ONTOGENY AND ASEXUAL REPRODUCTION OF DISCOIDAL EDIACARAN ORGANISMS: WHY SO MANY DISCS BUT SO FEW WITH FRONDS?

Centimeter-sized discoidal forms are ubiquitous among Ediacaran assemblages, and they are by far the most abundant fossils in several Ediacaran successions (e.g., the St. John’s Group in Newfoundland and the Khatyspyt Formation in northern Siberia). They may represent a diverse range of organisms including microbial colonies or algae, but the majority of them (particularly those with three-dimensional preservation) are plausibly interpreted as holdfast structures of frond-like Ediacaran organisms. This interpretation raises the question why in some Ediacaran assemblages there are so many discoidal holdfasts but so few with stalks and fronds. Selective preservation of discoidal holdfasts over fronds, because holdfasts were already buried in sediments in life, provides part of the answer. But new observations suggest that the local abundance of discoidal Ediacaran holdfasts is also related to their ontogenetic and reproductive mode. Discoidal holdfasts probably represent very early ontogentic stages, with stalks and fronds added in late ontogeny, -- an ontogenetic interpretation supported by the observation that smaller discoidal fossils are less likely to be stalked than larger ones in the same assemblage and that stalks appear to form before fronds during ontogeny. A higher mortality rate at younger ontogenetic ages partially explains the dominance of discoidal fossils in some Ediacaran successions. Additionally, asexual reproduction (e.g., fission and budding) of discoidal holdfasts may have also contributed to their local abundance and clustered occurrences. Twinned discoidal fossils have been previously interpreted as mutually deformed holdfasts, but new material from the Khatyspyt Formation indicates that such twinning may represent asexual fission.