| 1. |
- Grazhdankin, Dmitriy V., et al.
(author)
-
Carbonate-hosted Avalon-type fossils in arctic Siberia
- 2008
-
In: Geology. - 0091-7613 .- 1943-2682. ; 36:10, s. 803-806
-
Journal article (peer-reviewed)abstract
- Avalon-type fossils are crucial to understanding the origin of Phanerozoic ecosystems, but their usual occurrence in volcaniclastic and siliciclastic facies greatly limits their paleobiological resolution. The unique carbonate-hosted. assemblage of the Khatyspyt Formation, on the Olenek uplift of north-central Siberia, promises a much enhanced anatomical and paleoecological view of these enigmatic organisms. Avalon-type fossils are preserved by authigenic carbonate cementation in intervals of finely laminated nodular limestones (Khatyspyt-type taphonomic window). Interbedded silicified calcareous mudstones yield diverse carbonaceous compressions, occasionafly with taphonomic phantoms of Avalon-type taxa (Miaohetype taphonomic window). Styles of moldic preservation do not appear to be taxon selective, and therefore cannot alone be responsible for the morphological distinctiveness of Ediacaran macrofossils and the profound disparity in the taxonomic composition between fossil assemblages. On the other hand, the exclusion of Avalon-type fossils from carbonaceous compressions (Miaohe-type preservational window) is a real taphonomic signal that provides an important constraint on the properties of certain Ediacaran tissues.
|
|
| 2. |
- Xiao, Shuhai, et al.
(author)
-
Towards an Ediacaran Time Scale : Problems, Protocols, and Prospects
- 2016
-
In: Episodes. - : International Union of Geological Sciences. - 0705-3797 .- 2586-1298. ; 39:4, s. 540-555
-
Journal article (peer-reviewed)abstract
- The Ediacaran Period follows the Cryogenian Period in the wake of a snowball Earth glaciation and precedes the Cambrian Period with its rising tide of animal radiation. It is also the longest among all stratigraphically defined geological periods, lasting 94 million years (635-541 Ma). Hence, a good Ediacaran time scale is essential, not only to elucidate geological time, but also to provide a temporal context for extreme climatic events and transformative evolutionary transitions. Ediacaran fossils are known from many sections and boreholes around the world, permitting ready age recognition and stratigraphic correlation of Ediacaran strata. However, the Ediacaran fossil record is colored by taphonomic biases that variously affect the preservation of the soft-bodied organisms that dominated Ediacaran marine ecosystems, and the Phanerozoic approach of defining stratigraphic boundaries using the first appearance datum (FAD) of widely distributed, rapidly evolving, easily recognizable, and readily preservable species would have limited success in the Ediacaran System. The subdivision of the Ediacaran System must therefore be founded on a holistic approach integrating biostratigraphic, chemostratigraphic, and geochronometric data for correlation. Series-level subdivision of the Ediacaran System is a challenging task, and alternative models subdividing the Ediacaran System into two or three series can be recognized. Resolving these alternatives critically depends on obtaining further data to constrain the age, duration, and global extent of the Shuram negative delta C-13 excursion, to calibrate and correlate Ediacaran acanthomorph biozones, and to determine the temporal relationship among the Shuram excursion, the Gaskiers glaciation, and Ediacaran acanthomorph biozones. Stage-level subdivisions at the bottom and top of the Ediacaran System, however, are realistic goals in the near future, and we propose that the subdivision of the Ediacaran System should initially aim at the second Ediacaran stage (SES) and the terminal Ediacaran stage (TES) where stratigraphic information is relatively rich and consensus for stratigraphic correlation is emerging. Potential stratigraphic markers for the definition of the SES include the post-glacial radiation of eukaryotes as represented by the first appearance of acanthomorph acritarchs, the termination of the cap carbonate series, or the end of the negative delta C-13 excursion (EN1 = Ediacaran negative excursion 1) associated with the cap carbonate. Terminal Ediacaran strata are well dated and host several taxa of skeletal and tubular fossils that postdate the Shuram negative delta C-13 excursion (or its probable equivalent, EN3 = Ediacaran negative excursion 3) where their stratigraphic relationship can be determined; these biostratigraphic markers may be used to define the TES in a Phanerozoic fashion. Additional Ediacaran stages between the SES and TES can be envisioned. Through collaborative efforts in the Ediacaran community, we hope that the first Precambrian stage will be established in the near future to facilitate a better understanding of the geological aftermath of snowball Earth, the redox history of global oceans, the early evolution of multicellular life, and the evolutionary fuse of the Cambrian explosion.
|
|
