Development of the dermal skeleton in <i>Alligator mississippiensis</i> (Archosauria, Crocodylia) with comments on the homology of osteoderms
Bibliographic record
Abstract
The dermal skeleton (=exoskeleton) has long been recognized as a major determinant of vertebrate morphology. Until recently however, details of tissue development and diversity, particularly among amniotes, have been lacking. This investigation explores the development of the dermatocranium, gastralia, and osteoderms in the American alligator, Alligator mississippiensis. With the exception of osteoderms, elements of the dermal skeleton develop early during skeletogenesis, with most initiating ossification prior to mineralization of the endoskeleton. Characteristically, circumoral elements of the dermatocranium, including the pterygoid and dentigerous elements, are among the first to form. Unlike other axially arranged bones, gastralia develop in a caudolateral to craniomedial sequence. Osteoderms demonstrate a delayed onset of development compared with the rest of the skeleton, not appearing until well after hatching. Osteoderm development is asynchronous across the body, first forming dorsally adjacent to the cervical vertebrae; the majority of successive elements appear in caudal and lateral positions. Exclusive of osteoderms, the dermal skeleton initiates osteogenesis via intramembranous ossification. Following the establishment of skeletal condensations, some preossified spicules become engorged with many closely packed clusters of chondrocyte-like cells in a bone-like matrix. This combination of features is characteristic of chondroid bone, a tissue otherwise unreported among nonavian reptiles. No secondary cartilage was identified in any of the specimens examined. With continued growth, dermal bone (including chondroid bone) and osteoid are resorbed by multinucleated osteoclasts. However, there is no evidence that these cells contribute to the rugose pattern of bony ornamentation characteristic of the crocodylian dermatocranium. Instead, ornamentation develops as a result of localized concentrations of bone deposited by osteoblasts. Osteoderms develop in the absence of osteoblastic cells, osteoid, and periosteum; bone develops via the direct transformation of the preexisting dense irregular connective tissue. This mode of bone formation is identified as metaplasia. Importantly, it is also demonstrated that osteoderms are not histologically uniform but involve a range of tissues including calcified and uncalcified dense irregular connective tissue. Between taxa, not all osteoderms develop by homologous processes. However, it is concluded that all osteoderms may share a deep homology, connected by the structural and skeletogenic properties of the dermis.
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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 | 0.000 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".