A new study published in Nature reveals that Spinosaurus, among the largest carnivorous dinosaurs ever discovered, could hunt underwater.
New research by a multi-institution team including the Field Museum of Natural History, Chicago, the University of Oxford and the University of Cambridge used X-ray imaging of fossilised bones to analyse bone density. By comparing the bone structure of living animals and birds that forage in the water, researchers found strong evidence that dinosaurs from the spinosaurid family swam underwater to search for prey.
Spinosaurid dinosaurs look like T. rex, but with a long, narrow snout and short legs. They include species like Spinosaurus from northern Africa, and Baryonyx from England, and were first discovered in 1915. Spinosaurus was similar in size to T. rex, weighing up to 7 tonnes, with a large crest-like sail across its back.
Fossilised gut contents show that spinosaurids were amphibious hunters – eating both surf and turf – but it has been debated whether they caught fish in the water like a crocodile or hunted from the shallows like a heron.
Some palaeontologists have suggested that spinosaurids were specialist aquatic predators, based on their long snout, and a tail fin that might have been used for propulsion in water, as well as other evidence. However, fossil footprints show that most dinosaurs walked on land, and evidence for swimming is rare.
The new study used data on skeletal bone density to resolve whether spinosaurids swam underwater. Using CT scanning (a form of 3D x-ray imaging) the researchers collected femur and rib bone cross-sections from more than 200 living animals, including those that hunt in the water like otters and those that forage on land like badgers. The researchers compared these cross-sections to those of many dinosaurs, including three spinosaurids: Spinosaurus, Baryonyx and Suchomimus.
The scans showed that 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.
‘Animals that search for food underwater have high bone density to help them stay submerged without spending too much energy, including hippos, sea lions, cormorants and crocodiles,’ explained co-author Professor Roger Benson, Department of Earth Sciences, University of Oxford.
‘We found there is a very strong correlation between bone density and underwater foraging. This means that all the animals that have the behaviour where they are fully submerged have these dense bones, and that was the great news because it allows us to test which dinosaurs were swimming underwater,’ said lead author Dr Matteo Fabbri, Field Museum of Natural History, Chicago.
The Spinosaurus and Baryonyx both had the sort of dense bone associated with full submersion, whereas their close relative, Suchomimus, had hollower bones. It still lived by the water and ate fish, as evidenced by its crocodile-mimic snout and conical teeth, but based on its bone density, it was not swimming to hunt.
Dr Fabbri said ‘One of the big surprises from this study was how rare underwater foraging was for dinosaurs. We really only found evidence for this in spinosaurids, and their behaviour was much more diverse than we had thought.’
‘Dinosaurs are mysterious and inspiring animals, but we only have their bones and most skeletons are incomplete. Our study shows how much extra we could know about dinosaurs, using analysis of living animals. In the future, we expect this approach to continue giving new insights about dinosaurs and their world,’ said Professor Benson.
The full paper ‘Subaqueous foraging among carnivorous dinosaurs’ is available to read on Nature.
Illustration of the Spinosaurus, Credit: Davide Bonadonna.