Spinosaurusis not an aquatic dinosaur

A predominantly fish-eating diet was envisioned for the sail-backed theropod dinosaur,Spinosaurus aegyptiacus, when its elongate jaws with subconical teeth were unearthed a century ago in Egypt. Recent discovery of the high-spined tail of that skeleton, however, led to a bolder conjecture, thatS. aegyptiacuswas the first fully aquatic dinosaur. The ‘aquatic hypothesis’ posits thatS. aegyptiacuswas a slow quadruped on land but a capable pursuit predator in coastal waters, powered by an expanded tail. We test these functional claims with skeletal and flesh models ofS. aegyptiacus. We assembled a CT-based skeletal reconstruction based on the fossils, to which we added internal air and muscle to create a posable flesh model. That model shows that on landS. aegyptiacuswas bipedal and in deep water was an unstable, slow surface swimmer (<1m/s) too buoyant to dive. Living reptiles with similar spine-supported sails over trunk and tail in living reptiles are used for display rather than aquatic propulsion, and nearly all extant secondary swimmers have reduced limbs and fleshy tail flukes. New fossils also show thatSpinosaurusranged far inland. Two stages are clarified in the evolution ofSpinosaurus, which is best understood as a semiaquatic bipedal ambush piscivore that frequented the margins of coastal and inland waterways.