The hunting habits of Spinosaurus aegyptiacus, the largest known predatory dinosaur to roam the Earth, have been subject to intense scientific debate since detailed descriptions of its most complete fossils to date were published in 2014.
At the time, Spinosaurus was described as a “semiaquatic” predator that prowled the shoreline of Cretaceous-era rivers, wading into the muddy banks to ambush fish with its massive, crocodilian jaws and interlocking teeth.
More recent discoveries have pushed Spinosaurus further from the shore, with some researchers suggesting it was well-suited to pursuing prey out of the shallows and hunting deep underwater. These arguments are based on new fossils that suggest Spinosaurus had a fleshy, paddle-like tail for swimming and dense bones to help submerge it underwater.
A new paper by paleontologists from the University of Chicago and colleagues elsewhere, however, rejects this “aquatic hypothesis” as far-fetched. Using updated virtual reconstructions of its skeleton and body mass based on its fossils, they analyzed its ability to propel its prodigious bulk underwater and found that while Spinosaurus was indeed the scourge of the shoreline with many adaptations for life at the edge of the water, it would fail as a fully aquatic, agile, underwater predator.
“Do I think that this animal would have waded into water on a regular basis? Absolutely, but I don't think it was a good swimmer, nor capable of full submergence behavior,” said Paul Sereno, a professor of organismal biology and anatomy at UChicago and lead author of the new study who co-led the initial Spinosaurus discovery in 2014. “This is simply not an animal that in your wildest dreams would be dynamic above water as a swimmer, much less underwater.”
The new paper, titled matter-of-factly, “Spinosaurus is not an aquatic dinosaur,” was posted as a preprint at bioRxiv and published Nov. 30 in the journal eLife.
Elusive from the beginning
Spinosaurus aegyptiacus has been an elusive creature since German paleontologist Ernst von Stromer discovered its skeleton in Egypt in 1915. With its elongated jaws and large sail protruding from the spine, Stromer initially proposed Spinosaurus as a bipedal, fish-eating predator. He put the fossils on display in Munich’s Paleontological Museum, but they were destroyed during Allied bombing in World War II. All that was left were Stromer’s notes and drawings. The giant Spinosaurus seemed lost in time.
Decades later, miners in southeastern Morocco unearthed what appeared to be another Spinosaurus skeleton buried in sandstone rocks. Those fossils formed the basis of the 2014 study, published in Science, in which Sereno and his team analyzed the new fossils plus a partial skull and other specimens gathered from museums around the world, along with Stromer’s original records and journal articles.
Their findings brought the team worldwide acclaim. They depicted Spinosaurus as a nearly 50-foot-long monster, with giant, interlocking slanted teeth for snatching fish, a long neck and trunk, short hind legs with muscular thighs, and enormous dorsal spines covered in skin that created its signature sail. The skull had small nostrils located farther back on the skull to allow it to breathe when partially submerged and crocodile-like neurovascular openings on its snout that may have been for receptors to sense movement in the water. Yet the researchers still viewed the beast as “semiaquatic,” an animal best suited for land while venturing into shallow waters to feed.
Since those initial findings, researchers from the University of Portsmouth in the United Kingdom collected the nearly complete tail of the rediscovered skeleton, which was hidden in sediment to the side of the original bone quarry. The tail had tall, slender spines that that would have been covered with skin, a version of the sail along its back. In a 2020 Nature paper, the U.K. team proposed that Spinosaurus would have used this fleshy tail to propel itself like an eel through the water column as a fully aquatic predator.
This year, that same team published another paper in Nature that compared the density of Spinosaurus bones to a wide variety of living and extinct animals. They concluded that the dinosaur had very dense bone walls like penguins, suggesting it actually spent most of its time in water, using the heavier bones for ballast to submerge itself regularly in underwater pursuits.
A fearsome predator, but awkward swimmer
Meanwhile, Sereno and colleagues approached the question from several new angles, starting with a mistake in the 2014 paper (“I love to admit mistakes, especially when I can correct them myself,” Sereno said). When they calculated the animal’s center of gravity, the software didn’t properly deduct mass to account for the volume of its lungs, pitching it forward and leading the team to believe it walked on all fours.
In the new research, Sereno and team constructed a fresh model of Spinosaurus from CT scans of its skeleton and fleshed it out with its musculature and body mass, based on modern reptiles. This time they made sure to account for the volume of its lungs, which positioned the center of gravity over its hips, allowing the usual two-legged (bipedal) stance like all other large-bodied dinosaurian predators. Sereno says dense bones make sense for such a large animal walking on two legs.
“The solid limbs are not there for ballast while swimming, but rather to support the great weight of the beast,” he said.