Skeletal anatomy and systematic placement of the Permian actinopterygian Brachydegma caelatum based on new data from μCT
Bibliographic record
Abstract
The sparse Permian record of actinopterygian fishes remains relatively understudied. As a result, the origins of the morphologically diverse actinopterygian clades of the Triassic, and with them the origins of surviving major lineages, are incompletely understood. One of the few three-dimensionally preserved Permian actinopterygians is Brachydegma caelatum Dunkle 1939, an enigmatic species from the Artinskian (early Permian) of Texas. Brachydegma has had a volatile systematic history, being affiliated with Elonichthys, acrolepids, crown holosteans, and, more recently resolved as a crownward member of the neopterygian stem. This ambiguity in phylogenetic placement stems from limited character information available for Brachydegma, which is restricted to superficial details of the external skeleton. Using computed microtomography (μCT) I investigated the internal anatomy of the two known specimens. Surrounding matrix shows considerable growth of dense (likely iron) minerals that make segmentation difficult, but nevertheless μCT yields considerable new morphological data highlighting an unusual mosaic of characters. Widely distributed actinopterygian traits revealed in Brachydegma include: an aortic canal; a persistent oticooccipital fissure; a perforate hyomandibula; five branchial arches; clavicles capping the anteroventral processes of the cleithra; and a notochord partially constricted by paired neural and haemal elements. Derived features of Brachydegma with a more restricted distribution among actinopterygians include: a parasphenoid stalk that underlies the otic and the anterior part of the occipital regions; well-developed uncinate processes on the epibranchials; and multiple basibranchial ossifications. Distinctive—and potentially autapomorphic— traits include: a broad interorbital septum and a peculiar hyomandibula with a thin dorsal and a much wider ventral limb. These features reinforce recent verbal and cladistic arguments excluding Brachydegma from the neopterygian crown, but the unusual combination of features in this genus and the instability of relationships among ‘early’ actinopterygians precludes a more precise placement at present.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
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 | 0.000 | 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 | 0.000 |
| 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".