Shelly microfossils near the Precambrian–Cambrian boundary, Mackenzie Mountains, northwestern Canada

Abstract

Our understanding of early metazoan evolution is being revolutionized by recent discoveries of pre-trilobite faunas. A fascinating aspect of this evolution is the appearance and development of metazoan skeletal parts near the Precambrian–Cambrian boundary. Fossils of these parts are best known from the lowermost stage (Tommotian) of the Cambrian in Siberia which contain archaeocyathids and small shelly fossils, the latter commonly of phosphatic composition^1–3. Although the sudden, in terms of geological time, appearance of metazoan hard parts near the boundary is recognized as an outstanding evolutionary event, none of the proposals given to explain the ‘Cambrian explosion’ (see refs 4–6) has been widely accepted. Additional information from areas outside Siberia is now needed to broaden our knowledge on early Metazoa, and to assist a geological team, the Precambrian–Cambrian Boundary Working Group of the International Union of the Geological Sciences, that is locating, defining and correlating the Precambrian–Cambrian boundary on a world-wide basis. Here we report the discovery of a small fauna of microfossils near the boundary in the Mackenzie Mountains. New forms are present and a known species is used for correlation with the classic Precambrian–Cambrian sections in Siberia and successions elsewhere.