Skeletal Nets of the Ediacaran Fronds

The Ediacara fauna is traditionally regarded as the first complex, diverse and widespread macro-scopic 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".