Atypical morphology in the skull of Pachyrhinosaurus lakustai: contributions of pathology and taphonomy to observed variation within dinosaur death assemblages
Bibliographic record
Abstract
The centrosaurine ceratopsid dinosaur Pachyrhinosaurus lakustai is among the most richly represented of all the non-avian dinosaurs due to the large number of individuals preserved in the Pipestone Creek Bonebed of the Upper Cretaceous Wapiti Formation near Grande Prairie in northern Alberta, Canada. Although the material is disarticulated, multiple examples of virtually every bone in the body are present, across multiple ontogenetic stages. These bones appear to be the remains of a single population of P. lakustai that died in a single mass mortality event. Variation among individuals is prevalent amongst these remains, as demonstrated by the patterns of ornamentation seen in different skulls, unique as fingerprints. Many of the differences observed are likely phenotypic: they may represent different genotypes, secondary sexual traits, ontogenetic stages, or life history events. Others, like the lateral or dorsoventral compression that warps many of the bones, are taphonomic in nature. “Virtual paleontology” imaging modalities supplement direct observation of the disparate morphologies with additional information that traditional descriptions of gross morphology would be likely to omit. Tomographic imaging modalities like X-ray computed tomography (CT) allow paleontologists to peer inside fossil specimens, while surface scanning techniques offer quick and easy ways of making three-dimensional (3D) models. Both techniques were applied herein to examples of variation and atypical morphology in the skull of P. lakustai in order to infer their likely causes. CT imagery of UALVP 62634, a partial P. lakustai cranium with an atypical premaxilla, added information from the interior of the specimen to what could be observed about the deformation externally. The ventral margin of the right premaxilla is mediolaterally expanded and marked by two deep depressions. Lack of typical hallmarks of osteolysis or osteonecrosis under CT, paired with a dorsal offset of the expanded region, suggests that this abnormality was a pathologic feature caused by a trauma and resulting soft tissue granuloma(s) that lysed depressions in the bone. Additionally, surface scanning via photogrammetry paired with histology and geographic information system (GIS) technology indicates that one of the most salient variable features of P. lakustai, the eponymous nasal boss, owes its variability to taphonomic deformation: fragile bosses were preferentially eroded to form a gradient of shapes, from domed to bowl-shaped. These findings highlight the effects of pathology and taphonomy in producing the variation seen in dinosaur death assemblages. While pathology at times can lead to ecological inferences about the life experiences of extinct animals, the impact of taphonomy should be taken as a cautionary tale about the reconstruction of ecology based on atypical morphology.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 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".