Skeletal Nets of the Ediacaran Fronds

Abstract

The Ediacara fauna is traditionally regarded as the first complex, diverse and widespread macroscopic life. The uncertainty of systematic position of its members has led to very different views on the early evolution of metazoans. In part, this may be due to a lack of data on sclerotization: a hard skeleton is a part of the archetype of the most taxa known from the fossil record, whereas the Ediacara fauna as a whole is most often considered soft-bodied, which complicates comparison. Here we report the Late Precambrian frond-like fossils (Petalonamae) from the Vendian assemblage of the Southeastern White Sea area (∼555.3 Ma), which show evidence of elaborate skeleton composed of a regular meshwork reinforced by dense longitudinal and circular bands. Judging from the nature of preservation and the dynamics of the environment Ediacaran fronds secreted a relatively rigid but flexible skeleton. The fact that frond-like Petalonamae had a supporting structure similar to that of sponges and cnidarians seems to be a powerful argument in favor of their metazoan affinity. The new observations indicated also that the widespread skeletonization had occurred long before the “Cambrian skeletal revolution”.