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. (Find out what happened immediately after the impact that ended the dinosaurs’ reign.)
The new findings published in Nature strengthen the case that spinosaurids, the group containing Spinosaurus and its kin, adapted to a life in the water—the only known dinosaurs other than birds to have done so. Recently, for instance, the first fossil ever found of Spinosaurus’s tail revealed a strange paddle-like shape that the dinosaur may have used to swim.
“The bones don’t lie,” says National Geographic Explorer Nizar Ibrahim, the study’s senior author and a paleontologist at the University of Portsmouth in the United Kingdom. Spinosaurus “is even more aquatic than we anticipated,” he says.
Still, exactly how Spinosaurus swam and pursued its fishy prey remains an open question. The prehistoric predator, with its elongated body and gigantic back sail, continues to perplex scientists because it’s so unusual when compared to other aquatic creatures.
“I don’t doubt that Spinosaurus has very dense bones … [but] is it adding enough mass to help the animals sink?” asks Don Henderson, a paleontologist at Canada’s Royal Tyrrell Museum of Palaeontology whose 2018 study of Spinosaurus suggested that the animal would have been very buoyant.
“If you watch penguins swimming, as soon as they stop flapping their wings, they start to bob up to the surface,” Henderson says. “I just cannot see how Spinosaurus can stay underwater without extreme effort.”
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.
Though Spinosaurus and Baryonyx would have had an easier time hanging out in water than other dinosaurs, they also must have come up onto land from time to time. The first known fossil of Baryonyx contains fish scales as well as the bones of a juvenile plant-eating dinosaur—perhaps a sign of the predator’s opportunism both on land and in the water.
Spinosaurid expert Tom Holtz, a paleontologist at the University of Maryland who wasn't involved with the study, says that the new data could have implications far beyond Spinosaurus and Baryonyx. Some paleontologists have suggested that a few types of dicynodonts—extinct two-tusked creatures distantly related to the earliest mammals—could have led hippo-like lifestyles. New bone density analyses could help test these ideas and others like them, especially for fragmentary fossils from which little else is known.
“To my knowledge, no one’s tried this [analysis] on this scale before,” Holtz says. “They will inspire work [and] inform research on what are perhaps to the public less charismatic, but scientifically equally interesting, creatures.”
Bucking the trends of evolution
Beyond Baryonyx and Spinosaurus, the study also took a look at other candidate semi-aquatic dinosaurs, notably the duck-like Halszkaraptor. The team found that none of them had dense, penguin-like bones. However, this does not preclude these animals from a life with ties to the water. Plenty of modern dinosaurs that live around water, such as ducks, geese, and other waterfowl, don’t have penguin-like bone densities.
One of the study’s biggest surprises comes from another spinosaurid species: Unlike Spinosaurus and Baryonyx, a spinosaurid found in Niger called Suchomimus does not have dense, ballast-like bones, despite that animal’s otherwise close resemblance to Baryonyx.
“One would expect in a group that is not necessarily 100-percent committed to aquatic life the possibility of evolution moving back and forth,” says Holtz.
The overall findings underscore just how odd spinosaurids were in the grand scheme of evolutionary history. In most other cases where terrestrial animals adapted to life in the water, the pioneering species tended to have small bodies. According to Holtz, the first mosasaurs—a group of ancient, non-dinosaur marine reptiles—were no bigger than four to five feet long, about the size of modern Nile monitors. But the progenitors of Baryonyx and Spinosaurus were likely giants.
Paleontologist Kiersten Formoso, a Ph.D. candidate at the University of Southern California, says that while Spinosaurus has dense bones like other land animals that evolved to return to the water, it also “bucks some of the common paths,” evolutionarily speaking.
For example, most aquatic amniotes have fairly flat, round bodies, but Spinosaurus’s body was tall and skinny. In general, theropod dinosaurs including Spinosaurus had bodies that were stiffer than those of some other semi-aquatic animals, which would have affected potential swimming motions. Theropods’ vertebrae were also shot through with air sacs, which would have made it that much harder for spinosaurids to sink, though she adds that the creatures‘ extra bone density may have counteracted this effect.
And then there’s one of Spinosaurus’s hallmark traits: its hydrodynamically vexing, six-foot-tall dorsal sail.
“I think this animal was perfectly happy chilling and foraging in shallow water with its feet to the bottom in [most] instances,” Formoso says, though in her opinion, Spinosaurus “would have had a difficult time aquatically pursuing prey.”
Still, it’s possible that Spinosaurus’s unique habitat gave the dinosaur an easier time with aquatic hunting. The rock layers that preserved the Morocco Spinosaurus also bear fossils of large, slow-moving prehistoric fish. “Those are the kinds of things that Spinosaurus would have been going after—we’re obviously not talking about going after prey dolphin-style,” Ibrahim says.
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.
“The evidence is clearly pointing toward [Spinosaurus] being some sort of aquatic animal, and this new addition supports them being underwater, or at least largely underwater … OK, how?!” he wonders. “Figuring out what they were doing is going to be the fun part.”