BURGESS SHALE-TYPE PRESERVATION: A COMPARISON OF NARAOIIDS (ARTHROPODA) FROM THREE CAMBRIAN LOCALITIES

Abstract

The chemical composition of well-preserved naraoiids from the Chengjiang, Kaili, and Burgess Shale biotas is compared. Gut diverticulae in samples from all three biotas contain C, P, and Fe, indicating a primary composition of organic carbonaceous material, and the presence of apatite and pyrite as the result of authigenic mineralization in association with decay and early diagenetic processes. Gut traces from Burgess Shale specimens retain apatite and pyrite, as well as clay minerals, reflecting a history involving greenschist-grade metamorphism. Kaili specimens have been subjected to lower-grade metamorphism and, or thermal alteration, but alteration of pyrite pseudomorphs in the gut traces to limonite indicates the effect of weathering. Loss of sulfur and calcium, oxidation of pyrite, and the light color of the Chengjiang samples are the result of a greater degree of weathering than in specimens from the other two localities. As demonstrated here, Kaili samples serve as an important baseline for interpreting specimens from the Burgess Shale (high-grade metamorphism) and the Chengjiang (intense chemical weathering) deposits. Our study shows for the first time that the conservation of organic carbon is the common primary mode of soft-part preservation in naraoiid arthropods from these three signature Burgess Shale-type localities. Differences among these major deposits are a product of later diagenesis and weathering of the authigenic mineralization associated with the preservation of labile structures. Among the major constituents formed during diagenesis—carbonaceous material, calcium phosphate (e.g., apatite), and iron sulfide (e.g., pyrite)—the minerals are more susceptible to chemical weathering via hydrolysis.