The ins and outs of Ediacaran discs

Abundant discoid fossils referable to Aspidella and rarer specimens of Eoporpita and Hiemalora occur in the Ediacaran “June beds” deposits of the Mackenzie Mountains, Northwest Territories, Canada. Among them are specimens that preserve partial stem imprints, supporting recent interpretations of Ediacaran discs relatable to Aspidella as the basal holdfast structures of fronds. The range and morphological transitions among the June beds specimens accounted for by taphonomy have implications for our recognition and classification of Ediacaran discoidal fossils. We find that Aspidella represents the remains of an outer membranous cover of the holdfast structure and that Eoporpita, rather than being a distinct biological taxon, represents the remains of its internal contents. Here we reconstruct the internal structure of these holdfasts as a series of laterally-adjacent, hollow, club-like lobes arranged in tiers. This implies that Ediacaran discoid holdfasts may additionally have had structural and/or metabolic functions in addition to their role in anchoring fronds.