Spinosaurus, the largest known carnivorous dinosaur, and its closest relatives have long baffled scientists trying to understand how these unusual water-loving beasts lived their lives and hunted their prey. Did they wade through rivers and lakes like a heron? Or did they swim underwater like a hippopotamus or a croc?
It turns out the answer lay in their bones – their bone density, to be precise. Scientists said Wednesday that Spinosaurus and its cousin Baryonyx had extremely compact bones that would have helped them stay submerged to swim underwater as semi-aquatic predators targeting large prey.
Both were members of a group of Cretaceous period dinosaurs called spinosaurids that boasted anatomical adaptations such as elongated, crocodile-like snouts and tapered teeth for hunting aquatic prey. But the researchers found that another spinosaurid called Suchomimus lacked dense bones and was likely a wading predator – showing an unexpected degree of ecological diversity within this group.
What Makes Spinosaurus Weird
The Spinosaurus, about 15 meters long and weighing six tons, lived 95 million years ago in Africa. Its anatomy was unlike any other dinosaur, with a relatively small pelvis, short hind legs, paddle-like tail and feet for propulsion in water, and a curious sail-like structure of bony spines of two meters high on his back.
“I think this animal is just weird: we don’t have anything alive today that could be considered similar,” said Matteo Fabbri, postdoctoral researcher in paleontology at the Field Museum in Chicago and lead author of the study published in the journal Nature.
“I really like the idea of this giant animal, weighing many tons, submerging underwater to catch prey. I’m struck by the fact that it has a giant sail crest on its back. It would have made it difficult for him to hide underwater, but much more disturbing: a bit like a shark’s fin pointing above the waterline,” said the University of Oxford paleontologist and co-author of the study, Roger Benson.
Baryonyx, 10 meters long, lived 125 million years ago in Europe. Suchomimus, 11 meters long, lived 120 million years ago in Africa.
To determine that greater bone density is directly associated with an aquatic existence, researchers amassed data on 297 species of living and extinct animals, land and water dwellers.
Bone compactness has been shown to be a defining characteristic in animals adapted to life in water such as whales, seals, dugongs, hippos, crocodiles, penguins and various extinct marine reptiles. No other dinosaurs have been found with the bone density of Spinosaurus and Baryonyx, indicating that they were alone among the dinosaurs to conquer the aquatic kingdom.
“Spinosaurus has the highest bone density among the three,” Fabbri said. “Baryonyx has a slightly lower bone density, but still very similar to Spinosaurus. Suchomimus, found as a more terrestrial animal in our study, has bone density similar to other dinosaurs, reptiles, and terrestrial mammals.
Debate about how the aquatic Spinosaurus was
Spinosaurus even surpasses Tyrannosaurus rex in size, but its anatomy has long intrigued scientists. Its original fossils from Egypt were destroyed during World War II. But the discovery of a skeleton in Morocco in 2008 and other coccyxes unearthed afterwards have led some paleontologists to propose that Spinosaurus was semi-aquatic and an active swimmer. Other researchers examining the same fossils disagreed.
The new study was launched to try to settle the debate.
Large fish in rivers and lakes as well as dinosaurs walking along their banks may have been attractive prey for Spinosaurus.
“Spinosaurus may have moved along shallow water using a combination of ‘bottom walking’ – like modern hippos – and side-swiping its giant tail,” said Guillermo Navalón, postdoctoral researcher at the University of Cambridge and co-author of the study. “Probably used this means of locomotion not to pursue prey over long distances in open water, but to ambush and catch very large fish like lungfish or coelacanths.”