Late Ediacaran macrofossils in storm and debris flow deposits reveal the allochthonous nature of many Ediacaran palaeocommunities from the Nama Group, Namibia
Bibliographic record
Abstract
Our ability to compare late Ediacaran macrofossil assemblages between global localities and place firm constraints on the palaeoecology of Ediacaran biota is limited by a paucity of detailed palaeoenvironmental reconstruction and sedimentological facies analysis for many fossil-rich strata. Here, we present sedimentological evidence for ubiquitous transport of soft-bodied macro-organisms in storm deposits of the Aar Member, Nama Group, Namibia. Higher in the succession, transport of biomineralizing taxa in debris flows of the Spitzkop Member reveals further evidence for mass transport. Fossiliferous sandstone beds of the Aar Member often have sharp bases with erosional scours, flute casts, tool marks, and elongate parallel syneresis cracks. Upper plane stage beds pass both upward and along strike into 3D dunes and 3D ripples (hummocky cross-strata), to 2D wave-ripple lamination, and finally to silty sandstone beds. Facies and bedform transitions occur over short distances (m-scale to decameter-scale) before event beds pinch out into mudstone. Drawing on constraints from flume experiments, these beds require rapid deceleration and sediment aggradation under combined unidirectional and oscillatory flow conditions – common during storm events. Ediacaran soft-bodied macrofossils such as Pteridinium are aligned with flow or twisted, jumbled, and overturned in event beds – i.e., they are entrained and transported in storm deposits and are not preserved in situ. Other soft-bodied macrofossils such as Ernietta can colonize the sediment surface after events and may be in situ. In some Spitzkop Member localities, biomineralizers such as Cloudina and Namacalathus are preserved in cm- to m-scale stacked carbonate beds (recording many events), have scoured bases with tool marks, and are graded and capped by bypass surfaces. Individual fossils (clasts) within beds are chaotic in orientation or imbricated. Erosive bases and chaotic arrangement of clasts are typical of gravity-induced debris flows in deep-water and therefore many biomineralizing taxa are also not in situ. Our findings have implications for palaeoecological interpretations of Ediacaran taxa and for research into potential late Ediacaran extinction events. Consideration of palaeontological and sedimentological data from late Ediacaran shallow marine sedimentary sections in the U.K., Australia, and Canada further permits direct comparison between our Namibian data and the Ediacaran palaeocommunity compositions of comparable depositional environments.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".