Two new pre-trilobite faunas from western North America

Perhaps the greatest puzzle in the history of life is the nature and cause of events comprising the Precambrian–Cambrian evolutionary radiation of Metazoa, The major stumbling blocks to resolving this puzzle are a scarcity of late Precambrian and earliest Cambrian faunas in the fossil record and difficulties in correlating the few faunas that are known. The most diverse and best-known basal Cambrian (Tommotian) faunas are from Siberia1–3, and other pre-trilobite shelly faunas are known from the UK4,5, Newfoundland6,7, Mongolia8, China9–11, Scandinavia12 and northwestern Canada13. Here we report the discovery of two new early Cambrian shelly faunas from western North America, one from Nevada and the other from Sonora, Mexico. These faunas consist of small calcitic tubes and cones, some of which are closely related to previously known fossils from other areas. Both faunas are found hundreds of metres below the first trilobite occurrences, below the first presumed trilobite trace fossils, and seem to be earliest Cambrian in age. This is the first discovery of a diverse pre-trilobite shelly fauna in the classic Precambrian–Cambrian sequence of western Nevada and the White-Inyo Mountains of California, and the first report of pre-trilobite small shelly fossils from Mexico. These new finds may lead to an understanding of the pattern and dynamics of the Precambrian–Cambrian metazoan radiation, and to improved biostratigraphical correlations of the strata recording that event.