian shales of the Mount Cap Formation in the Mackenzie Mountains, northwestern Canada, host a variety of Burgess Shale-type macrofossils, including anomalocarid claws, several taxa of bivalved arthropod, articulated hyolithids, and articulated chancelloriids. Hydrofluoric acid processing has also yielded a broad range of organic-walled fossils, most of which are derived from forms more typically known as shelly fossils; e.g., trilobites, inarticulate brachiopods, small shelly fossils (SSF), hyolithids, and chancelloriids. Organic-walled hyolithids include conchs, opercula and helens; the proximal articulation of the helens is erosive, suggesting that they were formed “instantaneously” and periodically replaced. Organic-walled chancelloriid sclerites exhibit a polygonal surface texture and an inner “pith” of dark granular material with distally oriented conoidal divisions; such a pattern is similar to that seen in the fibers of some modern horny sponges and points to a poriferan relationship for the chancelloriids. The robust nature but minimal relief of most of these fossils suggests that primary biomineralization was minimal.