Integrative
and Comparative Biology 43,
114-126 (2003) Jerzy
Dzik Synopsis. Various modes of preservation of Ediacaran
fossils in different sediments, quartz sand at Zimnie Gory in northern Russia
and lime mud at Khorbusuonka in northern Yakutia, show that the sediment was
liquid long after formation of the imprints and that its mineralogy did not
matter. A laminated 2 mm thick microbial mat is preserved intact at Zimnie
Gory. It stabilized the sediment surface allowing formation of imprints on it.
The soft body impressions on the under surface of the sand bed and within it
developed owing to formation of a less than 1 mm thin ‘death mask’
by precipitation of iron sulphide in the sediment. Fossils of the same species
or even parts of the same organism may be preserved differently. Internal
organs either collapsed, their cavities being filled with sediment from above,
or resisted compression more effectively than the rest of the body. This
allows restoration of the original internal anatomy of Ediacaran organisms. At
Zimnie Gory numerous series of imprints of Yorgia
on the clay bottom surface with the collapsed body at their end represent
death tracks. The environment of formation of the Ediacaran fossils was thus
inhospitable to most organisms. Those adapted to it, namely the radially
organized frondose Petalonamae (of possible ctenophoran affinities), anchored
in the mat with their basal bulbs. They evolved towards sessile life possibly
in symbiosis with photo- or chemoautotrophic microorganisms. Vagile Ediacaran
organisms belong mostly to the Dipleurozoa (somewhat resembling chordates and
nemerteans), characterized by a segmented dorsal hydraulic skeleton, intestine
with metameric caeca, and serial gonads. Only a fraction of the actual
Precambrian faunal diversity is represented in the Ediacaran biota. |