Burgess Shale fossils shed light on the agnostid problem
Bibliographic record
Abstract
Agnostids (agnostinids and eodiscinids) are a widespread and biostratigraphically important group of Cambro-Ordovician euarthropods whose evolutionary affinities have been highly controversial. Their dumbbell-shaped calcified tergum was traditionally suggested to unite them with trilobites, but agnostinids have alternatively been interpreted as stem-crustaceans, based on Orsten larval material from the Cambrian of Sweden. We describe exceptionally preserved soft tissues from mature individuals of the agnostinids Peronopsis and Ptychagnostus from the middle Cambrian (Wuliuan Stage) Burgess Shale (Walcott Quarry and Marble Canyon, British Columbia, Canada), facilitating the testing of alternative hypotheses. The digestive tract includes conspicuous ramifying cephalic diverticulae. The cephalon carries one pair of elongate spinous antennules projecting to the front, two pairs of appendages with distally setose, oar-like exopods, and three pairs of presumably biramous appendages with endopods sporting club-shaped exites. The trunk bears five appendage pairs, at least the first two of which are similar to the posteriormost cephalic pairs. The combined evidence supports a nektobenthic and detritivorous lifestyle for agnostinids. A head with six appendiferous segments contrasts strikingly with the four known in trilobites and five typical of mandibulates. Agnostinids are retrieved as the sister group to polymeroid trilobites in our phylogeny, implying that crustacean-like morphologies evolved homoplastically. This result highlights the variability in segmental composition of the artiopodan head. Finally, our study emphasizes the continued role of Burgess Shale-type fossils in resolving the affinities of problematic biomineralizing taxa.
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 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 | 0.001 | 0.003 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.002 | 0.001 |
| Science and technology studies | 0.002 | 0.006 |
| Scholarly communication | 0.002 | 0.002 |
| Open science | 0.001 | 0.002 |
| Research integrity | 0.002 | 0.001 |
| Insufficient payload (model declined to judge) | 0.005 | 0.001 |
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".