Extant aquatic reptiles are rather scarce – only about 0.8% of the living fauna but, they are much more diverse and abundant in the fossil record, especially during the Mesozoic. And some of the ancient reptiles show distinct morphologies and degrees of adaptation to aquatic life styles were secondarily adapted to aquatic environments. Bone histology is one of the major sources of information about life history traits and histological features in fossil reptiles and several aquatic reptile taxa have been examined for bone histology. These studies have generally focused on a single group but a review of the data from multiple clades provides better understanding what bone histology can tell us about the process of secondary adaptation to an aquatic life.
In a recent paper, Alexandra Houssaye (2013) focuses on two main histological features: (1) the organization of the collagenous weave [to distinguish between lamellar, parallel-fibred and fibrous (woven-fibred) bone]; and (2) the organization of the vascular network.
She finds they show the absence of interaction between these histological features on the one hand and body size, mode of swimming, type of microanatomical specialization and phylogeny on the other. Instead these histological features provide information about the growth rate and basal metabolic rate of these taxa. The growth rate seems to have been rather high in most marine reptiles, when compared with terrestrial ectotherms. Moreover, distinct metabolic abilities are suggested. Some clades probably displayed a peculiarly high body temperature, and some show trends towards endothermy. The study also emphasizes the need for homologous comparisons in histology and shows that much remains to be done to better understand the relationship between histological features, growth rate and metabolism in extant taxa in order to make inferences in the fossil groups
Houssaye, A. (2013) Bone histology of aquatic reptiles: what does it tell us about secondary adaptation to an aquatic life?. Biological Journal of the Linnean Society, 108: 3–21. doi: 10.1111/j.1095-8312.2012.02002.x