Development of the dermal skeleton in <i>Alligator mississippiensis</i> (Archosauria, Crocodylia) with comments on the homology of osteoderms
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Notice bibliographique
Résumé
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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Prédiction distillée sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,001 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,001 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découle