New evidence suggests dinosaurs may have ruled not only the land, but the waters too.

Ground-breaking research provides “unambiguous evidence” that the dinosaur Spinosaurus aegyptiacus possessed a uniquely-shaped tail which facilitated swimming. This, combined with its other aquatic adaptations, proves that Spinosaurus was an aquatic dinosaur, and opens the door to the theory that dinosaurs invaded the water not just as visitors, but as inhabitants.

However wacky you choose to imagine a terrestrial dinosaur, something like it probably exists. From the turkey-sized predator Compsognathus, to the giant Mamenchisaurus, whose 46-foot neck alone is longer than the rest of its body put together, to the monstrous (and toothless) Gigantoraptor, confirmed dinosaur morphologies range from the improbable to the seemingly ridiculous. So, with diversity this immense, what makes aquatic dinosaurs such a problematic idea?

Really, it’s all in the name. The concept of living in water isn’t new at all; in fact, many well-known marine reptiles, including plesiosaurs, ichthyosaurs, and crocodilians, lived during the time of the dinosaurs. However, these water-dwelling beasts were not dinosaurs.

The name “dinosaur” is not just a general term; it’s also the name of a scientific group. Strictly, all dinosaurs fall into the group Dinosauria: the ‘terrible lizards’. Species contained in a scientific group all share a set of clearly-defined features, and share a common ancestor. For Dinosauria these features include some reptilian traits – having a spine comprised of vertebrae, and producing eggs, for example, as well as more complex morphological specifications.

Critically, all members of Dinosauria have legs that are positioned under directly under their bodies, unlike other reptiles, whose legs are positioned to the side. This is possible due to the presence of a hole in dinosaur hip sockets, which allows the leg to sit below. Because of this evolutionary development, dinosaurs could walk and run effectively, and their legs were able to bear the weight of their heavy bodies. This distinction is what makes species like Spinosaurus (as well as modern day birds), dinosaurs. It is also what excludes prehistoric marine reptiles (and indeed avian reptiles) from the group: they lack this key evolutionary development.

Thus, scientists considered dinosaurs to be entirely terrestrial. While was agreed that many dinosaurs were able to venture into the water seeking prey, perhaps wading through on long legs, theories surrounding aquatic dinosaurs fell by the wayside, abandoned due to controversy and a lack of conclusive results.

A previous imagining of how Spinosaurus
may have looked.

That is, until recently. In 2014, Ibrahim and his team found that the sail-backed Spinosaurus, an unequivocal dinosaur, possessed bones with unusually thick walls, similar to those that help modern animals like penguins control their buoyancy. Their suggestion that this could indicate an aquatic lifestyle was met with pushback at the time. Scientific peers found the results polarizing – for some, it was just more evidence that the dinosaur may have waded into the water after its prey. Luckily, Ibrahim was not deterred.

In 2018, the team discovered a Spinosaurus tail that was 80% complete. Modelling this fossil allowed them to produce a plastic replica, which was used in underwater experiments designed to identify the strength of forces generated by the tail. If the forces generated a 
strong thrust, it would suggest that Spinosaurus used its wide, fin-shaped tail to propel it through the water like a paddle.

The results, reported in Nature in April 2020, were more than Ibrahim could possibly have hoped for. Spinosaurus’ tail was found to exert more than eight times the amount of thrust that was achieved by tails of other dinosaurs, such as Coelophysis or Allosaurus.

This finding cemented Ibrahim’s belief that the tail was used for more than just stability while wading. It is a paddle, used for tail-propelled swimming, which would have allowed the 16m long Spinosaurus to navigate aquatic habitats much like a grossly oversized (and deadly) newt.

The tails efficiency, coupled with the previous findings suggesting buoyancy control and a pescatarian diet, implies that Spinosaurus not only ventured into the water, but certainly swam, and even spent time submerged. However, these adaptations wouldn’t have precluded life on land, either. Fossil evidence of Spinosaurus has been found far inland; so, while the dinosaur was certainly a swimmer, it’s likely that it also spent at least some time ashore.

By discovering that Spinosaurus was a swimming dinosaur, Ibrahim’s team have begun to redefine long-standing beliefs about what a dinosaur can be. What’s more, these results also hint at a much larger trend – potential for a “persistent and widespread” presence of dinosaurs in aquatic habitats.

A modern day swamp, that may not be dissimilar to the Cretaceous swamps where Spinosaurus roamed.

Article Link: Ibrahim, N., Maganuco, S., Dal Sasso, C. et al. Tail-propelled aquatic locomotion in a theropod dinosaur. Nature 581, 67–70 (2020). https://doi.org/10.1038/s41586-020-2190-3

See also: https://eurekalert.org/pub_releases/2020-04/ngs-nfr042920.php