TAPHONOMIC ANALYSIS OF CAMBRIAN VERMIFORM FOSSILS OF UTAH AND NEVADA, AND IMPLICATIONS FOR THE CHEMISTRY OF BURGESS SHALE-TYPE PRESERVATION

Abstract

Burgess Shale-type preservation is a predominantly Paleozoic style of exceptional fossilization via the process of kerogenization, through which organic tissues are converted to more geologically stable forms of carbon. It is often associated with two additional modes of mineralization: (1) pyritization, collectively the precipitation of pyrite on, near, or replacing decaying organic matter; and (2) aluminosilicification, the association of clay minerals, frequently templating the fossil material. Further, some organisms preserved through the Burgess Shale-type pathway also show limited phosphatization, the replication of tissues by apatite, which is usually restricted to digestive tracts/organs. Here, sixteen Cambrian vermiform (worm-like) fossils, preserved via Burgess Shale-type preservation in three formations of the Great Basin, western US, were analyzed using scanning electron microscopy-based methodologies. These fossils display a wide range of taphonomic character, with visual differences in kerogenous-, pyrite-, and aluminosilicate-associated preservation, in addition to some preserved medial structures presumed to be gut tracts. Microchemical analyses indicate additional unique mineral associations, including barite and monazite, which can be broadly attributed to later-stage diagenetic alteration of the initial preservational mineralization. A consistent model of the chronology and drivers of mineralization is presented, and may prove useful for considering Burgess Shale-type preservation at other localities.