Evolution and Ecology of Neoproterozoic-Lower Cambrian Trace Fossils, NW Canada

Abstract

Neoproterozoic to subtrilobite Cambrian strata in the Mackenzie Mountains, Canada, provide a test case for the current paradigm of evolutionary stages in the development of the early infauna. Three predominantly siliciclastic formations (Ingta, Backbone Ranges, and Vampire formations) collectively exhibit 13 facies associations representing nonmarine to distal shelf environments, and can be subdivided into 18 simple sequences, each of 800,000-1,000,000 years duration. Trace fossil occurrences were digitized at the level of simple sequences and plotted on a matrix with facies associations (environment) on the x-axis and simple sequences (time) on the y-axis. This analysis shows that evolution was a first-order control on trace fossil distribution, and that environmental factors exerted an important second-order control. Trace fossils were most abundant in normal marine environments and less so in marginal-marine settings with fluctuating or brackish salinity. Eolian and fresh-water deposits were barren, presumably because animals able to tolerate these conditions had not evolved. Turbulence excluded organisms from shoreface environments. Several biostratigraphically important ichnotaxa (e.g., Treptichnus and Rusophycus), however, show broad environmental tolerance across the marine shelf Taphonomic factors led to fine preservation in interbedded sandstone-mudstone facies, but to diagenetic obliteration of trace fossils in most carbonate facies. Trace fossils provide a consistent indicator of the Neoproterozoic-Cambrian boundary. Four evolutionary zones can be recognized: (I) Simple burrows, (II) Treptichnus pedum Zone, (III) Rusophycus avalonensis Zone, and (IV) Cruziana tenella Zone. Global biostratigraphic and chemostratigraphic correlations imply that Zone I is terminal Neoproterozoic and the overlying three zones subtrilobite Cambrian in age. These zones are present in the Newfoundland Neoproterozoic-Cambrian boundary GSSP, the East European Platform, and elsewhere.