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 ∼600–544 Myr BP) are unique in consisting of soft-bodied animals, which are typically preserved as impressions in coarse-grained sediments1,2,3,4. These Lagerstätten are also special because they pre-date the major burst of skeletonization, which occurred near the start of the Cambrian period3. Most Ediacara-type fossils are interpreted to be cnidarians, but higher metazoans such as annelids and molluscs may also be represented1,2,3,4. 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 Vendobionta5. In addition, the rarity of Ediacara-type fossils in younger sediments has led to suggestions of a terminal Proterozoic mass extinction6. 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.