TRACE FOSSILS AND SEDIMENTARY STRUCTURES FROM THE EDIACARAN-CAMBRIAN TRANSITION, WESTERN NEVADA-EASTERN CALIFORNIA

The Ediacaran-Cambrian transition was a time of dramatic evolutionary changes including the diversification of early metazoans, development of resistant skeletons, and escalation of prey-predator systems. In North America, the transition is well recorded in the terminal Neoproterozoic to Cambrian strata of Esmeralda County, Nevada, and Inyo County, California. Localities studied are in the Deep Spring Formation (Ediacaran-Cambrian), and the Campito, Poleta, and Harkless formations (Cambrian).

The Deep Spring Formation shows the transition from a microorganism-dominated evolutionary record to a more diverse and complex record of the Phanerozoic. Stromatolites are common in lower Deep Spring Formation, and fossilized microbial mats (“elephant skin”) are present in siltstone layers of the middle member. Traces preserved in the Ediacaran part of the Deep Spring Formation (Middle Member) include Bergaueria, Palaeophycus, Planolites, and other simple resting, dwelling, and perhaps foraging traces. Microbially stabilized substrates and microbial mat structures have played a role in the detailed preservation of Ediacaran trace fossils and sedimentary structures.

In Cambrian strata, microbial related structures are mostly “wrinkled” structures and gas-escape structures. Chalcopyrite and limonite are commonly observable within burrows. The minerals might be related to decay processes within burrows, however, microbial mats do not seem to be a necessary condition for their formation. Trace fossils include Bergaueria, Cruziana, Monomorphichnus, Palaeophycus, Planolites, Rusophycus, and Trichophycus pedum.

The most dramatic changes in Cambrian trace fossils are correlated with dramatic faunal changes: increases in size, behavioral diversity, complexity, and abundance. Despite the evolutionary changes in the Cambrian, the Ediacaran-Cambrian transition of Nevada-California show that basic behavioral patterns of feeding and dwelling were already developed in the Ediacaran.