The Development of the Chimaeroid Pelvic Skeleton and the Evolution of Chondrichthyan Pelvic Fins
Bibliographic record
Abstract
Pelvic girdles, fins and claspers are evolutionary novelties first recorded in jawed vertebrates. Over the course of the evolution of chondrichthyans (cartilaginous fish) two trends in the morphology of the pelvic skeleton have been suggested to have occurred. These evolutionary shifts involved both an enlargement of the metapterygium (basipterygium) and a transition of fin radial articulation from the pelvic girdle to the metapterygium. To determine how these changes in morphology have occurred it is essential to understand the development of extant taxa as this can indicate potential developmental mechanisms that may have been responsible for these changes. The study of the morphology of the appendicular skeleton across development in chondrichthyans is almost entirely restricted to the historical literature with little contemporary research. Here, we have examined the morphology and development of the pelvic skeleton of a holocephalan chondrichthyan, the elephant shark (Callorhinchus milii), through a combination of dissections, histology, and nanoCT imaging and redescribed the pelvic skeleton of Cladoselache kepleri (NHMUK PV P 9269), a stem holocephalan. To put our findings in their evolutionary context we compare them with the fossil record of chondrichthyans and the literature on pelvic development in elasmobranchs from the late 19th century. Our findings demonstrate that the pelvic skeleton of C. milii initially forms as a single mesenchymal condensation, consisting of the pelvic girdle and a series of fin rays, which fuse to form the basipterygium. The girdle and fin skeleton subsequently segment into distinct components whilst chondrifying. This confirms descriptions of the early pelvic development in Scyliorhinid sharks from the historical literature and suggests that chimaeras and elasmobranchs share common developmental patterns in their pelvic anatomy. Alterations in the location and degree of radial fusion during early development may be the mechanism responsible for changes in pelvic fin morphology over the course of the evolution of both elasmobranchs and holocephalans, which appears to be an example of parallel evolution.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.002 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".