Could Spinosaurus swim? The fierce dinosaur ignites debate

Scene: A small patrol boat cruises through the water, just offshore of an island somewhere in the Caribbean. Cue the pounding drums, movie-trailer speak for danger approaching.

Enter: Spinosaurus. Three large spiny sails slice through the cerulean sea and begin to circle the boat. The water roils.

“What the hell are those!?” a passenger asks with trepidation.

Cut to: Another passenger clinging precariously to the rigging as the boat lists. Suddenly, one of the spiny-sailed terrors surges from the ocean, jaws snapping.

That scene from the trailer for Jurassic World Rebirth, set to appear in theaters this summer, brings the controversial dinosaur back to the Jurassic Park franchise. During its previous cameo — in Jurassic Park III, released in 2001 — Spinosaurus stalked the movie’s heroes through the jungle. 

But this time, based on a recent wave of scientific evidence, Spinosaurus gets to swim.

The concept of a swimming dinosaur is a game changer to paleontologists. The Age of Dinosaurs lasted from about 240 million years ago to 66 million years ago, or most of the Mesozoic Era. That’s when dinosaurs dominated Earth, stomping and grazing and scampering across every continent, including Antarctica. There are around 700 known species of extinct dinosaur (excluding birds). And every one of them was a landlubber. The seas, the rivers — those were the domains of other creatures.

That all changed in the last decade, when a group of paleontologists proposed that Spinosaurus’ anatomical peculiarities make the most sense when you look at them through a watery lens. This dinosaur, they argued, lived its life mostly submerged. 

It’s a hypothesis that remains highly controversial — but there’s no doubt it has fueled ongoing research, along with popular interest in this quirky creature.

Spinosaurus didn’t need to be a swimmer to stand out from other dinosaurs. It was a striking animal, the kind of fierce-looking, sharp-toothed predator that draws large museum crowds. To begin with, it was huge; at 15 meters long, it is the longest predatory dinosaur ever found, with a body bigger than a Tyrannosaurus rex. It had a narrow, crocodile-like snout, a great big sail on its back that was taller than a human and a long, flat, paddlelike tail.

“It’s bizarre looking, even by dinosaur standards,” says Thomas Holtz, a vertebrate paleontologist at the University of Maryland in College Park. It looked absolutely nothing like T. rex or Velociraptor or other familiar predatory dinosaurs. Even among its closest relatives, collectively called spinosaurids, Spinosaurus was … kind of extra.

“The proportions — what we know of them — are weird, even for a spinosaurid. It’s not quite like any of the others,” Holtz says. “It’s not the sort of dinosaur most people are used to.” 

A swimming Spinosaurus, like the version that’s going to be on-screen a lot this year, would be a blockbuster discovery — if it could be confirmed.

Reading the bones 

Any attempt to reimagine an extinct creature’s habitat and lifestyle must begin with the bones. That’s a problem, because “unlike other well-known carnivorous dinosaurs, we do not yet have a single good Spinosaurus skeleton,” Holtz says. 

The first Spinosaurus fossils were found in 1912 in the Bahariya Oasis of western Egypt. It was just a partial skeleton — a lower jaw, some crocodile-like teeth and a handful of vertebrae bearing spines up to 2 meters tall. The fossils were so distinctive that German paleontologist Ernst Stromer determined that they must belong to a newfound creature. He named it Spinosaurus aegyptiacus, which to date remains the only agreed-upon species in the Spinosaurus genus.

For decades, those fossils, installed at Munich’s natural history museum, were the only remnants of Spinosaurus. Then Allied forces bombed the city in 1944, destroying much of the museum and the fossils along with it.

Fortunately for future paleontologists and dinosaur enthusiasts, Stromer made detailed sketches of the bones. “Stromer did such beautiful illustrations, and often in multiple views, that we can reasonably reconstruct the size of Spinosaurus,” says Paul Sereno, a vertebrate paleontologist at the University of Chicago. 

Until about 2009, those sketches were nearly all that researchers had to go on. New Spinosaurus bones proved elusive. Fragmentary fossils — isolated teeth and bits of jaw and snout — that were ascribed to Spinosaurus turned up in Tunisia, Morocco and Niger between the 1970s and 2000s.

Species with similar croclike snouts but lacking such tall neural spines — long extensions of the vertebrae — were found and grouped into a larger spinosaurid family tree. One 2003 study even proposed that, given the difficulty of replicating the creature, Stromer’s original Spinosaurus skeleton might have been a chimera, a mishmash of bones from other dinosaurs.

The extreme scarcity of bones might have something to do with the fact that Spinosaurus lived in a part of the world that now, some 95 million years later, is the Sahara, says Nizar Ibrahim, a vertebrate paleontologist at the University of Portsmouth in England. Paleontologists returned to the Bahariya Oasis again and again, searching in vain for more evidence.

Then, Ibrahim says, he was visiting a museum in Milan in 2008 and spied a set of recently acquired jawbones that he recognized as belonging to Spinosaurus. The bones, he learned, had been purchased from a Moroccan freelance fossil hunter. Ibrahim was determined to track this man down — although he knew little of him aside that he was mustachioed. Against the odds, Ibrahim spotted the dealer in a bazaar in Morocco and persuaded the man to lead him to the remote escarpment in Morocco’s Kem Kem region where the bones had been found.

It was a site deep in the desert, near the border with Algeria. Ibrahim followed the fossil hunter’s puttering motorcycle “up these very steep slopes, up again and down again,” he says. Hours passed, gas began running low and he wondered if they’d be able to get back out again.

But it was well worth the trip: The Kem Kem beds turned out to harbor a wealth of fossils that revealed fresh surprises about Spinosaurus.

The life aquatic 

Ibrahim and colleagues described their analyses of the newfound Spinosaurus bones in a 2014 study in Science. New Spinosaurus material was exciting enough, after a century-long gap. But the real headline-grabber was the team’s suggestion that the creature might have been largely aquatic.

Water was almost certainly nearby. Today’s arid, windswept Sahara was much wetter in the past. Around 100 million years ago, Earth’s average sea level was about 200 meters higher than it is today, giving rise to vast inland seas and waterways. North America was split in two by the Western Interior Seaway, extending from Mexico to Canada. Across the Atlantic Ocean, western Africa was divided by the Trans-Saharan Seaway, which covered much of Algeria, Mali and Niger.

Chemical analyses of previously discovered teeth had already suggested that Spinosaurus ate a lot of fish. Ibrahim and his team outlined various lines of new evidence that they said pointed to a primarily aquatic lifestyle: Spinosaurus’ limb bones were dense, like the bones of penguins or manatees, animals that evolved to become water denizens. Such dense bones help those animals control their buoyancy.

Spinosaurus also had smaller hip bones than other large predatory dinosaurs, along with short, muscular hind limbs (at the time, no forelimbs had yet been found or described). These features suggested that it wasn’t actually bipedal, like other carnivorous dinosaurs, but used all four limbs for locomotion, as might be needed in the water.

Other possible aquatic adaptations included its cone-shaped teeth, which would have been adept at snagging slippery fish, and the position of its nostrils, well back from the tip of the snout, which could have helped Spinosaurus breathe easy while swimming.

Over the next few years, Ibrahim went back again and again to the Kem Kem site. In 2020, he announced another headline-grabbing find: a nearly complete tail. Staying on-brand, it was a very weird tail, nearly the same length as the creature’s body, with its own set of tall spines forming a tail fin.

And to Ibrahim and colleagues, that tail helped flesh out the picture they were building of a dinosaur that was especially adapted to spend its time in the water. The tail was ideal for water propulsion, they reported in a 2020 study in Nature: Surprisingly flexible, it allowed for a wide range of movement, such as a sideways swinging motion. A robotic version of the tail, tested in a water tank, outperformed simulated tails of other dinosaurs when it came to propulsion and was close to the performance of the tails of semiaquatic swimmers such as crocodiles. 

Spinosaurus, the team concluded, used its tail to slice powerfully through the water, actively swimming to pursue its prey. It was, in short, a water monster, rather than a land terror.

Artistic sketches of this version of Spinosaurus leaned even more heavily into the idea of a watery lifestyle: The dinosaur was depicted chasing prey underwater, jaws snatching, legs paddling, long tail powerfully slashing sideways to propel it forward. The public snapped up this version of Spinosaurus: It was compelling and fun and dynamic.

But to some paleontologists, this vision went a bit too far.

Watery whiplash 

The tail paper was too much for Holtz and vertebrate paleontologist David Hone of Queen Mary University of London. The pair penned a swift response to it in 2021. The evidence was compelling but far from conclusive, they wrote in Palaeontologia Electronica. Sure, Spinosaurus may have been semiaquatic, living near and hunting in the water, but there simply isn’t enough evidence that Spinosaurus would have been capable of full-on swimming or underwater prey pursuit.

Instead, they proposed that the dinosaur was more like a heron than a crocodile: wading in the water and fishing from the shoreline or from the shallows. As for the tall sail and the paddle-shaped tail, those weren’t so much adaptations for swimming as flamboyant displays for mating or other social behaviors, they argued.

Sereno was also skeptical that this dinosaur could swim. In a separate study, he and coauthors reexamined Spinosaurus’ buoyancy using skeletal and flesh models based on the fossils and interpolated muscles.

Dense bones, in and of themselves, aren’t necessarily indicative that an animal can swim. Hippos have dense bones too, but they walk on riverbeds or lake bottoms. At any rate, Sereno says, their examination of Spinosaurus’ bones suggested they weren’t actually as dense as thought.

“We wrote a long rebuttal,” he says. The bones are solid, yes, but also contain significant air pockets. And dense as the hind limbs might have been, they were relatively small — reducing their effectiveness as ballast or buoyancy control. The models, Sereno’s team reported in 2022 in eLife, showed an animal that walked on two legs on land, was unstable and slow in the water and was too buoyant to be able to dive. 

The debate rages in part because there’s both science and art in extrapolating from bits of bone to an animal’s life: how it moved, how it ate, how it interacted with other animals.

Without one single complete skeleton, every time a new bit of Spinosaurus turns up, there’s a frenzy as people scramble to reimagine what the whole dinosaur might have looked like or how it behaved.

“In dino fandom, that’s been a running gag over the last 11 years,” Holtz says. Honey, wake up, new Spinosaurus fossils just dropped! “What radical changes of anatomy will there be this time?” 

Any attempts to reconstruct how Spinosaurus moved and lived, including whether it swam, are going to run up against uncertainties about the missing bits of soft tissue — the volume of fat, muscles, and so forth — that help researchers reconstruct accurate body mass. That is compounded by the lack of a complete skeleton that could put it all together, showing how all the different bodily components fit to create a whole animal.

And the utter weirdness of Spinosaurus means that there really aren’t any obvious living analogs to help things along. Researchers have compared its various body parts and functions with everything from crocodiles to herons to newts to eels, but none are a solid match. “It is an animal that’s so different from anything alive today, and that’s a wonderful thing,” Ibrahim says. “If extinct dinosaurs were simply replicating living animals, I wouldn’t be a paleontologist.” 

For example, “how many muscles do you pack on a Spinosaurus tail?” he asks. “We can go feature by feature: Yes, it kind of looks like it could be used this way, maybe it could be this, maybe it could be that. With Spinosaurus, we have got a paddlelike tail, we have lots of aquatic features. And yet someone might publish a paper and say maybe the tail was used to play Ping-Pong. Can I disprove that? No. With extinct animals, people can make all sorts of claims that they can never prove.” 

Ibrahim sees the debate on just how aquatic Spinosaurus was as, to a degree, hair-splitting. “It’s semantics,” Ibrahim says. “What do people mean when they say semiaquatic? How do we define a polar bear versus a seal?” Dolphins, marine iguanas, seals — all have varying degrees of adaptations to spending time in the water. The bottom line, he says, is that Spinosaurus “is an animal with lots and lots of aquatic adaptations.” 

Offbeat family 

There is one other line of research that might help illuminate Spinosaurus: studying its extinct family members.

As Tyrannosaurus rex is to tyrannosaurids, so Spinosaurus is to the spinosaurids. It’s the one you think of when you picture this group of dinosaurs. It’s bigger, stranger, more mysterious than its cousins. Like T. rex, it’s just a little bit more.

Spinosaurids were theropods, that fierce, charismatic branch of the dinosaur family tree that includes other sharp-tooths like tyrannosaurs, allosaurs and velociraptors. There are about a dozen or more different types of spinosaurids, including Spinosaurus, Suchomimus, Baryonyx and the aptly named Irritator, so dubbed because the dinosaur’s only known skull was doctored up with extra teeth by fossil poachers, vastly annoying the scientists trying to study it.

This whole family of dinos is an enigmatic bunch, says Jingmai O’Connor, a vertebrate paleontologist at the Field Museum in Chicago. “There’s a lot of things that we don’t know about this group.” 

Spinosaurus’ relatives also seem to be extraordinarily unlucky when it comes to preservation. For example, the only known fossils of the spinosaurid Oxalaia quilombensis, found at the edge of the Amazon, were destroyed in a 2018 fire at the National Museum of Brazil in Rio de Janeiro.

What we do know is that all spinosaurids showed some semiaquatic adaptations. The other spinosaurids have been found all around the globe, except in North America. Like Spinosaurus, they all had a long, narrow, crocodilian skull, with nostrils positioned well back on the head — farther back than other theropods — and a bony crest near the middle of the skull. Many, but not all, spinosaurids also had a distinct feature that gave the group their name: a row of elongated neural spines along the back, sketching out to form a sail on the back — though none had one as tall as Spinosaurus‘.

Then there are the teeth: conical and straight, without obvious serrations, designed for snatching onto food rather than sawing through it. “They’re an unusual group of dinosaurs because they’re the only group of dinosaurs that seems to be specialized to primarily eat fish,” O’Connor says. 

In the absence of other options, researchers have used these close relatives, particularly Suchomimus and Baryonyx, to try to better understand Spinosaurus’ ecology and behavior. Vertebrate paleontologist María Ciudad-Real of the National University of Distance Education in Madrid and colleagues created a brain cast for Spinosaurus with some missing parts of the cranial cavity filled in by Suchomimus skull material. Compared with other dinosaurs living in the same area at the time, Spinosaurus had reduced olfactory bulbs but larger eye sockets, possibly supporting an aquatic lifestyle. Moreover, Spinosaurus appeared to incline its head downward like a heron or other wading bird would as it scans the water for food, but not at all like a crocodile would, Ciudad-Real reported in 2023 in Cincinnati at the annual meeting of the Society for Vertebrate Paleontology.

Tooth analyses also seem to support the idea that the dinosaur was not particularly croclike. Both Spinosaurus and Baryonyx had relatively skinny teeth that weren’t particularly powerful, and most of Spinosaurus’ crushing bite force was toward the back of the jaws, according to Evan Johnson-Ransom, a paleontologist at the University of Chicago who presented his team’s analysis at the 2023 paleontology meeting. That suggests, he said, that crocodiles aren’t a great analog for spinosaurids in general. 

Analyzing the skull alone isn’t enough to interpret feeding behavior. Feeding is a whole-body effort: skull, neck, hind limbs, Sereno says. “If it’s a heron, it should have a skull, neck and hind limbs that allow it to be a heron.” So he and colleagues measured these different skeletal features for Spinosaurus and Suchomimus and plotted the measurements, along with similar measurements for other nonbird dinosaurs, modern reptiles including crocodiles, and modern birds of prey.

That analysis revealed clusters of anatomy and feeding behaviors among the different groups, Sereno and colleagues report in an upcoming study in PLoS ONE. Both spinosaurids plotted right alongside long-necked, semiaquatic birds like herons. 

Taken together, these lines of evidence suggest that spinosaurids were, perhaps, spending a lot of time near the water, but they were waders, Sereno says. It may be that the group was becoming more and more aquatic over time — but they never quite made it to swimming.

“Spinosaurus was the glory, the capstone of this group,” he adds. “It’s the farthest it’s gone in this direction.” But as for swimming, “I just don’t think it’s gone that far.” 

Enduring mystery 

Ask any paleontologist what’s needed to better understand an extinct animal, and they’ll all tell you the same thing: more bones. 

For Ibrahim, part of the appeal of Spinosaurus is its enduring mystery. It has no close living analog to make things easier, and just bits and pieces that create a tantalizing puzzle, with no picture on the box cover to illuminate what it should look like.

“The thrill of working on Spinosaurus is discovering all these news things. You go into this to see where the evidence leads you,” Ibrahim says.

His repeated voyages to Morocco’s Kem Kem beds continue to turn up treasures, comprising hundreds of new bones that he and colleagues are working to put together into a whole. The puzzle-box cover may be elusive, but he is determined. “We can’t discard anything,” Ibrahim says. “Even the smallest fragment can actually be the missing piece.” 

Other researchers are also on still the hunt. In November, Sereno announced at the 2024 Society for Vertebrate Paleontology meeting the discovery of a partial skeleton of a brand-new, never-before-seen Spinosaurus species. The fossils, found in Niger, included a skull with a narrow-snouted jaw, a forearm with long claws and portions of the hind limb. The skull also had a scimitar-like crest, the tallest ever reported on a predatory dinosaur. That lends credence, Sereno says, to the idea that Spinosaurus’ tall sail and tail fin might have been for sociosexual display, rather than propulsion.

This new Spinosaurus species was also, uncontrovertibly, no swimmer, Sereno adds. The site was near a riverbed, but thousands of kilometers from any water deep enough for such a large animal to swim in.

Meanwhile, in popular imagination, Spinosaurus’s star continues to rise, fueled by its ongoing mystique. In addition to the Jurassic World movie, Spinosaurus is getting its own dedicated episode in the BBC’s upcoming nature documentary Walking With Dinosaurs 2. The new series, whose release date hasn’t yet been announced, is a sequel to the 1999 miniseries, which combined computer-generated imagery with live-action footage to simulate the lives and times of dinosaurs.

The follow-up series was, in part, built around the desire to feature this oddball animal, says Ibrahim, who served as scientific consultant for the Spinosaurus episode. It was the first dinosaur the documentary crew chose to include in the new series, Ibrahim says. “Spinosaurus is very much the superstar.”