Ediacara-type fossils in Cambrian sediments

Fossil assemblages that preserve soft-bodied organisms are essential for our understanding of the composition and diversity of past life. The worldwide terminal Proterozoic Ediacara-type fossils (from similar to 600-544 Myr BP) are unique in consisting of soft-bodied animals, which are typically preserved as impressions in coarse-grained sediments(1-4). These Lagerstatten are also special because they pre-date the major burst of skeletonization, which occurred near the start Of the Cambrian period(3). Most Ediacara-type fossils are interpreted to be cnidarians, but higher metazoans such as annelids and molluscs may also be represented(1-4). However, the unique style of preservation and difficulties in finding convincing morphological homologies with definite animals have led some specialists to prefer non-metazoan interpretations, such as Vendobionta(5). In addition, the rarity of Ediacara-type fossils in younger sediments has led to suggestions of a terminal Proterozoic 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 Uratanna Formation, South Australia. This occurrence bridges the apparent divide 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 extinction in the disappearance of Ediacara-type organisms.