COMPARATIVE TAPHONOMY OF BURGESS SHALE-TYPE FOSSILS: VERMIFORM SOFT TISSUE PRESERVATION IN THE LOWER–MIDDLE CAMBRIAN OF THE WESTERN UNITED STATES

With numerous Ediacaran–Cambrian localities worldwide, Burgess Shale-Type (BST) lagerstätten, defined by preservation of organisms as carbonaceous compressions, have afforded paleontologists a spectacular view of the evolution and ecosystems of early complex metazoans. Even through compression, BST fossils notably retain an exceptional flattened likeness, replete with anatomical detail, of their originally three-dimensional precursors, a likely result of rapid collapse of structurally integral cellular tissues. These fossils are mostly remnants of recalcitrant extracellular structures, and are sometimes expressed with apparent pyrite or clay associations. Although the constructive role, if any, of pyrite and aluminosilicates in preserving soft tissues under the BST taphonomic regime is still debated, comparative taphonomy between organisms of varying original composition and tissue types can provide insights onto the chemical pathways leading to this style of preservation.

While tissues of BST animal fossils, such as recalcitrant arthropod cuticles, have been examined in detail with advanced techniques, fewer analyses have focused on metazoans comprised chiefly of more labile tissues. In this study, eight exceptionally preserved vermiform fossils, including two specimens of Ottoia prolifica, from the Lower–Middle Cambrian Pioche, Spence, and Marjum deposits of the Western U.S. were analyzed using environmental scanning electron microscopy (ESEM) to acquire microtaphonomic and micromorphological data. Evaluated against previous studies of contemporaneous cuticular metazoan tissues and macroalgae, vermiform organisms are ideal candidates for comparative tissue-type preservation studies along the kerogenization–aluminosilification–pyritization taphonomic gradient. ESEM techniques permit nondestructive determination of surface and micrometer-scale subsurface composition, and, when paired with high-magnification topographic and compositional imaging signals on both part and counterpart, allow for high-resolution microtaphonomic characterization. This approach helps to provide a process-oriented understanding of BST preservation and expression, and may additionally provide clues to the affinities of unassuming worm and worm-like fossils.