EDIACARA-TYPE FOSSILS IN CAMBRIAN SEDIMENTS

Fossil assemblages that preserve soft-bodied organisms are essential f or our understanding of the composition and diversity of past life. Th e worldwide terminal Proterozoic Ediacara-type fossils (from similar t o 600-544 Myr BP) are unique in consisting of soft-bodied animals, whi ch are typically preserved as impressions in coarse-grained sediments( 1-4). These Lagerstatten are also special because they pre-date the ma jor burst of skeletonization, which occurred near the start Of the Cam brian period(3). Most Ediacara-type fossils are interpreted to be cnid arians, but higher metazoans such as annelids and molluscs may also be represented(1-4). However, the unique style of preservation and diffi culties in finding convincing morphological homologies with definite a nimals have led some specialists to prefer non-metazoan interpretation s, such as Vendobionta(5). In addition, the rarity of Ediacara-type fo ssils in younger sediments has led to suggestions of a terminal Proter ozoic mass extinction(6). Here we report typical Ediarcara-type frond- shaped fossils that occur together with an assemblage of Cambrian-type trace fossils in unequivocally Cambrian-aged sediments of the Uratann a Formation, South Australia. This occurrence bridges the apparent div ide between the terminal Proterozoic and Cambrian fossil assemblages, and also suggests that closure of a taphonomic window (an interval of time with unique preservational conditions) was as important as extinc tion in the disappearance of Ediacara-type organisms.