A new predatory actinopterygian from the Tournaisian of Nova Scotia provides insight into the evolution of actinopterygian feeding
Bibliographic record
Abstract
The Devonian–Carboniferous transition represents a fundamental shift in vertebrate faunal composition and ocean ecology. Tournaisian-aged outcrops of the Horton Bluff Formation from Blue Beach, Nova Scotia capture this moment and yield a diverse fauna of actinopterygians and other vertebrates. Here, we report an actinopterygian mandible preserved in 3D, representing a new genus and species, Sphyragnathus tyche. This mandible is elongate, deeply curved, and bears a primary dentition of heterodont fangs. Actinopterygian identity is established by the characteristic ornamentation, dentition, and overall mandible construction observed in the specimen. Analysis of the relationship between mandible and body length in Paleozoic actinopterygians establishes S. tyche as a relatively large actinopterygian. Mandible length, curvature, and fang morphology combine to produce a functionally differentiated dentition with distinct regions for prey capture and prey processing. Comparison with modern actinopterygians places S. tyche as a back-fanged macrodont, distinguishing it from front-fanged macrodont actinopterygians of the Late Devonian. This earliest known instance of back-fanged macrodonty in the actinopterygian fossil record provides further evidence of actinopterygian morphological differentiation post-Devonian and implies experimentation in feeding mode. Apparent changes in feeding mode are underscored by analysis of stress distribution across the dentition of Devonian front-fanged macrodont actinopterygians and S. tyche. Although this specimen is compatible with a previous ‘head-first’ model of morphological diversification in early Carboniferous actinopterygians, we argue that a ‘feeding-first’ model is a better fit.
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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 | 0.001 | 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) | 0.003 | 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".