A NEW MODE OF PRESERVATION IN A BURGESS SHALE-TYPE LAGERSTÄTTE, GUIZHOU PROVINCE, SOUTH CHINA

Previous observations on preservation of nonmineralizing tissues in the Burgess Shale-type deposits include organic carbon films, replication of clay minerals, pyritization, and phosphatization. Here we report silicification as a new mode of preservation from the Kaili biota, South China. The middle portion of the Kaili Formation is composed primarily of silty mudstone. It contains a Burgess Shale-type Lagerstätte dominated by trilobites (> 70 species), echinoderms, and the discoidal animal Pararotadiscus guizhouensis. Nonmineralizing taxa, including arthropods, discoidal animals, worms, and algae, are commonly preserved as organic carbon films. Phosphatization is rare and restricted to internal volatile tissues, such as gut glands.

Several clusters containing uncompacted silicified spheroids have been recovered from beds containing elements of the Kaili biota. These spheroids are interpreted as putative marine invertebrate eggs based on the normal distribution of the spheroid size, 500 to 850 micron in averaging diameter, and the analogue of modern invertebrate eggs. Fecal material is also present in the same deposit, but they are preserved as organic compressions and are different from the putative egg spheroids based on composition and relief. Pellet interpretation for some clusters, which consist of much smaller-sized spheroids and ellipsoids, cannot be completely ruled out at this moment.

These spheroids are unlike Cambrian and/or Precambrian fossil embryos reported in the past decade, which are exquisitely preserved as either phosphate encrustation, replacement in carbonates and phosphorites, or as silica permineralization in chert. This study documents putative eggs preserved by silica replacement with pore spaces and iron oxide impurities in a fine-grained siliciclastic setting. Silicification can occur in low pH and high solubility conditions. Decalcification of trilobite exoskeletons and echinoderm plates further supports the low pH environment during diagenesis. Silicified spheroids are three-dimensional and show little sign of compaction. We suggest that the silicification took place prior to mesodiagenesis, which is indicated by compaction and dehydration. Co-occurring siliceous sponge spicules may have been the source for silicification.