Did the 'river monster' Spinosaurus hunt like a stork?

A new study pushes back on claims that the fearsome 50-foot-long dinosaur could actively pursue its prey underwater.

Published 27 Jan 2021, 10:10 GMT
Photograph by Illustration by Robert Nicholls

More than 95 million years after it prowled North Africa’s ancient river systems, the predatory dinosaur Spinosaurus is still raising a ruckus—by fuelling a long-running scientific debate over how it lived and hunted.

Last year, researchers led by National Geographic Explorer Nizar Ibrahim made the case that Spinosaurus, a 50-foot-long scaly beast with a six-foot-tall sail on its back, was a “river monster” that actively pursued prey while swimming. But in a new study published in the journal Palaeontologia Electronica on Tuesday, two of the world’s leading experts on spinosaurids—the group containing Spinosaurus and its kin—argue for a different model.

Their review of the anatomical evidence instead advocates for the competing notion that Spinosaurus was a jack-of-all-trades shoreline predator that jabbed its head into water for prey, somewhat similar to a modern stork or heron.

Spinosaurus had ties to both land and water. Evidence, from chemical signatures in isolated fossil teeth to the structure of its jaw, implies that Spinosaurus often ate fish and other shoreline snacks. At the same time, some fossil records suggest that spinosaurids had no issues gobbling up land-dwelling dinosaurs and even winged pterosaurs. In addition, dinosaur young inside eggs would have drowned if submerged, which means that at a minimum, Spinosaurus would’ve come up onto land to lay its eggs.

But the new study shows that experts remain divided on how Spinosaurus split its time between terra firma and the life aquatic—and how it moved and hunted in the water once it got there.

In an email to National Geographic, study coauthor Tom Holtz, a palaeontologist at the University of Maryland in College Park, said that he agrees with the idea that Spinosaurus is the most water-based dinosaur of its kind yet discovered, even among its fish-eating kin. “But semiaquatic and aquatic animals fall in a spectrum,” he writes.

No living animal makes for a perfect Spinosaurus analogy, but some scientists think that the dinosaur may have waded through shallows like today's saddle-billed stork (Ephippiorhynchus senegalensis), seen here in Tanzania.

Photograph by Richard J. Green, Science Source

“In our interpretation,” Holtz adds, “Spinosaurus was probably a better swimmer than a polar bear but not a sea lion.”

In an email to National Geographic, Ibrahim welcomed the study’s alternative proposal but cautioned that Spinosaurus wouldn’t necessarily require fish-like agility to hunt while swimming—and that his team hadn’t interpreted Spinosaurus that way.

“Nobody suggested Spinosaurus was a dolphin-like, lightning-speed predator … You have to look at the prey animals in Spinosaurus's river system, which include enormous coelacanths and other slow-moving aquatic animals,” Ibrahim writes.

T. rex was not a fast runner, but it was fast enough to pursue a Triceratops or Ankylosaurus,” he adds, “and that's all that matters, folks.”

A vexing mystery

The fact that scientists can’t agree on how exactly Spinosaurus lived and behaved shouldn’t come as a surprise. It’s part of the inescapable interpretive challenge of palaeontology: Experts have limited fossil fragments to work with, and without the luxury of a time machine, there’s no way to check their best efforts.

Making matters more difficult, Spinosaurus is an especially vexing creature. The animal is bizarre by dinosaur standards and unlike anything alive today: a 50-foot-long scaly predator with a six-foot-tall sail on its back. In addition, scientists no longer have access to some key pieces of evidence. The Egyptian fossils that first defined Spinosaurus in the early 1900s were destroyed in a bombing raid on Munich, Germany, during World War II.

Still, by 2014, palaeontologists were generally comfortable with the idea of a fish-eating Spinosaurus. After decades of finding and studying close relatives of Spinosaurus across Africa, Asia, Europe, and South America, palaeontologists regarded spinosaurids as fish-eating specialists, perhaps living along shorelines and riverbeds and feeding in the shallows. Ibrahim’s team pushed this idea further, arguing that Spinosaurus was adapted to spending most of its time in the water.

Not all researchers followed Ibrahim’s lead. In 2018 Canadian palaeontologist Donald Henderson used computer simulations to argue that the dinosaur’s buoyancy, centre of mass, and large back sail would have made Spinosaurus an awkward swimmer.

However, Spinosaurus had more surprises in store. The skeleton at the heart of Ibrahim’s study—found in a sandstone outcrop in the Moroccan Sahara—was already known to sport an array of intriguing features, including a crocodile-like skull, unusually short hind limbs, and dense-walled bones similar to those in penguins. In April 2020, Ibrahim and his colleagues revealed in the journal Nature that this specimen also had a bizarrely shaped and very flexible tail, which they interpreted as a paddle that could have propelled the animal through water.

Tests in a bio-robotics lab for fish at Harvard showed that the outline of Spinosaurus’s tail was more efficient at generating thrust in water than the tails of other related dinosaurs, though it was less efficient than modern crocodile tails. Adding to the aquatic tie, a separate study published in September found disproportionately large numbers of Spinosaurus teeth within Morocco’s ancient river sediments.

Hunting for answers

With these new fossils out in the open, Holtz and his colleague David Hone, a palaeontologist at Queen Mary University of London, reassessed the wading model by looking at Spinosaurus’s anatomy, from its snout to the tip of its tail. The duo checked each trait to see whether it pointed toward the dinosaur being a wader or an active aquatic hunter, or whether the feature was consistent with both interpretations.

Hone and Holtz argue that the dinosaur’s long, S-shaped neck points toward Spinosaurus being a predator best suited to ambushing its prey from above, either while swimming on the surface or while standing, heron-like, in the shallows. Predators that actively pursue prey in water, such as modern sea lions, tend to have shorter, stubbier necks.

What’s more, the team notes, Spinosaurus’s eyes and nostrils hadn’t evolved to sit on the top of its skull, which means that to breathe and see, it needed to keep most of its head out of the water. However, the dinosaur’s nostrils evolved to be much farther back on its snout, which means the dinosaur could have breathed easily while keeping the end of its snout in water for extended periods, a position well-suited to waiting for and ambushing prey in the shallows.

The duo adds that while Spinosaurus’s tail could have helped it swim, it may not have been muscular and efficient enough to pack the punch needed to dart after fish while submerged. “We agree that the tail of Spinosaurus could have aided in its swimming,” Holtz writes. “But its efficiency doesn't even reach the level of burst ambushers like crocodilians, much less pursuers.”

Hone and Holtz suggest the tail could have served different purposes. Other animals, such as the modern green basilisk lizard, have tall, paddle-like tails that function more as social and sexual billboards than as swimming structures.

Ibrahim and his colleagues are pushing back on Hone and Holtz’s interpretation, arguing that the study didn’t present any new data that explicitly refuted the Nature study.

“To me, the combination of anatomical features indicates strong aquatic adaptations, a semiaquatic animal able to swim in water,” Harvard palaeobiologist Stephanie Pierce, a senior author of the 2020 Spinosaurus study, writes in an email. “I don't think it was a pursuit predator as they very narrowly define it in the paper, but an animal that could presumably swim and lunge burst after its prey, catching it in the water column. They are getting swept up in definition.”

Hone and Holtz’s study certainly isn’t the last word on how Spinosaurus hunted, especially with additional fossils on the horizon. In 2019 and 2020, Ibrahim and his colleagues collected more parts of the Moroccan Spinosaurus, including foot and ankle bones, which may allow scientists to test whether the dinosaur’s feet were webbed.

“So if you want to know what Spinosaurus was really like—stay tuned,” Ibrahim writes. “Because … we've got the bones!”

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