Attachment Adaptations of Ediacaran Sedentary Organisms  

Fixation to the substrate is thought to be an important aspect of the biology and ecology of benthic organisms. However, Ediacaran attachment structures were not considered as a significant object for investigation, due to organism’s disjunction during the fossilization process.  Nevertheless, the analysis of attachment patterns makes it possible to recognize some morphological peculiarities of organisms which do not appear to have evident modern analogues.  The attachment structures, being preserved in situ, serve as unambiguous ecological markers, enabling reliable comparisons between Phanerozoic and Ediacaran communities. 

There are seven basic strategies of attachment employed by Phanerozoic organisms. Some were typical for Phanerozoic-style soft substrates with separate hard ground environments; others for rather firm Proterozoic- and Early-Middle Cambrian-style substrates. Based on a taphonomic analysis of representative samples of specimens collected from a single fossiliferous layer, some Ediacaran attachment adaptations were recognized: (1) fixation on the substrate surface by organic gluing (Fedomia); (2) attachment to the substrate with a sucker-like structures (Vaveliksia) and peculiar incrustations (Palaeophragmodictya); (3) fixation with pointed basal parts inserting into the microbially bound sediment (tubiform Cloudina and conical Thectardis); (4) anchoring into the sediment by discoidal (Ediacaria) and rhizoid-like (Hiemalora) holdfasts; (5) partial submergence into the sediment (Nemiana) and infaunal life habit (Petalonama – Pteridinium, Namalia, Ernietta, Swartpuntia, Rangea).

Taking into account the adaptations listed above, one can suppose that both Proterozoic-style fixation (1-3) and Phanerozoic-style ones (4-5) were widespread in the Ediacaran communities.  If the data on taphonomy and morphology of Ediacaran sedentary organisms are interpreted correctly, we can conclude that most attachment adaptations to the soft and hard substrates had already emerged before the Cambrian. 

The observations of Ediacaran community succession suggest that a significant change of attachment strategies took place.  For example, Avalon sequences are characterized by minor variants of attachments at the beginning, reaching maximum diversity before a reduction of the variants at the terminal part of the sequence.  A similar trend seems to be observed during the existence of the Ediacara biota, although the ambiguity of the interpretation of cyclic fossils makes this scheme less reliable.   Such a trend can be explained by intrinsic causes of features’ evolution as well as global environmental changes which have been occurred at the beginning of Cambrian.