Spinosaurus Was An Aquatic, Tail-Propelled Theropod Dinosaur — Or Was It?
Anatomy, biomechanics, and ichnology tell us that dinosaurs were terrestrial beasts. We didn’t always think so — there was a time when people believed that sauropods and hadrosaurs were semi-aquatic animals. Eventually has been proposed on the scientific literature some dinosaur species with possible semi-aquatic lifestyle, such as Halskaraptor escuilliei, and of course: spinosaurids.
A new paper published on Nature and authored by Ibrahim et al. deals specifically with Spinosaurus aegyptiacus, and claims, on the basis of the study of a well preserved tail collected from the Kem Kem beds in Morocco, that they found evidence for the hypothesis according to which “the highly specialized morphology of the Spinosaurus tail allowed it to function as a propulsive structure for aquatic locomotion”. Well, actually Ibrahim et al. were the ones who proposed this hypothesis.
The presered tail is pretty weird. The neural spines of the caudal vertebrae are quite tall; the first ones project dorsally, but towards the tip of the tail they become more posterodorsally projected. Chevrons on the ventral aspect of the vertebrae project slightly posteroventrally. As pointed out by the authors “Taken together, the elongate neural and haemal arches result in a tail shape that is markedly vertically expanded and has an extensive lateral surface area”.
Additionally, pre- and postzygapophyses (bony structures that interlock adjacent vertebrae together) are less and less developed towards the tip of the tail. Such particular condition implies in more flexibility on the caudal region. This is quite odd since spinosaurids are tetanuran, and these dinosaurs had “stiffened tail in which the degree of overlap in articulation between pre- and postzygapophyses increases along the caudal series, greatly diminishing the range of motion between individual vertebrae”, we are told by Ibrahim et al.
To test the propulsive-structure hypothesis, Ibrahim et al. built models of tails of Spinosarus, two other theropods (Coelophysis bauri and Allosaurus fragilis), the crested newt Triturus dobrogicus, the crocodile Crocodylus niloticus, and a rectangular tail for control. The tails were then “attached to a robotic controller and actuated in a water flume” and the authors evaluated the swimming performance by measuring thrust and efficiency, the latter meaning “the extent to which input power is translated into thrust”.
In short: the Spinosaurus tail generated about 8 times more thrust and were 2.6 times more efficient than the other theropod tails. However, the newt and the crocodile tails (in descending order) generated more thurst than the tail of Spinosaurus; with regard to efficiency, crocodile was followed by the newt and then Spinosaurus. The authors interpreted these results as supporting the “inference that Spinosaurus used tail-propelled swimming”.
Apparently, Ibrahim told Gizimodo “that the tail was a “highly specialized propulsive structure” that would have allowed this dinosaur to actively pursue prey in the water”.
Well, not everybody is happy with that. Many are skeptical about the actual flexibility of the tail, and other aspects, such as if the thrust generated by the tail would make Spinosaurus a tail-propelled-swimming theropod. I encourage you to read some thoughts on the issue here, and here.
You can read a lot more on Spinosaurus and its tail on:
Ibrahim, N., Maganuco, S., Dal Sasso, C. et al. Tail-propelled aquatic locomotion in a theropod dinosaur. Nature (2020). https://doi.org/10.1038/s41586-020-2190-3
GRESHKO, M., 2020. Bizarre Spinosaurus Makes History As First Known Swimming Dinosaur. [online] National Geographic. Available at: <https://www.nationalgeographic.com/science/2020/04/first-spinosaurus-tail-found-confirms-dinosaur-was-swimming/> [Accessed 30 April 2020].
Dvorsky, G., 2020. Well-Preserved Tail Of Terrifying Aquatic Dinosaur Shows It Was A Formidable Swimmer. [online] Gizmodo. Available at: <https://gizmodo.com/well-preserved-tail-of-terrifying-aquatic-dinosaur-show-1843155314> [Accessed 30 April 2020].
