Semiaquatic adaptations in a giant predatory dinosaur

TLDR

The study describes semiaquatic adaptations in *Spinosaurus aegyptiacus*. The authors identify morphological features—retracted nostrils, elongated neck and trunk, reduced pelvis and short, solid hindlimbs, robust femur, and flat‑bottomed claws—that facilitate buoyancy control and foot‑propelled swimming in *Spinosaurus*. Bone evidence indicates the dorsal sail was likely skin‑covered and served mainly for display rather than hydrodynamic function.

Abstract

We describe adaptations for a semiaquatic lifestyle in the dinosaur Spinosaurus aegyptiacus. These adaptations include retraction of the fleshy nostrils to a position near the mid-region of the skull and an elongate neck and trunk that shift the center of body mass anterior to the knee joint. Unlike terrestrial theropods, the pelvic girdle is downsized, the hindlimbs are short, and all of the limb bones are solid without an open medullary cavity, for buoyancy control in water. The short, robust femur with hypertrophied flexor attachment and the low, flat-bottomed pedal claws are consistent with aquatic foot-propelled locomotion. Surface striations and bone microstructure suggest that the dorsal "sail" may have been enveloped in skin that functioned primarily for display on land and in water.