TAPHONOMIC BIASES INFLUENCING EXCEPTIONALLY PRESERVED NARAOIA FROM THE BURGESS SHALE

The identification of fossilized species relies on morphological characteristics. Accurate morphological observations must take preservational biases into account because, as an organism decays and fossilizes, soft tissues are typically lost, creating a bias towards biomineralized tissues such as shells, bones, and teeth. Cases of exceptional soft-tissue preservation, as in the Burgess Shale, offer unique insights into fossil morphology and diversity of the middle Cambrian. The Burgess Shale is best known for exquisitely preserved specimens; however, its preservation quality varies, presumably as a result of tissue decay. Naraoia is a Nektaspid arthropod first described from the Burgess Shale. It is an ideal experimental proxy for Burgess Shale preservation because of its easily defined characteristics and range of morphologies, which may represent different stages of decay. Using 203 specimens from the Smithsonian Institution National Museum of Natural History collections, we constructed a detailed decay series ranging from 1 (poor) to 10 (exceptional), then assessed the preservational state of each individual. We compared approximately 30 attributes of Naraoia to the decay series and, with confirmation from MDS ordination techniques, identified those that were highly dependent on the organism's level of decay, and thus strongly taphonomically biased. Strongly biased attributes, such as the presence of genal spines and the overall shape of the carapace, are variable due to decay, and must be considered carefully in species designation. Our analyses also identified a clear correlation between the quality of preservation and the orientation of the specimen in reference to the bedding plane. Oblique specimens were typically poorly preserved. Identifying taphonomic biases influencing Naraoia is essential for accurately describing its true morphological variation, as well as preservational processes influencing all organisms found within the Burgess Shale. Understanding taphonomic processes resulting in Burgess Shale-type preservation is vital for accurate interpretations of Cambrian fossils at the dawn of complex animal life.