A new plastomenid trionychid (Testudines: Pan-Trionychidae) from Milk River Formation of southern Alberta (Cretaceous: Santonian)
Bibliographic record
Abstract
The pre-Campanian trionychid fossil record in North America is composed of highly fragmentary specimens, which are often not identifiable beyond Pan-Trionychidae. Here, we describe a new species of plastomenid soft-shelled turtle based on a partial shell (ROM 56647) from the Santonian Milk River Formation of southern Alberta, estimated at approximately 84 Ma. This represents the oldest relatively complete shell of a trionychid from North America. Plastomenidae, a pan-trionychid clade known only from the fossil record, is classically characterized by the complete suturing of its posterior plastral bones along the midline, a crescent-shaped entoplastron, and enlarged costals VIII. ROM 56647 has a unique combination of plastomenid characters (i.e., mid-line contact of hypoplastra and xiphiplastra, anteroposteriorly long eighth costal) and apomorphies (an emarginate nuchal, enlarged tubercles on the carapace, a wide pygal notch with a straight anterior edge, and a fused hyo-hypoplastron) that allows us to identify it as a new taxon, Jimemys glaebosus gen. et sp. nov. Phylogenetic analysis places J. glaebosus within Plastomenidae as the sister taxon to a clade containing Plastomenus, Helopanoplia, and Hutchemys. This phylogenetic position implies that Aspideretoides foveatus Leidy, 1856, Atoposemys, and Gilmoremys, all of which are more basal within Plastomenidae, had ghost lineages extending at least to the Santonian. As the oldest pan-trionychid that is diagnostic to the species level in North America, J. glaebosus provides new insights into both the early evolution of trionychids in North America and the biodiversity of southern Alberta during a poorly sampled time in the Late Cretaceous of North America.
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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 | 0.001 | 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".