Preservation and diagenesis of soft-bodied fossils and the occurrence of phosphate-associated rare earth elements in the Cambrian (Wuliuan) Spence Shale Lagerstätte

Abstract

The Spence Shale Lagerstätte is a middle Cambrian (Miaolingian Series; Wuliuan Stage) fossil deposit in northern Utah and southern Idaho, USA. At present, it is known to preserve 89 species, from at least 10 phyla, of biomineralizing and soft-bodied taxa, and represents the only major Lagerstätte of Wuliuan age in Laurentia outside the Canadian Rockies. In addition to taxonomic diversity, the Spence Shale exposures represent a variety of depositional environments along a continental shelf, which have been affected by several diagenetic events, making it a compelling setting to improve our understanding of taphonomic and diagenetic pathways as they affect the broader Burgess Shale-type (BST) preservation. We present analyses of 28 specimens from six localities using scanning electron microscopy (SEM) and integrated energy dispersive X-ray spectrometry (EDS), together with comparisons of published data. Taxa are predominantly preserved as a combination of carbonaceous compressions, iron oxide and pyrite replacement, but eldoniids of the genus Eldonia consistently show enrichment in rare earth elements (REE). The REE, predominantly lanthanum and cerium, are enriched in the phosphate mineral monazite, which occurs in the inner coil of Eldonia ludwigi . This pattern, seen in all sampled Eldonia , differs from previous reports of REE association in BST preservation in being limited to one tissue type within a specific identifiable taxon. REE enrichment of tissues can occur during the lifetime of an individual or during fossilization, and we present both possible pathways along with an outline of taphonomic processes leading to the preservation of soft-bodied fossils from the Spence Shale. • Main preservation modes are carbonaceous compressions and iron oxides/pyrite. • Phosphatized Eldonia ludwigi have unique REE associations. • Preservation of tissue types is biased, as is found in other Paleozoic Lagerstätten.