Ancestral Body Plan and Adaptive Radiation of Sauropterygian Marine Reptiles
Bibliographic record
Abstract
SUMMARY Mesozoic was the age of reptiles that not only occupied the land and sky but also adapted to the sea. Sauropterygia, from turtle-like placodonts, lizard-like pachypleurosaurs, and predatory nothosaurs to long-necked pistosaurs including plesiosaurs, is one of the most predominant lineages among secondarily aquatic reptiles. However, we know little about the early evolution of sauropterygians involving the morphological gaps among aforementioned sub-groups and their ancestral body-plan, because the Early Triassic fossil remains are scarce shortly after their origin. Here we report a skeleton from the Early Triassic in South China, representing the oldest known complete specimen related to Sauropterygia. It can be referred to Hanosaurus hupehensis and shows a mosaic morphology combining the characters of multiple sauropterygian sub-lineages. We constructed an updated character matrix obtaining the hitherto most comprehensive phylogeny of Triassic sauropterygians, and Hanosaurus hupehensis is stably resolved as the basal-most member of the Sauropterygiformes (clade nov.). Sauropterygians were previously considered using limb propulsion for the movement that is distinguishable from ichthyosaurs and other marine reptiles, but this skeleton reflects an ancestral body plan of sauropterygians unexpectedly developing an elongate trunk and four short limbs, being different from most sauropterygians but more convergently similar to the basal members of other marine reptilian lineages in body shape. After this convergence, we confirm the rapid adaptive radiation of sauropterygiform reptiles following the end-Permian mass extinction. Our results provide an evolutionary framework of sauropterygian marine reptiles and highlight the integrations of both convergences and divergences when an emerging animal lineage arises.
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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 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".