Forbidden fruits in the Garden of Ediacara

Abstract

The expulsion of jellyfish from the "Garden of Ediacara", as described by Adolf Seilacher, has challenged the “gelatinous ocean” stereotype; however, not all discoidal fossils can be interpreted as holdfast structures, sandy skeletons of benthic organisms, microbial colonies, fungal fairy rings, or erosional scratch circles. Here I describe a late Ediacaran (~550 Ma) medusiform organism, Bjarmia cycloplerusa gen. et sp. nov., preserved as a composite mould in a steep crescentic erosional scour cast in fine-laminated sandstone from the Erga Formation in the Southeast White Sea area. Biostratinomic features point to an allochthonous burial of a bowl-shaped body as it trapped mud pebbles when it was suspended in a sediment-laden flow. An unprecedented range of preserved characters, including moulds of a coronal and longitudinal muscles, suggests affinities with scyphomedusae. The organism is reconstructed as a coronate-like jellyfish, with numerous pedalia separated one from another by deep radiating slits, four deep subgenital pits in the floor of the subumbrella, and a skirt of poorly differentiated tentacle-like structures surrounding the large four-cornered mouth opening. Rhopalia and marginal lappets are not preserved in the specimen. Bjarmia cycloplerusa gen. et sp. nov., if borne out by future research, can be used as evidence for a substantial branching by the late Ediacaran within-stem cnidarian lineages—a largely cryptic component of the pre-Cambrian biota—and raises questions about the nature of late Ediacaran food webs.