HITTING THE SLOPES: THE KEY TO TRILOBITE TAPHONOMY

Paleoecological conditions such as rapid burial, depositional energy levels and anoxic settings control the degree and quality of preservation throughout the fossil record. Trilobites of the Spence Shale in Idaho were first discovered by Charles Doolittle Walcott in 1908. One year later, Walcott discovered the now recognized UNESCO World Heritage Burgess Shale site located in Yoho National Park. Although both are marine shales of Cambrian age, fossils in the Burgess Shale are exceptional, having preserved soft tissues. The Spence Shale, by contrast, preserves only mineralized skeletal elements. This raises the question as to what paleoecological conditions account for these different preservational styles. Analyzing the preservation of trilobites, hyolithids, and brachiopods from the Spence Shale indicates a high frequency of disarticulation and breakage of skeletons compared to the Burgess Shale. These observations, together with consideration of the regional geology, point towards a difference in the angle of the slope of the marine depositional setting. The depositional setting of the Burgess Shale was a steep slope providing anoxic conditions and rapid burial, while the Spence Shale was a gradual slope bisected by the storm wave base, leading to variations in depositional energy and oxygenation levels.