Tail-propelled aquatic locomotion in a theropod dinosaur

Abstract

Intensive research on non-avian dinosaurs in recent decades strongly suggests that these animals were restricted to terrestrial environments1. Historical views proposing that some groups, such as sauropods and hadrosaurs, lived in aquatic environments2,3 were abandoned decades ago4,5,6. Recently, however, it has been argued that at least some spinosaurids, an unusual group of large-bodied Cretaceous theropods, were semi-aquatic7,8, but this idea has been challenged on anatomical, biomechanical, and taphonomic grounds and remains controversial9,10,11. Here we present the first unambiguous evidence for an aquatic propulsive structure in a dinosaur, the giant theropod Spinosaurus aegyptiacus7, 12. This dinosaur has a tail with an unexpected and unique shape consisting of extremely tall neural spines and elongate chevrons forming a large, flexible, fin-like organ capable of extensive lateral excursion. Using a robotic flapping apparatus to measure undulatory forces in physical tail models, we show that the tail shape of Spinosaurus produces greater thrust and efficiency in water than the tail shapes of terrestrial dinosaurs, comparable to that of extant aquatic vertebrates that use vertically expanded tails to generate forward propulsion while swimming. This conclusion is consistent with a suite of adaptations for an aquatic lifestyle and a piscivorous diet in Spinosaurus7,13,14. Although developed to a lesser degree, similar aquatic adaptations are found in other spinosaurids15,16, a clade with a near global distribution and a stratigraphic range of more than 50 million years14, documenting a significant invasion of aquatic environments by dinosaurs.