Growth and life history of freshwater chelydrid turtles (Testudines: Cryptodira): A bone histological approach

The current study examines the growth pattern and lifestyle habits of the freshwater snapping turtles Chelydra and Macrochelys based on limb bone histology. Femora, humeri, and tibiae of 25 individuals selected from a range of ontogenetic stages were assessed to determine inter-element and intraskeletal histological variation. Osteohistological assessment of multiple elements is consistent with overall moderate growth rates as revealed by the dominance of parallel-fibered bone. However, the growth was cyclical as shown by deposition of multiple lines of arrested growths in the compacta. It appears that the bone tissue of C. serpentina is more variable through ontogeny with intermittent higher growth rates. M. temminckii appears to grow more slowly than C. serpentina possessing compact and thick cortices in accordance with their larger size. Overall, vascularization decreases through ontogeny with humeri and femora being well-vascularized in both species. Contrarily, epipodials are poorly vascularized, though simple longitudinal and radial canals are present, suggesting differences in growth patterns when compared with associated diaphyseal sections. The tibiae were found to be the least remodeled of the limb bones and therefore better suited for skeletochronology for snapping turtles. Intra-elementally, femora and humeri preserved higher cortical vascularity ventrally, suggestive of faster relative growth. We hypothesize that the differential growth pattern in limb bones of snapping turtles may relate to differential functional constraints, where forelimbs are operational in swimming while the hindlimbs provide stability. This is the first-ever detailed study that has examined the bone histology of freshwater snapping turtles Chelydra and Macrochelys. In this study, femora, humeri, and tibiae of 25 individuals were selected from a range of ontogenetic stages and histologically analyzed. Our histological results reveal overall moderate growth rates for snapping turtles; however, it appears that the bone tissue of C. serpentina is more variable through ontogeny with intermittent higher growth rates as compared to M. temminckii. In addition, intra-elemental (potentially allometric) and inter-elemental growth differences also exist in the limb bones due to differential functional constraints. We also document important differences in the relative preservation of growth marks between elements and between the sampled species, providing useful insights into the interpretation of histological growth indicators (and associated tissue types).image