Dinosaurs were also marine creatures, not just land dwellers

Researchers have found evidence of dinosaurs that spent much of their time in water, which shows that the prehistoric beasts were not in fact restricted to land as had been previously thought.

Romain Amiot at the University of Lyon in France and a team of colleagues came across the results.

According to a report in Nature News, the discovery, made by analysing oxygen isotopes found in the fossils of a spinosaur that fed on fish, shows how the dinosaur might have coexisted with other large predators such as tyrannosaurs.

Baryonyx walkeri, from the spinosaur family, had a long, crocodile-like skull littered with iconic cone-shaped teeth.

When it was found, theories swirled that with piercing teeth, rather than the serrated teeth so often found in closely related meat eaters such as Tyrannosaurus rex, and a long snout, the dinosaur was a fish feeder.

Evidence of fish-eating behaviour came with the discovery of partially digested fish scales inside the fossilized gut contents within a Baryonyx skeleton unearthed in England in 1983.

But the gut contents also contained dinosaur remains and other evidence since has shown that pterosaurs also formed part of spinosaur diets, muddling the issue.

The lack of noticeable fins, webbing or propelling tails also did not suggest an aquatic way of life.

This led Amiot and his colleagues to look at oxygen isotopes locked away inside the enamel of the spinosaur's teeth and compare them to the oxygen isotopes found in the teeth of crocodiles and other dinosaurs, and turtle-shell fragments from the same period.

Animals that spend a lot of time in a dry environment lose water through breathing and through evaporation from skin.

Because oxygen-16 is lighter than another isotope - oxygen-18 - it is more readily released with water vapour.

As a result, oxygen-18 becomes more concentrated in tissues and when tooth enamel is formed.

The researchers reasoned that if spinosaurs were aquatic, then the concentration of oxygen-18 in their tissues would closely match aquatic animals such as crocodiles and turtles and be lower than the isotope values of other dinosaurs.

To see if this was the case, the team collected isotope data from 133 Cretaceous specimens - a mix of spinosaurs, other dinosaurs, crocodiles, and turtles - on four different continents.

They report that spinosaurs had oxygen-18 values that were 1.3 percent lower than those found in terrestrial dinosaurs - a statistically significant difference.

In contrast, oxygen-18 values in crocodiles and spinosaurids were not significantly different.

The team argues that this indicates that spinosaurs were dwelling in aquatic habitats.