BIOSTRATIGRAPHIC AND GEOCHRONOLOGICAL CONSTRAINTS ON EARLY ANIMAL EVOLUTION

Two distinct evolutionary pulses, represented by the Vendian Ediacaran fauna and Cambrian small shelly faunas, are generally thought to char acterize the emergence of macroscopic animals at the end of Precambria n time. Biostratigraphic and uranium-lead zircon age data from Namibia indicate that most globally distributed Ediacaran fossils are no olde r than 549 million years old and some are as young as 543 million year s old, essentially coincident with the Precambrian-Cambrian boundary. These data suggest that the most diverse assemblages of Ediacaran anim als existed within 6 million years of the Precambrian-Cambrian boundar y and that simple discoid animals may have appeared at least 50 millio n years earlier.