Bone density reveals lifestyle –

Were they more likely to chase shallow waters like waders, or to swim and dive like the largest carnivorous dinosaurs? Paleontologists report that this previously controversial question now sheds light on the bone characteristics of spinosaurs: according to their analyzes and comparisons, the giants had a high bone density that is typical of aquatic animals. Similar to the crocodile and its colleagues, this feature can be said to have stabilized spinosaurs in the water and enabled them to dive when searching for fish. Scientists say it is now clear that these were in fact aquatic dinosaurs.

The evolutionary history of all animals began in the water, then the ancestors of the inhabitants of the Earth crawled out of the element and finally settled the terrestrial habitat. Later, some species returned: animals of different lines of evolution again adapted 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 aquatic animals that once evolved from Earth’s inhabitants. This is how marine iguana swimming appeared in the Jurassic and Cretaceous periods. However, mosasaurs, plesiosaurs, and the like were descendants of special development lines and are not representatives of dinosaurs. On the other hand, the inhabitants of the waters were not known about these for a long time.

How did the wonder giants live?

But in 2014, researchers led by Nizar Ibrahim of the University of Portsmouth described spinosaurs as aquatic dinosaurs. It is a genus of legged predatory dinosaurs that includes record-breaking giants that overshadowed T. rex and co. Ibrahim and colleagues identified traits in a sample from North Africa that they interpreted as clear evidence of a predominantly aquatic lifestyle. They hypothesized that spinosaurs swim and dive in the water, using their legs to press the bottom and the tail of the rudder to propel themselves forward. But this has not convinced other paleontologists – a controversy that has arisen over the actual way of life of spinosaurs. Some researchers have interpreted anatomical features as evidence that the animals did not actively hunt in the water, but may have only walked in shallow water. They could dip their heads in order to search for prey, similar to herons, storks and the like.

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This discussion prompted Ibrahim and an international team of paleontologists to explore another way to prove 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 control their buoyancy. Scientists have now sought precisely this leadership through extensive investigations. They compiled a data set of femur and rib cross-sections from 250 extinct and animal species, including both terrestrial and aquatic species. “We made sure there was a lot of diversity: 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 Fabry.

The researchers were then able to categorize the results of the analysis of the bones of Spinosaurus and its relatives Baryonyx and Schomemus into the results. The first step in the study was to confirm the existence of a global relationship between bone density and lifestyle. The second is to derive ecological adaptations in extinct animals,” the paleontologist explains.

monstrous water creatures

The team reported that the results of the analysis showed a clear correlation between bone density and the animal’s lifestyle: species that dive underwater in search of forage have bones that are nearly universally solid, while cross-sections of bone from land dwellers tend to be similar to muffins. – They have a hollow heart mostly. We found a very clear relationship. So it can be said that all animals that feed on completely submerged water have dense bones,” says Fabry.

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This finally allowed the researchers to interpret the results of analyzing the bone sections of Spinosaurus and its Baryonyx lineage unearthed in England. Both types of dense bone were found associated with complete immersion. This is confirmed by the fact that they swim and dive while hunting. Interestingly, however, this was apparently not the case for Schomemus discovered in North Africa, which is also thought to be a relative of Spinosaurus. He possessed a relatively hollow bone material. He supposedly lived in beach areas and grabbed fish from the water there, as his crocodile-like nose and conical teeth suggest. But judging by the density of his bones, he likely wasn’t in the water or underwater.

As for the notable Spinosaurus, Abraham concludes, “I believe that with this additional evidence, ideas that the Spinosaurus was a giant shorebird can now be dismissed. We now know that even the internal structure of the bones is consistent with our interpretation of this animal as a gigantic river predator that hunted and used its paddle-like tail for propulsion.” As he recently announced, the exciting dinosaur exploration is far from over: “We are currently working on a much more detailed reconstruction of how the feral water dwellers moved,” says the paleontologist.

Source: Field Museum of Natural History, University of Portsmouth, Article: Nature, doi: 10.1038/s41586-022-04528-0

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