Spinosaurus is the largest predatory dinosaur known – over two metres longer than the longest Tyrannosaurus rex – but the way it hunted has been a subject of debate for decades.

In a new paper, published in Nature, a group of palaeontologists have taken a different approach to decipher the lifestyle of long-extinct creatures: examining the density of their bones.

By analysing the density of Spinosaurid bones and comparing them to other animals like penguins, hippos, and alligators, the team found that Spinosaurus and its close relative Baryonyx from the Cretaceous of the UK both had dense bones that would have allowed them to submerge themselves underwater to hunt.

Scientists already knew that Spinosaurids had certain affinities with water – their elongate jaws and cone-shaped teeth are similar to those of fish-eating predators, and the ribcage of Baryonyx, from Surrey, even contained half-digested fish scales.

In the last decade University of Portsmouth palaeontologist and National Geographic Explorer Dr Nizar Ibrahim unearthed different parts of a Spinosaurus skeleton in North Africa’s Sahara Desert. The skeleton Dr Ibrahim and his team described had retracted nostrils, short hind legs, paddle-like feet, and a fin-like tail: all signs that firmly pointed to an aquatic lifestyle.

Dr Ibrahim said: “We battled sandstorms, flooding, snakes, scorpions and more to excavate the most enigmatic dinosaur in the world 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 specialised anatomy, Dr Ibrahim and his team previously suggested that Spinosaurus could swim and actively pursue prey in the water, but others claimed that it was not much of a swimmer and instead waded in the water like a giant heron.

Researchers have continued to debate whether Spinosaurus spent much of its time submerged, pursuing prey in the water, or if it just stood in the shallows and dipped its jaws in to snap up prey.

“In part this is probably because we were challenging decade-old dogma – so even if you have a very strong case, you kind of expect a certain degree of pushback,” Dr Ibrahim said.

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

Dr Fabbri said: “The idea for our study was, okay, clearly we can interpret the fossil data in different ways. But what about the general physical laws? There are certain laws that are applicable to any organism on this planet. One of these laws regards density and the capability of submerging into water.”

Across the animal kingdom, bone density can tell us whether an animal is able to sink beneath the surface and swim.

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

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

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

Dr Ibrahim said: “The scope of our study kept expanding because we kept thinking of more and more groups of vertebrates to include.”

The scientists found a clear link between bone density and aquatic foraging behavior: animals that submerge themselves underwater to find food have bones that are almost completely solid throughout, whereas cross-sections of land-dwellers’ bones look more like doughnuts, with hollow centres.

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

Meanwhile, the closely related African Suchomimus had hollower bones. It still lived by water and ate fish, as evidenced by its crocodile-like snout and conical teeth, but based on its bone density, it wasn’t actually swimming much. “That was a bit of a surprise” according to Ibrahim, “because Baryonyx and Suchomimus look rather similar”. But the team soon realised that it was not out of the ordinary and similar patterns can be seen in other groups.

Other dinosaurs, like the giant long-necked sauropods also had some dense bones in their limbs, but this simply 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, a curator at the Field Museum and co-author of this study, says that collaborative studies like this one that draw from hundreds of specimens, are “the future of palaeontology. They’re very time-consuming to do, but they let scientists shed light onto big patterns.”

Dr Ibrahim is already thinking about the next questions. “I think that, with this additional line of evidence, speculative notions that envisage Spinosaurus as some sort of giant wader lack evidential support and can be safely excluded. The bones don’t lie, and now we know than even the internal architecture of the bones is entirely consistent with our interpretation of this animal as a giant predator hunting fish in vast rivers, using its paddle-like tail for propulsion. It will be interesting to reconstruct in a lot more detail how these river monsters moved around – something we are already working on.”