Meeting Abstract
Skeletal plasticity has been extensively documented in mammals, and some evidence of plasticity exists in modern birds. In contrast, little is known about skeletal responses to physiologic and environmental perturbation in “reptiles”. Our understanding of skeletal biology of modern crocodilians is critical for confident functional interpretation of fossil specimens of extinct archosaurs. Here I review the available experimental evidence of how various factors affect bone growth and remodelling (or lack thereof) in modern crocodilians, focusing on laboratory-based studies of the American alligator (Alligator mississipiensis) and the estuarine crocodile (Crocodylus porosus). I consider potential influences of cardiovascular design (in-series versus in-parallel), locomotor activity (walking and swimming), muscle disuse (via tenotomy), atmospheric oxygen composition (12-36%), thyroid hormone level, and calcium supply. Most of these factors exert at best a limited effect on bone tissue in crocodilian hatchlings and juveniles, although chronic exposure to severe hypoxia and extreme hyperthyroidism can significantly alter post-hatching bone growth trajectory and microstructure. Developmental constraint on calcium supply, however, has a pronounced effect on macro- and microstructural properties of the embryonic skeleton. This suggests that skeletal plasticity in crocodilians may be temporally restricted to the pre-hatching period. Reduced post-hatching plasticity may be of selective advantage, as it allows these successful sit-and-wait predators to maintain a robust skeleton during long periods of musculoskeletal disuse. Whether it is an ancestral or derived character of crocodilians hinges on documentation of skeletal plasticity in other “reptiles”, data for which are currently lacking.
