FORTUNIAN SOFT-BODIED FOSSILS AND PLUG-SHAPED BURROWS FROM THE SYHARGALAKH FORMATION, NE SIBERIAN PLATFORM

There are three hypotheses explaining the disappearance of the soft-bodied Ediacaran biota at the Precambrian-Cambrian boundary: 1) mass extinction of most Ediacaran forms; 2) biotic/ecological displacement of the Ediacaran forms by the emerging Cambrian biota; 3) ‘the Cheshire Cat’ hypothesis – the closure of the unique taphonomic window necessary for the preservation of non-mineralized organisms. Here, we are testing the third hypothesis by investigating the fossil record of the Syhargalakh Formation, early Fortunian, Cambrian in age (~543-530 Ma).

The Syhargalakh Formation encompasses besides typical Fortunian ichnoassemblage, also plug-shaped vertical burrows, and soft-bodied organisms, which we are nominally assigning to Ediacara-like holdfasts and one possible frond-like organism. There are 16 specimens of plug-shaped burrows, which were identified as Berguaria hemispherica and B. perata. Furthermore, 17 specimens of discoidal fossils, which could be tentatively named Aspidella-type holdfasts, and 11 specimens of discoidal fossils with root-like appendages, which more closely remind the Mawsonites-type holdfasts. Absolutely standing out one specimen of a “frond”-like fossil, which could be either a body fossil or a piece of a microbial mat, however, one specimen is not enough to draw definite conclusions.

Based on the petrographic and electron microscope investigations three-dimensional preservation of the holdfasts was provoked by an authigenic crystallization of calcite and its further early-diagenetic dolomitization. Unlike, the Berguaria, holdfast fossils have a statistically meaningful difference between dolomite crystal size inside the fossil and in the sediment around them. Thus, the Syhargalakh Formation is the unique taphonomic window, which reveals that some Ediacara-type soft-bodied organisms could tolerate moderate bioturbation and exist through at least the early Fortunian time. Thus, even though ‘the Cheshire Cat’ hypothesis of the disappearance of the Ediacara-like organisms in the Phanerozoic is valid in general, it probably was more complicated and took more time for the ecosystem reconstruction during this pivotal time of the Earth’s history.

The research was funded by RFBR # 20-35-70016, RFS # 21-17-00052, and government contract 0331-2019-0002 (Ministry of Education and Science of the Russian Federation).