Fossil Tail Suggests Spinosaurus Is The First Known Swimming Dinosaur

Spinosaurus was known to be semi-aquatic, hunting prey possibly in the shallows. Now, it's powerful tail suggests it was quite the accomplished swimmer. Daniel Eskridge/Shutterstock

The long-held belief that many non-avian dinosaurs were restricted to terrestrial environments has been challenged by a well-preserved fossil tail of Spinosaurus aegyptiacus. The findings, published this week in the journal Nature, describe the unique structure of the tail and how this paddle-like appendage would’ve been the perfect propellant for aquatic hunting, suggesting Spinosaurus is the first known swimming dinosaur.

The well-preserved tail of a subadult Spinosaurus aegyptiacus was found in the 95-million-year-old Kem Kem beds in south-eastern Morocco. The fossil reveals that Spinosaurus had a flexible tail with a series of tall neural spines. The tail would have been able to move laterally to create thrust, propelling the predatory dinosaur through the water in a similar way to the locomotion we see in modern-day crocodiles.

Reconstruction of the tail skeleton of Spinosaurus (missing bones shown in white). Center: cross-sections through the tail showing changes in the vertebrae, tail volume, and arrangement of major muscles. Bottom: the new - and surprising - look of Spinosaurus (black, soft parts/body outline; red, bones collected in 2008 by a local fossil collector; green, bones from recent scientific excavations; yellow, bone fragments collected in the debris around the main excavation area). Drawings: Marco Auditore. Photos: Gabriele Bindellini.

The researchers used a robotic flapping apparatus to model the movement of the recovered specimen, testing the undulatory forces created by different tail shapes. Their results showed that the tail shape of Spinosaurus was far better equipped for marine environments compared to the tail shapes of terrestrial dinosaurs.

The performance of the model indicates the tail was an adaptation to an aquatic lifestyle in keeping with the piscivorous diet that’s previously been proposed for Spinosaurus. The idea that Spinosaurids took to aquatic environments is not new, as previous studies have suggested these predatory dinosaurs may have waded in marine environments for fish. This new research, however, is the first unambiguous evidence of a specialized tail for moving in water.

Spinosaurids’ fossil records span over 50 million years, but these enormous dinosaurs, at around 15 meters in length (50 feet), are only known from incomplete fossils. Not only that but the most complete Spinosaurus fossil was destroyed during World War II. The specimen found by Nizar Ibrahim, from the University of Detroit Mercy, and colleagues on this dig represents the most complete skeleton of a Cretaceous theropod recovered to date in mainland Africa.

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