ON BIOTURBATION AND BURGESS SHALE-TYPE DEPOSITS - A CASE STUDY OF TRACE FOSSIL-BODY FOSSIL ASSOCIATION IN THE KAILI BIOTA (MIDDLE CAMBRIAN), GUIZHOU, CHINA

More deposits of exceptional preservation occur in the Cambrian than in the remainder of the Phanerozoic. However, the abrupt decline of exceptional fossil deposits is a subject of considerable debate. Among others, bioturbation has been suggested as one of the major limiting factors for preservation of Burgess Shale-type deposits. Trace fossils commonly occur in the Kaili Biota (middle Cambrian), which is an important transitional community between the older Chengjiang Biota and the younger Burgess Shale Biota; thus, a comprehensive survey of both trace fossils and body fossils can tell us more about the relations between bioturbation and fossil preservation in a Burgess Shale-type deposit.

New discoveries based on 323 specimens examined include not only additional specimens of eldoniids associated with trace fossils that have been reported before, but also echinoderms, trilobites, monoplacophorans, and non-biomineralizing arthropods all preserved in various degrees of preservation and associated with disturbed bedding surfaces. This new information allows us to test previous hypotheses on the absence of trace fossils with Burgess Shale-type preservation.

Common ichnologic elements of the Kaili Biota include Arenicolites, Cochlichnus, Cruziana, Gordia, Planolites, Rusophycus, and Trichophycus. Based on the observed effects of bioturbation on the preservation of seven different organisms, it is clear that infaunal scavengers/deposit feeders were actively plowing through the sediments on the Kaili sea floor. In general, the average infaunal activity is low to moderate in the Kaili substratum, and the Ichnofabric Index ranges from i.i. = 1 to 3, except for the “Phycodes Beds” that are completely disturbed by infaunal activity (i.i. = 5). Compared with either the older Chengjiang Biota or the younger Burgess Shale Biota, relative scarcity of volatile soft-part preservation in the Kaili Biota may be caused by post-burial bio-disturbance due to the relatively high intensity of local bioturbation. It is plausible that oxygenation in deeper sediments made possible with bioturbation allows carcasses undergo a relatively more extensive period of normal decay prior to final burial.