Case for 'river monster' Spinosaurus strengthened by new fossil teeth

More than 95 million years ago, a mighty river system roared through what is now the Moroccan Sahara, providing a home to one of the most unusual river monsters known to science, the predatory dinosaur Spinosaurus. Fully grown, the 50-foot-long, seven-ton beast stretched longer than an adult Tyrannosaurus rex and had an elongated snout similar to a crocodile’s that bristled with sharp, conical teeth.

Now, paleontologists plumbing these ancient sediments have found large abundances of those conical teeth at two sites in southeastern Morocco. In one bone bed, teeth from Spinosaurus outnumber those of other dinosaurs by roughly 150 to one. Because these rocks formed from river sediments, the discovery implies that Spinosaurus lost its teeth in the water far more often than other dinosaurs that lived in the region—further bolstering the case that this animal was a unique aquatic killer.

“With such an abundance of Spinosaurus teeth, it is highly likely that this animal was living mostly within the river rather than along its banks,” lead study author Thomas Beevor, a graduate student at the U.K.’s University of Portsmouth, says in a press release.

The new study—recently published in the journal Cretaceous Research— builds on previous work arguing that Spinosaurus was an avid swimmer. One 2010 chemical study found tentative evidence that Spinosaurus and its kin spent a large portion of their days in water, like modern crocodiles or hippos. And studies of a Moroccan Spinosaurus skeleton published this year and in 2014 found evidence of features seen in other swimming animals, including a paddle-shaped tail that may have helped propel the dinosaur through water. (Read more about Spinosaurus’s tail and the cutting edge of dinosaur research.)

“When you study the bones, it’s very difficult to understand how these animals were actually interacting with their ecosystem,” says paleontologist and Yale Ph.D. candidate Matteo Fabbri, a coauthor on the 2014 and 2020 skeletal studies who wasn’t involved in the new study. “This study is important because it’s looking at the ecosystem itself.”

Tale of a tail

Spinosaurus is one of the most unusual dinosaurs yet found: a predator that was longer, snout to tail, than an adult T. rex, with a six-foot-tall sail on its back.

Discovered in the 1910s in Egypt, the first known fossils of the animal were destroyed in a World War II bombing raid, making it extraordinarily difficult for paleontologists to make sense of the creature’s anatomy after the fact.

In the decades since, paleontologists have found sister species to Spinosaurus around the world, including in Asia, South America, Europe, and other parts of Africa. The animals’ crocodile-like skulls hint at a similarly croc-like ability to nab fast-moving prey such as fish. In addition, a sister species to Spinosaurus found in 1983 was preserved with fish scales in its ribcage—evidence that suggested, but didn’t prove, that the predators ate fish, along with pterosaurs and smaller dinosaurs.

In the decades since these discoveries, the group of so-called spinosaurids has stood out for its unusual “crocodile mimic” anatomy. But as weird as these spinosaurids are as a group, the mysterious Spinosaurus remains in a class of its own.

In 2014, researchers led by National Geographic Explorer Nizar Ibrahim, a co-author of the new study, announced that a site in Morocco preserved a surprisingly complete Spinosaurus skeleton. The new bones revealed that its hindlimbs were unusually short relative to its forelimbs, and like modern hippos and penguins, the walls of its bones were notably thick and dense. These adaptations pointed to a semiaquatic lifestyle.

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This April, evidence for the “river monster” Spinosaurus mounted when Ibrahim’s team announced that they had found the first fossilized tail of a Spinosaurus. Unlike the tails of practically every other related dinosaur, Spinosaurus’s tail looked like a paddle. Early robotic tests suggest that this tail structure would have propelled Spinosaurus through water more effectively than the tails of related, landlubbing dinosaurs.

Treasure trove of teeth

To further examine Spinosaurus’s ties to the water, Ibrahim and his colleagues combed through two sites in October and November 2019 in southeastern Morocco near the village of Tarda. The rocks there belong to the Kem Kem formation, a 150-mile-long escarpment containing rocks formed in an ancient river system that flowed 95 million to 100 million years ago.

Though researchers working in the Kem Kem beds keep their fingers crossed in hopes of discovering full skeletons, picture-perfect fossils are extraordinarily rare there, since ancient rivers were rough-and-tumble places. Nearly all Kem Kem fossils are either isolated bone fragments or teeth, which dinosaurs lost and replaced throughout their lives, like crocodiles do today.

Both sites also had been mined by locals for fossils who sell them to exporters, wholesalers, collectors, and researchers, an artisanal trade that employs thousands of people in southeastern Morocco.

The first site had been abandoned before the researchers’ arrival, but it still contained large chunks of sandstone filled with teeth and bones, enough for researchers to recover 926 fossils in 2019. The second site, less than a mile away, was an active mining site. To get a comprehensive sample of the site’s fossils, study co-author David Martill, a paleontologist at the U.K.’s University of Portsmouth, bought 1,261 fossils that one digger had found there.

At the first site, nearly half of all the recovered fossil teeth—and a sixth of the site’s total fossils—were Spinosaurus teeth. At the second site, more than two of every five fossil teeth recovered there belonged to Spinosaurus. In total, Spinosaurus teeth made up nearly a third of the second site’s vertebrate fossils, an extremely unusual find. “We know of no other location where such a mass of dinosaur teeth have been found in bone-bearing rock,” Martill says in a statement.

The next most common “teeth” at the two sites weren’t technically teeth at all; they were the highly modified, tooth-like scales that lined the snout of the ancient sawfish Onchopristis. The teeth of dinosaurs other than Spinosaurus were barely represented.

A mighty swimmer

The researchers acknowledge one alternative to their notions of a swimming Spinosaurus: Rather than full-fledged swimming, Spinosaurus might have nabbed fish at the water’s edges and shallows, like a modern heron, losing teeth that then fell in the water. However, the scientists note that wading birds tend to have disproportionately long hindlimbs, while the Moroccan Spinosaurus has unusually short ones.

“Not only are these hindlimb proportions inconsistent with those of a wading animal, they suggest that Spinosaurus is more poorly adapted to a wading mode of life than [nearly] any other non-avian theropod,” the study’s authors write. Instead, they argue, the more viable explanation is that Spinosaurus was actively swimming in the rivers of the Kem Kem, shedding teeth as it swam through and hunted in the water.

More fossils will surely come from the rocks of the Kem Kem—and these teeth are far from the last word on the beguiling Spinosaurus. But for now, at least, the data match up with a provocative image of the distant past: a massive, crocodile-like dinosaur swimming through mighty rivers where the Sahara now lies.

Fabbri says the new study’s findings are a welcome treat: more evidence consistent with the idea, decades in the making, that Spinosaurus was a water-dwelling river monster. “Science is auto-correcting itself continuously,” he says. “But it’s also good to have good news that we were right!”