A severe drop in Eurasian ichthyosaur diversity prior to their late Cenomanian extinction: local or global signal?
Bibliographic record
Abstract
During the last decade, our knowledge of the taxonomic diversity of the Early Cretaceous ichthyosaurs (Mesozoic marine reptiles) has increased significantly, with the recognition of new species, genera, and subfamilies from Canada, Europe, and Russia. New data from England, France, and western Russia suggest ichthyosaurs remained diverse and abundant in western Eurasian marine ecosystems up to the late Albian–early Cenomanian, with the co-occurrence of three to four taxa occupying two to three distinct ecological niches in each formation considered (Cambridge Greensand Member, England; Marnes Bleues Formation, France; Stoïlensky quarry, Russia). However, the overlying formations (middle–late Cenomanian), consisting of chalk or glauconiferous sands, have yielded a very depauperate ichthyosaur fauna. These ichthyosaur assemblages are monospecific and comprise medium to large-sized, presumably opportunistic predators belonging the genus Platypterygius. This suggests a severe drop in ichthyosaur diversity some 5 millions years before their final extinction, which presumably occurred at or near the Cenomanian–Turonian boundary. However, it is difficult to know if this pattern is biased or genuine: the diversity drop may very well be an effect of preservational/ecological biases as well as a genuine extinction linked to the profound environmental changes occurring during the Cenomanian. The presence of similar impoverished assemblages in Cenomanian sediments worldwide favours the latter hypothesis, but the question remains open for now.
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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 | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.002 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| 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".