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Spinosaurus had penguin-like bones, a sign of hunting underwater

More than 95 million years ago, a mighty river system flowed through what is now the Moroccan Sahara, providing a home to one of the most unusual dinosaurs known to science: Spinosaurus, a 49-foot-long, seven-ton beast with a crocodile-like snout that bristled with conical teeth.

Paleontologists agree that Spinosaurus and its kin had strong ties to water, but for years, they have debated whether this bizarre, crested creature swam through the water as a fish-nabbing “river monster,” prowled the shores like a reptilian mega-heron, or something in between.

Now, using the biggest dataset of its kind, researchers have compared the density of the prehistoric carnivore’s bones to a wide array of living and extinct animals. The analysis found that Spinosaurus—and, surprisingly, its British cousin Baryonyx—had highly dense bone walls like penguins do, suggesting they likely spent much of their time in the water and hunted down aquatic prey.

“Spinosaurus might also have been a wading animal sometimes, but its ecology is characterized by full immersion in water,” says lead study author Matteo Fabbri, a postdoctoral researcher at the Field Museum of Natural History in Chicago, Illinois.

Spinosaurus hunting underwater.

Catapulted into recent fame by Jurassic Park III, Spinosaurus is one of the most mysterious dinosaurs ever found. Researchers discovered the first known fossils of the animal in the 1910s in Egypt, but the bones were destroyed in a World War II bombing raid, which has made it extraordinarily difficult for paleontologists to study the creature’s anatomy.

In 2014 researchers led by Ibrahim announced that a site in Morocco preserved a surprisingly complete Spinosaurus skeleton. The fossil had unusually stubby hindlimbs, as well as dense limb bones. These traits pointed to a semi-aquatic lifestyle in which Spinosaurus was actively going out into the water at least part of the time. The 2020 announcement of the same fossil’s paddle-like tail only added to the intrigue.

To further investigate how much Spinosaurus could swim, Fabbri and Ibrahim’s team started working on the new study in 2015, and the project ended up taking six years to complete. Much of the time was devoted to Fabbri’s quest to collect as much data as possible on a wide variety of animals. He contacted museum after museum, persuading an ever-growing list of curators to help him collect specimens for his study.

In other land animal groups that made the evolutionary transition to the water, such as whales, increased bone density was one of the first traits to appear, with the bones acting as energy-saving ballast. So to test whether spinosaurids were adapted to the water, the researchers compiled digitized cross sections of the ribs and femurs of more than 200 different types of amniotes, the broad group of animals that includes modern reptiles, birds, and mammals, as well as a mix of ancient, extinct creatures including dinosaurs.

The team then compared the bone densities with the animals’ known abilities to forage underwater or fly. The analysis showed that highly dense bones were tightly correlated with an animal frequently diving underwater. Spinosaurus and Baryonyx were the only two of the non-avian dinosaurs in the study with bones this dense.

Not every animal with dense bones lives an aquatic or even partially aquatic lifestyle. Elephants and extinct sauropod dinosaurs, for example, have dense limb bones to support their large weights.

However, these weight-bearing “graviportal” bone structures visibly differ from the “osteosclerotic” ones that penguins and other diving creatures have. Highly dense bones can give animals a unique edge when it comes to staying in the water for extended periods without eliminating their ability to navigate dry terrain.

“While a penguin or a crocodile are capable of swimming and diving underwater, they are also able to walk on land,” Fabbri says.

Spinosaurus skeleton.

Future studies promise to reveal more details about this strange predator. Formoso and Henderson are working together on a forthcoming study that will compare Spinosaurus with aquatic mammals and reptiles, and Ibrahim’s team is working on follow-up studies of Spinosaurus’s hindlimbs and feet, which show signs of having been widely splayed, possibly to support webbing.

For Holtz, the most exciting times for research into Spinosaurus and its kin lie ahead.

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