Soft Tissue Decay

The most destructive process to affect the bodies of organisms is the decay of soft parts. The most volatile parts are tissues, such as internal organs and muscle, and as a result such tissues are very rarely encountered as fossils. Where they are preserved, it is usually (but not always: Butterfield 1990) the result of early dia-genetic mineralization (Allison 1988b).

Anoxia historically has been viewed as a prerequisite for the preservation of such tissues (e.g., Whittington 1971b). However, the removal of environmental oxygen does not prevent microbial activity (Berner 1981b; Allison 1988a; Allison and Briggs 1991a). Microbes simply utilize a variety of alternative oxidants for carbon degradation. In fact, it has been suggested that anoxia may retard the decay process by only a factor of two or three (Canfield and Raiswell 1991a). More importantly, anoxia prevents scavenging and favors early mineralization. The microbial reactions that are involved in anaerobic decay also generate a series of reactive elements, which in some circumstances can go on to produce early diagenetic minerals. These minerals, in turn, may preserve the decaying tissues themselves. The minerals most frequently associated with soft-part preservation are pyrite (Allison 1988a), phosphate (Allison 1988b; Briggs and Kear 1993), and carbonates. The activity of anaerobic bacteria is a necessity for the formation of all three. Formation of organic-clay complexes or clay coatings may also be important in soft-part preservation (Butterfield 1990).

Most well-preserved benthic fossils are preserved approximately at their life sites, so bottom-water anoxia can be ruled out as a preservational factor. Allison's (1990) calculations demonstrate that most organism bodies become anoxic microenvironments internally during early phases of decay. Inhibition of scavenging in these environments may prolong the association of skeletal elements. 1 lowever, even under conditions of anaerobio-sis or anoxia, bacterial decay of ligaments is rapid and the slightest currents will serve to disarray pieces (Allison 1988a, 1990).

Not surprisingly, most cases of soft-part preservation among trilobites are associated with dysoxic mudrock facies, typically dark, slightly organic-rich shales. The Burgess Shale (Middle Cambrian, British Columbia), Frankfort Shale (Beecher's Trilo-bite Bed, Upper Ordovician, New York), and the Hunsriick Shale (Lower Devonian, Germany) are a few of the most notable examples (Allison and Briggs 1991a, 1991b, 1993; see below).

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