After being declared the first known swimming dinosaur in 2020, Spinosaurus was subsequently suspected of being a shore-wader. A new analysis of its bones, however, has swung the needle back towards the concept of it going underwater to hunt its prey.
Spinosaurus was initially believed to be an adept swimmer due to its paddle-shaped tail, its crocodile-like long snout full of pointy teeth (ideal for catching fish), and the fact that its center of gravity was too far forward to support its weight if walked bipedally on land.
The discovery of a huge deposit of fossilized Spinosaurus teeth in an ancient riverbed further supported the idea that it lived an aquatic life.
It would have been a fearsome predator, with an estimated body length of 18 m (59 ft) and a weight of over 20 tons (18 tonnes). That's larger than Tyrannosaurus rex. And no, the plesiosaurs and mosasaurs don't count as swimming dinosaurs – although they were prehistoric reptiles, they were not part of the dinosaur group.
In 2021, scientists from Queen Mary University of London and the University of Maryland suggested that while Spinosaurus was likely indeed capable of swimming, it probably fed more like a stork, snatching up fish as it waded along the water's edge. Among other things, the researchers pointed to the facts that its eyes and nostrils weren't on top of its head, whereas those of a crocodile are, plus its tail wasn't as muscular.
Now, a team from the University of Cambridge, University of Oxford and Chicago's Field Museum is stating that if nothing else, Spinosaurus did at least fully submerge itself while hunting for food. Such is reportedly indicated by its solid, dense bones, which would have allowed it to sink under the surface of the water.
The scientists are basing this theory on an analysis of femur and rib bone cross-sections from 250 species of living and extinct animals. It was found that while the bones of land-dwelling creatures have hollow centers, the bones of animals known to forage underwater are completely solid. In cases where animals started out on land then evolved to an aquatic life – such as was the case with whales – their bone density changed accordingly.
"Spinosaurus might have moved through shallow water using a combination of 'bottom-walking' – like modern hippos – and side-to-side strokes of its giant tail," said U Cambridge's Dr. Guillermo Navalón, co-lead author of a paper on the study. "It probably used this means of locomotion not to pursue prey for long distances in open water, but to ambush and catch very large fish like lungfishes or coelacanths that lived in the same environment."
The paper was recently published in the journal Nature.
Source: University of Cambridge