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Spinosaurus is the largest known carnivorous dinosaur, even larger than the fearsome Tyrannosaurus rex. In addition to its imposing presence -more than twelve meters long and an unmistakable sail on its back-, little is known about this giant from 100 million years ago, since there are very few remains in the world. The only well-preserved ones were destroyed in an Allied air raid on Munich, Germany, during World War II.
With so few clues, speculating on the life of this formidable animal has been really difficult. One of the most debated aspects by paleontologists is whether spinosaurus was able to swim. In its skeleton some have seen a true swimmer, while others have proposed that it simply waded through the water like a heron.
To solve the mystery, a group of paleontologists have taken a different approach: examining the density of the dinosaur’s bones. Comparing them to those of other animals such as penguins, hippos and alligators, the team found that Spinosaurus and his close relative Baryonyx they had dense bones that would probably have allowed them to dive underwater to hunt. Meanwhile, another related dinosaur named Suchomimus it had lighter bones that would have made it harder for it to swim, so it probably waded or spent more time on land like other dinosaurs.
aquatic traits
As the researchers explain, all life initially came from water, but in most groups of terrestrial vertebrates there are members that have returned to it. Among mammals, whales and seals returned to the ocean, while otters, tapirs and hippos are semiaquatic. Among the birds, penguins and cormorants did. And among the reptiles, alligators, crocodiles, marine iguanas and sea snakes. For a long time, no non-avian dinosaurs (those that didn’t branch into birds) were believed to be aquatic, until in 2014 Nizar Ibrahim of the University of Portsmouth described a new spinosaur skeleton.
Scientists already knew that spinosaurids spent some time in the water: their long, crocodile-like jaws and cone-shaped teeth are similar to those of other aquatic predators, and some fossils had been found with bellies full of fish. . But the new specimen described in 2014 had retracted nostrils, short hind legs, paddle-like feet and a fin-like tail — all signs that pointed to an aquatic lifestyle. But the researchers continued to debate whether spinosaurids swam in search of food or simply stood in the shallows and poked their heads in to catch their prey.
laws of physics
To end the debate, Matteo Fabbri, a researcher at the Field Museum, decided to look at general physical laws. “There are certain laws that are applicable to any organism on this planet. One of these laws refers to the density and the ability to submerge in water », he explains.
Throughout the animal kingdom, bone density is an indicator of whether that animal can sink below the surface and swim. “Previous studies have shown that water-adapted mammals have dense, compact bones in their postcranial skeletons,” says Fabbri. The dense bone functions as buoyancy control and allows the animal to submerge.
Fabbri and colleagues, including researchers from the universities of Cambridge and Oxford, assembled a dataset of femur and rib cross-sections from 250 living and extinct species, both terrestrial and aquatic. Among them seals, whales, elephants, mice and hummingbirds. Also dinosaurs of different sizes and extinct marine reptiles. From animals that weigh several tons to others that weigh just a few grams. The researchers compared the samples with those of spinosaurus and its relatives. Baryonyx and Suchomimus.
This collection of animals revealed a clear link between bone density and aquatic foraging behavior: Animals that dive underwater to find food have bones that are almost completely solid, while cross sections of the bones of land dwellers They have hollow centers. “All animals that have the behavior of completely submerging have these dense bones, and that was the big news,” says Fabbri.
The researchers found that Spinosaurus and Baryonyx they had the kind of dense bone associated with total immersion. Meanwhile, the close relative Suchomimus had more hollow bones. It still lived in the water and ate fish, as evidenced by its crocodile-like snout and conical teeth, but based on the density of its bones, it wasn’t actually swimming.
Other dinosaurs, like giant long-necked sauropods, also had dense bones, but the researchers don’t think that meant they swam. “Very heavy animals like elephants and rhinos, and like sauropod dinosaurs, have very dense limb bones, because there is a lot of stress on the limbs,” explains Fabbri. “That said, the other bones are pretty light. So it was important for us to look at a variety of bones from each of the study animals.” And while there are limitations to this type of analysis, Fabbri is excited about the potential of this study to tell us how dinosaurs lived.
“One of the big surprises of this study was how rare underwater foraging was for dinosaurs, and that even among spinosaurids, their behavior was much more diverse than we thought.”
goodbye to the mosquito
For Jingmai O’Connor, from the Field Museum and co-author of the article published in the journal ‘Nature’, collaborative studies like this one, based on hundreds of specimens, are “the future of paleontology. They are time-consuming, but they allow scientists to shed light on large patterns, rather than making qualitative observations based on a fossil. It’s really amazing that Matteo was able to pull this off, and it takes a lot of patience.”
“I think that, with this additional line of evidence, speculative notions regarding Spinosaurus as some kind of giant gnat lack evidential support and can be safely ruled out,” Ibrahim says. “The bones don’t lie, and we now know that even the internal architecture of the bones is fully consistent with our interpretation of this animal as a giant predator that hunts fish in vast rivers, using its paddle-shaped tail to propel itself. It will be interesting to reconstruct in much more detail how these river monsters moved, something we are already working on.” The sail of this animal sticking out of the water must have been an impressive sight.
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