Were they more likely to stalk the shallows like waders, or swim and dive like the largest of all carnivorous dinosaurs? Paleontologists report that this previously controversial question is now being shed light on the bone characteristics of the spinosaurs: According to their analyzes and comparisons, the giants had a high bone density that is typical of aquatic animals. Similar to crocodile and co, this feature arguably stabilized spinosaurs in the water and enabled them to dive when hunting for fish. It is now clear that these were actually aquatic dinosaurs, the scientists say.
The evolutionary history of all animals began in the water, then the ancestors of the land dwellers crawled out of the element and finally settled the terrestrial habitats. Later, however, some species returned: animals from different lines of development adapted again to life in the water. Among the mammals, these are whales and seals, as well as some semi-aquatic species. Among birds, penguins and among reptiles, crocodiles are examples of mainly aquatic animals that once evolved from land dwellers. This is how the swimming marine iguanas of the Jurassic and Cretaceous periods came into being. However, mosasaurus, plesiosaurs and the like were descendants of special lines of development and not representatives of the dinosaurs. On the other hand, no water inhabitants were known of these for a long time.
How did the bizarre giants live?
But in 2014, researchers led by Nizar Ibrahim from the University of Portsmouth described the spinosaurs as aquatic dinosaurs. It is a genus of two-legged predatory dinosaurs that included record-breaking giants that eclipsed T. rex and co. Ibrahim and his colleagues identified features in a specimen from North Africa that they interpreted as clear evidence of a predominantly aquatic lifestyle. They assumed that the spinosaurs swam and dived in the water, pushing themselves against the bottom with their legs and propelling themselves forward with their oar tails. But this did not convince other paleontologists – a controversy developed as to the actual way of life of spinosaurs. Some researchers interpreted the anatomical features as evidence that the animals did not actively hunt in the water, but probably only walked in shallow water. They could have dipped their heads in order to rummage and snap for prey like herons, storks and the like.
This discussion has prompted Ibrahim and an international team of paleontologists to explore another way of proving the way of life. “Previous studies have shown that aquatic animals have dense, compact bones in their skeletons,” says lead author Matteo Fabbri of the Field Museum of Natural History in Chicago. It is assumed that these dense and therefore heavy bones serve to stabilize the animals in the water and to control their buoyancy. The scientists have now specifically pursued this lead through extensive investigations. They compiled a data set of cross-sections of femurs and rib bones from 250 extinct and living animal species, including both terrestrial and aquatic species. “We made sure there was a lot of variety: the selection included seals, whales, elephants, mice and even hummingbirds. We also included dinosaurs of various sizes and extinct marine reptiles like mosasaurs and plesiosaurs,” says Fabbri.
The researchers were then able to classify the analysis results of bones from Spinosaurus and its relatives Baryonyx and Suchomimus into the results. “The first step in the study was to confirm that there is indeed a universal correlation between bone density and lifestyle. And the second was to derive ecological adaptations in extinct animals,” explains the paleontologist.
Monstrous water creatures
The team reports that the results of the analysis showed a clear link between bone density and animal lifestyle: Species that dive underwater to forage have bones that are almost universally solid, while cross-sections of bones from land-dwellers tend to be similar Donuts look – they have a predominantly hollow core. “We found a very clear correlation. So it can be said that all animals that forage completely submerged have these dense bones,” says Fabbri.
This finally allowed the researchers to interpret the analysis results of the bone sections of Spinosaurus and its relative Baryonyx discovered in England. Both were found to have the kind of dense bones associated with full submersion. This is confirmed by the fact that they swam and dived when hunting. Interestingly, however, this was apparently not the case for Suchomimus discovered in North Africa, which is also believed to be a relative of Spinosaurus. He possessed a comparatively hollow bone material. He probably lived in the shore areas and snatched fish out of the water there, as his crocodile-like snout and his conical teeth suggest. But according to his bone density, he probably wasn’t in and under water.
As for the prominent Spinosaurus, Ibrahim concludes, “I think that with this additional evidence, ideas that Spinosaurus was some kind of giant shorebird can now be ruled out. Now we know that even the internal architecture of the bones is consistent with our interpretation of this animal as a giant predator that fished in rivers and used its paddle-like tail for propulsion.” However, as he finally announced, research into the exciting dinosaur is not complete: “We are currently working on reconstructing in much more detail how these monstrous water dwellers once moved,” says the paleontologist.
Source: Field Museum of Natural History, University of Portsmouth, Article: Nature, doi: 10.1038/s41586-022-04528-0