15 meters long and 20 tons heavy: Spinosaurus is considered the largest of all known predatory dinosaurs. However, in contrast to Tyrannosaurus rex and Co, he was probably not targeting other dinosaurs, but rather fish: new finds of remains of his tail now complete the picture of an aquatic animal. The features and tests with reconstructions show that the fin-like extension was able to provide effective propulsion in the water – similar to today’s crocodiles.
Spinosaurus aegyptiacus was for a long time a mystery of paleontology, because the first known fossil from Egypt was tragically lost to modern research: the find, which was stored in Munich, was completely destroyed in a bomb attack in 1944. Only fragmentary finds in North Africa were added to the photos and recordings received – for a long time many questions about the spectacular dinosaur were therefore unanswered. That only changed in 2014: paleontologists presented a very well preserved fossil from Spinosaurus, which came from Morocco. On the basis of the results of the investigation, they were able to reconstruct many features of the animal – with one important exception: because of the initially missing remnants of the tail, it has so far been presented analogously to the features of the extensions of other two-legged dinosaurs.
The results of the fossil investigation to date had already suggested that Spinosaurus had at least a semi-aquatic lifestyle. This is supported by a number of adjustments, such as the rather small hind limbs, wide feet, and features of the skull and bone density. It was believed that the dinosaur was hunting large fish in the waters of Cretaceous North Africa. His striking back sail may have served him as a visible viewing element for fellow species in the water.
A fin-like tail is emerging
However, the fact that the animals were actually good swimmers was previously controversial. Because a plausible drive method seemed to be missing and according to models, the giant dinosaur would even have to do what
overturned if he lost contact with the feet to the bottom. But as the researchers led by Nizar Ibrahim from the University of Detroit Mercy report, new discoveries from Morocco now provide a coherent overall picture. These are the almost complete series of tail vertebrae that belong to the 2014 fossil.
As the investigations showed, the tail of Spinosaurus differed significantly from that of other dinosaurs: While the extension of T. rex and Co tended to be evenly pointed and quite stiff, the appendage of Spinosaurus had a flat fin-like shape: the vertebrae had long extensions which gave the body part a slim and tall shape. Further characteristics of the bone structures show that the tail must have been very flexible laterally. These results suggest a wave-like movement, the scientists explain.
Drive similar to a crocodile
In order to investigate the possible propulsion of the tail when moving in the water, the scientists carried out tests with a robot-assisted “flapping device”: They recorded the wave forces of models of different tail shapes. In this way, they were able to confirm that, in contrast to the versions of other dinosaurs, the tail of the Spinosaurus provided strong thrust in the water. The effect was comparable to that of the tails of today’s aquatic animals such as crocodiles or newts. The swimming tail probably also stabilized the position of Spinosaurus in the water, the researchers say. The animal did not run the risk of toppling over in the open water, as was previously assumed. “The results are clear evidence that the dinosaur was an aquatic propulsion structure,” Ibrahim and his colleagues summarize.
The knowledge of the characteristics of Spinosaurus is now also changing the traditional image of dinosaurs as pure land animals, say the paleontologists. For a long time the Cretaceous world was clearly divided: the fish-like ichthyosaurs lived in the water, the pterosaurs whizzed through the air and the dinosaurs were considered the rulers of the country. As can now be seen, the distribution was obviously not so clear. Spinosaurus aegyptiacus was probably not an isolated case either. As the researchers report, there is evidence that other representatives of the Spinosauroidea group have had at least some adjustments to an aquatic lifestyle.