An artist's impression of Spinosaurus hunting a large Onchopristis underwater.

The bone density of Spinosaurus is similar to modern diving animals, suggesting it may have hunted prey underwater. Image © Davide Bonadonna

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Spinosaurus and Baryonyx may have hunted fish underwater

The carnivorous dinosaurs Spinosaurus and Baryonyx may have hunted prey underwater, a lifestyle very different from those of any other dinosaur.

Analysis of hundreds of living and extinct species builds on previous suggestions that the spinosaurs were some of the only dinosaurs that spent much of their lives in water.

The question of whether there were semi-aquatic dinosaurs may have finally been put to rest.

After the discovery of a Moroccan Spinosaurus which showed adaptations to swimming, palaeontologists knew that spinosaurs spent a lot of time near water, but there was strong debate over whether or not they ever got in.

Now, a study comparing the density of bones across the animal kingdom suggests that Spinosaurus and its close relative, Baryonyx, could submerge themselves underwater to hunt aquatic prey, while another spinosaur, Suchomimus, may have waded on the shoreline.

Dr Matteo Fabbri, the study's lead author, says 'There are certain laws that are applicable to any organism on this planet and one of them regards density and the capability of submerging into water.

'Previous studies have shown that mammals adapted to water have dense, compact bone in their postcranial skeletons. We used this as a proxy to determine if spinosaurs were actually aquatic.

'One of the big surprises was how rare underwater foraging was for dinosaurs, and that even among spinosaurs, their behaviour was much more diverse that we'd thought.'

Prof Paul Barrett, a Museum palaeontologist who was not involved with the study, says, 'Why the dinosaurs didn't reinvade the water is one of the mysteries that surrounds them. It's something that a lot of animal groups have done, so dinosaurs have always been something of an outlier in that respect. It's often been debated, but there's never been a good reason as to why. 

'While these reasons are still unclear, this study builds on a body of evidence which suggests spinosaurs had a semi-aquatic lifestyle. It still seems to be unique amongst the dinosaurs, even if it is much more common in their living relatives, the birds.'

The findings of the study, which made use of the Museum's Baryonyx specimen, were published in Nature

A mounted Baryonyx specimen

The type specimen of Baryonyx is held at the Museum. Image © The Trustees of the Natural History Museum, London, All Rights Reserved.

Did any dinosaurs live in water?

Until recently, it was thought that all dinosaurs were confined to land. While early studies of sauropods likened them to whales, and suggested that their huge size could only be supported by water, this idea has now been thrown out as the weight of water around their deeply submerged chests would probably have crushed their lungs.

Instead, during the Mesozoic the seas were dominated by marine reptiles such as the ichthyosaurs, plesiosaurs and mosasaurs, while rivers were full of species of fish as well as crocodiles and turtles, whose relatives are still alive today.

Some of these fish are known to have been eaten by some spinosaurs, such as Baryonyx. The type specimen of this species, held at the Museum, was found with the remains of Lepidotes fish in its stomach, and other spinosaurs are also thought to have been piscivores.

This is reinforced by characteristics such as their long and narrow jaws - similar to those of crocodiles - which further suggest a fish-based diet. Teeth found in areas that would have historically been floodplains, river beds and lake shoreline have also been used to suggest that dinosaurs such as Ceratosaurus may also have eaten fish.

However, the idea that dinosaurs went one step further and took the plunge into the water is controversial. While the concept has been suggested several times, the evidence has historically been weak enough that alternative explanations can also be offered.

But in 2014 a relatively complete Spinosaurus skeleton was uncovered in Morocco. Sharing features such as shortened back legs, broad feet and dense bones with modern semi-aquatic mammals, it was suggested that the dinosaur may have lived in water

Over the next six years, various papers were published refuting the hypothesis, with analysis of their skull suggesting they couldn't breathe while resting their head on the water's surface, like crocodiles, as well as computer models suggesting that Spinosaurus could not sink

Eventually, the team uncovered the tail of the same specimen, which shed more light on its potential semi-aquatic lifestyle.

'This skeleton indicated to the scientists who discovered it that Spinosaurus could have been more aquatic than was thought,' Paul says. 'Its tail was so deep from top to bottom that it may have been useful for propulsion in water. It also had skeletal proportions that hinted it may spend a lot of time there. 

'Other scientists also conducted oxygen isotope analysis on the bones of Spinosaurus and found that the composition was more similar to that of animals which spend a lot of time in water, or close to it, than not.  All of these different strands of evidence have been building up over time, pointing towards the possibility of a more aquatic lifestyle. 

'This accumulation of evidence also feeds into theories about the environment Spinosaurus would have lived in, which is suggested to be a mangrove swamp-like habitat.'

Members of the team which discovered the Moroccan Spinosaurus specimen have also been involved in this new study looking at a wide range of species both living and dead to provide further evidence that not all dinosaurs were afraid to get wet. 

A skeleton of Suchomimus

Suchomimus was found to have lower bone densities than its relatives, suggesting a greater reliance on land. Image © James St. John, licensed under CC BY 2.0 via Flickr

What do we now know about Spinosaurus and Baryonyx?

The study built on research from a range of scientific disciplines that have used bone density to assess the life history of different animals.

'Scientists have previously used bone density as a way of suggesting that animals were aquatic, but normally in a fairly limited way,' says Paul. 'This study has gone beyond that to look at bone densities across a wide range of reptiles, mammals and birds, compared with the habits of those animals. 

'This allowed them to use a variety of statistical approaches to tease the signals apart, with clear signals in bone density that could show the type of life an animal had, from living on land to diving and flying. 

'This analysis was used to make predictions about dinosaurs, with most being found to live on land. However, among dinosaurs, the exceptions were Spinosaurus and its closest relatives, which were found to have strong links to water.

'Interestingly, another relative, Suchomimus, had stronger links to land, which shows that not all spinosaurs were semi-aquatic.'

Based on family trees built between dinosaur groups, the researchers behind this paper suggest that underwater hunting could be common to all spinosaurs, with Suchomimus having later lost the ability to do so. 

While these dinosaurs probably wouldn't have been able to take to the sea, Spinosaurus and Baryonyx would have likely prowled lakes, swamps and rivers to hunt, using their dense bones to submerge themselves underwater where they could capture fish.

Even if they spent a lot of time in the water, these dinosaurs would have had to return to land to mate and reproduce, as all dinosaurs are currently thought to have laid hard eggs which are not suitable for aquatic environments.

Though the researchers are confident in their findings, the study is likely to be the subject of scrutiny from the palaeontological community as scientists discuss its conclusions in the years to come. 

Dr David Hone, a carnivorous dinosaur expert at Queen Mary, University of London who has previously raised issues with the idea that Spinosaurus was a highly specialised aquatic predator, says, 'This study provides more convincing evidence that spinosaurs are spending a lot of time around water. 

'However, it doesn't address many of the problems with a swimming or diving model for these dinosaurs. While dense bones may have allowed them to stand in deeper water, they could still be able to strike down at prey without having to dive to hunt them.'