Bone histology of the ichthyosaurs: comparative data and functional interpretation

Abstract

The periosteal cortex in the shaft of limb bones is described histologically in three ichthyosaurian genera, Omphalosaurus, Stenopterygius, and Ichthyosaurus. The primary periosteal deposits are composed of typical woven-fibered tissue that was accreted as spongy bone in young individuals, and more or less compact bone in older individuals. During growth, the bone tissue was extensively remodeled with a quantitative imbalance between resorption and redeposition. As a result, the cortex was made cancellous, if previously compact, or still more spongy, if already cancellous. This pattern of remodeling explains why compact cortices are generally lacking in the long bones of ichthyosaurs. The presence of woven-fibered tissue strongly suggests that the limb bones, and probably also the body as a whole, had a rapid postnatal growth in ichthyosaurs, that might have been related to a high, “endotherm-like” metabolic rate. This hypothesis bears on the ecological interpretation of the ichthyosaurs: they could have been capable of sustained, fast swimming and long-range movements, rather than being slow-moving creatures as commonly supposed.