A fierce group of predatory dinosaurs may have done much of their hunting in the water.
An analysis of the bone density of several sharp-toothed spinosaurs suggests that several members of this dino group were predominantly aquatic, researchers report March 23 in Nature.
That finding is the latest salvo in an ongoing challenge to the prevailing view that all dinosaurs were land-based animals that left the realms of water and air to marine reptiles such as Mosasaurus and flying reptiles such as Pteranodon. But, other researchers say, it still doesn’t prove that Spinosaurus and its kin actually swam.
Back in 2014, Nizar Ibrahim, a vertebrate paleontologist now at the University of Portsmouth in England, and colleagues pieced together the fossil of a 15-meter-long Spinosaurus from what’s now Morocco. The dinosaur’s odd collection of features — a massive sail-like structure on its back, short and muscular legs, nostrils set well back from its snout and needlelike teeth seemingly designed for snagging fish — suggested to the researchers that the predator might have been a swimmer (SN: 9/11/14). In particular, it had very dense leg bones, a feature of some aquatic creatures like manatees that need the bones for ballast to stay submerged.
In the new study, Ibrahim and his team returned to that question of bone density to assess whether it’s a reliable proxy for how much time a creature spends in the water. The team assembled “a massive dataset” of femur and dorsal rib bone densities from “an incredible menagerie of extinct and living animals, reaching out to museum curators all around the world,” Ibrahim says.
That menagerie includes spinosaurs like showy, sail-backed Spinosaurus as well as its equally sharp-toothed cousins Baryonyx and Suchomimus. It also includes other groups of dinosaurs, extinct marine reptiles, pterosaurs, birds, modern crocodiles and marine mammals.
The team then compared these bone analyses with the water-dwelling habits of the various creatures in the study. That work confirms that density is “an excellent indicator” for species in the early stages of a transition from land-dwelling to water-dwelling, the team reports. Those compact bones can aid such transitional creatures, which might not yet have features like fins or flippers to help them maneuver in the water more easily, in hunting underwater — what the team calls “subaqueous foraging.”
The analyses also show that not only did Spinosaurus have very dense bones, but Baryonyx did too. That suggests that both of these dinos were subaqueous foragers, the team says. That idea builds on previous work by Ibrahim and colleagues that proposed that Spinosaurus didn’t just spend much of its time in the water, but could actually swim in pursuit of prey, thanks to its odd, paddle-shaped tail (SN: 4/29/20).