Dense bones allowed Spinosaurus to hunt underwater

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Spinosaurus is the largest known predatory dinosaur – more than two meters longer than the longest tyrannosaurus rex – but how he hunts has been debated for decades.

In a new article, published today in Nature, a group of paleontologists have taken a different approach to deciphering the way of life of long-extinct creatures: examining their bone density.

By analyzing the bone density of spinosaurids and comparing them to other animals like penguins, hippos and alligators, the team found that Spinosaurus and his next of kin Baryonyx from the Cretaceous of the UK both had dense bones that would have allowed them to dive underwater to hunt.

Scientists already knew that spinosaurids had certain affinities with water – their elongated jaws and cone-shaped teeth are similar to those of piscivorous predators, and the rib cage of Baryonyxfrom Surrey, even contained half-digested fish scales.

Over the past decade, University of Portsmouth paleontologist and National Geographic explorer Dr Nizar Ibrahim has unearthed different parts of a Spinosaurus skeleton in the Sahara desert in North Africa. The skeleton described by Dr. Ibrahim and his team had retracted nostrils, short hind legs, paddle-like feet and a fin-like tail: all signs that strongly pointed to an aquatic lifestyle.

Dr Ibrahim said: “We’ve battled sandstorms, floods, snakes, scorpions and more to unearth the world’s most enigmatic dinosaur and now we have multiple lines of evidence all pointing in the same direction. – the skeleton really has “water-loving dinosaur” written all over it!

Based on its highly specialized anatomy, Dr. Ibrahim and his team have previously suggested that Spinosaurus could swim and actively pursue prey in the water, but others have claimed that it was not much of a swimmer and instead waded through the water like a giant heron.

Researchers have continued to wonder if Spinosaurus spent much of its time submerged, chasing prey through the water, or if it just stood in the shallows and plunged its jaws in to grab prey.

“It’s probably partly because we were challenging decade-old dogma – so even if you have a very strong case, you kind of expect some degree of pushback,” Dr Ibrahim said.

This ongoing debate led lead author Dr. Matteo Fabbri, based at the Field Museum in Chicago, lead author Dr. Ibrahim, and an international team of researchers to try to find another way to infer the way of life. and the ecology of long-extinct creatures like Spinosaurus.

Dr Fabbri said: “The idea of ​​our study was, okay, we can clearly interpret the fossil data in different ways. But what about general physical laws? There are certain laws that apply to any organism on this planet. One of those laws concerning density and the ability to submerge in water.”

Throughout the animal kingdom, bone density can tell us if an animal is able to sink below the surface and swim.

“Previous studies have shown that water-adapted mammals have dense, compact bone in their postcranial (behind the skull) skeleton,” said Fabbri, an expert on the internal structure of bone. The dense bone helps with buoyancy control and allows the animal to submerge.

The team collected a very large data set of femur and rib cross-sections from 250 species of extinct and living animals, including land and water dwellers, and covering animals ranging in weight from a few grams to several tons, including seals, whales, elephants, mice and even hummingbirds.

They also collected data on extinct marine reptiles like mosasaurs and plesiosaurs. The researchers compared bone sections from these animals to bone sections from Spinosaurus and his relatives Baryonyx and Suchomimus.

Dr Ibrahim said: “The scope of our study has grown steadily because we have continued to think of more and more vertebrate groups to include.”

Scientists have found a clear link between bone density and aquatic foraging behavior: animals that submerge underwater to find food have almost completely solid bones, while cross-sections of the bones of Earth dwellers are more like donuts, with hollow centers.

When researchers applied the bones of spinosaurid dinosaurs to this paradigm, they found that Spinosaurus and Baryonyx both had the type of dense bone associated with complete submersion.

Meanwhile, the closely-knit Africans Suchomimus had hollow bones. It always lived near water and ate fish, as evidenced by its crocodile-like snout and conical teeth, but judging by its bone density, it didn’t swim much. “It was a bit of a surprise” according to Ibrahim, “because Baryonyx and Suchomimus seem rather similar. But the team soon realized that this was not out of the ordinary and that similar patterns can be seen in other groups.

Other dinosaurs, like the giant long-necked sauropods, also had dense bones in their limbs, but that just reflects the huge amount of stress on those limb bones.

Dr Ibrahim said: “Some of these animals would have weighed as much as several elephants, so adding extra load-bearing capacity to the bones makes a lot of sense!”.

Dr Jingmai O’Connor, curator at the Field Museum and co-author of the study, says collaborative studies like this, which draw on hundreds of specimens, are “the future of paleontology. They take lots of time to do.” , but they have allowed scientists to shed some light on large models.”

Dr. Ibrahim is already thinking about the next questions. “I think with this additional piece of evidence, speculative notions that consider Spinosaurus for some kind of giant wading bird lacks evidence and can be safely ruled out. Bones don’t lie, and we now know that even the internal architecture of bones is entirely consistent with our interpretation of this animal as a giant predator hunting fish in vast rivers, using its paddle-like 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.”

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