Bacterial symbiosis: The driver for morphological peculiarities of the Vendian organisms?

Category Biostratigraphy
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Serezhnikova, Ekaterina
Holding Date 29 September 2008

Currently, modern symbiotic systems and those of the past are under careful consideration. There is no doubt that symbiosis was a key factor in development of the early biosphere, especially with regard to the origin and evolution of eukaryotes. Detailed understanding of the real morphological characteristics of Vendian fossils has radically altered our understanding of these organisms, many previously reconstructed as medusoids. These forms are now reinterpreted as attachment disks of organisms, whose relationships are not yet understood (Gehling, 2000). This reinterpretation raises questions concerning relations between these attached organisms and the substrate in which they lived.
Generally, Vendian structures have four main morphotypes: cyclic discoid attachments, e.g. Ediacaria; cone-like attachments bearing cyclic radial processes, e.g. Mawsonites; cone-like attachments bearing radial processes rather randomly arranged, e.g. Hiemalora; composite forms showing a combination of some features of the above three morphotypes, e.g. Palaeophragmodictya. These morphotypes seem to be ecological and preservational groups rather than true taxa. The diversity of Vendian basal structures situated within and atop sediments in life position appears to reflect the variety of attachment modes and substrate interactions. Development of microbial mats on ocean floors and minor, and shallow, bioturbation were characteristic of Vendian environments. This resulted in an abrupt transition from oxygenic conditions of the bottom waters to fully anoxic conditions within the sediments below (Seilacher, 1999). The close association and likely symbiosis of these attachments present in the Vendian is noteworthy.
Evidence supporting such an interpretation include: the considerable morphological diversity and complex form of these basal structures; the rapid cyclic isometric growth of the organisms; the absence of distinguishing features of a digestive system; the sedentary life-style; the large contact area with the substrate; the preferential fossilization of the basal parts; abrupt changes of fossil community with these sedimentary sequences; the absence of apparent modern analogues; traces of H2S in sediments; simultaneous extinction of many Vendian forms in concert with an abrupt decline of microbially affected substrates.
There are set of sentences about taxonomy of the Vendian cyclic organisms, such as true Metazoa (Glaessner, 1984, Fedonkin, 1985); giant protists (Seilacher, 1989); lichens (Retallack, 1994); bacterial colonies (Grazhdankin, 2003); fungi (Peterson et al., 2003). As taphonomic and morphological studies indicate that Vendian attachments could represent symbiotrophic structures, bacterial symbiosis may indeed be the driver for morphological peculiarities of the Vendian Metazoan organisms. This idea is supported by newly discovered symbiotic relationships between some living animals and sulphate-reducing, methanothrophic and photosynthetic bacteria.