A Dinosaur-Sized Mystery

In September 2012, the paleontologists Nazir Ibrahim and Paul Sereno at the University of Chicago opened a package containing a collection of fossils pulled from the Cretaceous rock beds of the Kem Kem formation in Morocco. These paleontological treasures, the researchers believed, had the potential to solve a mystery that had vexed dinosaur paleontologists since World War II: the true shape of the enigmatic Spinosaurus, one of the largest and strangest predatory dinosaurs ever discovered.

Spinosaurus has long been one of the great riddles of paleontology. The first remains—an enormous lower jaw, some vertebrae, and the long neural spines that give it its name—were discovered in 1912 at the oasis of El Bahariya, Egypt. Carefully described in 1915 by the German paleontologist Ernst Stromer, the fossils were shipped to a Munich museum for permanent display. There they were destroyed, obliterated by an Allied bombing raid that pummeled the building in 1944. All that remained were Stromer’s detailed illustrations and a few photographs. (Other possible Spinosaurus remains from Stromer’s collection have survived, but no one linked them to the species for some time.) Over the decades, a few other scraps of jaw and neck have been identified, but nothing has been substantial enough to further scientists’ understanding of what the animal looked like.

The two researchers had prepared themselves for a momentous discovery—but Sereno opened the crate and surveyed the contents, he was still shocked. “I was like, what?” he recalled. “I pulled out the limb bones, and the femur was longer than the [shin] … That single fact indicated that the proportions of the hind ­limb were like something we’d never seen before.”

The team published their findings in in the journal Science in September 2014, amid a media storm funded by the National Geographic Society. They argued that the shape of Spinosaurus had been settled: It was an animal unique among predatory dinosaurs, they wrote, with tiny hind ­limbs, massive forelimbs, and a penchant for subaquatic life. But other scientists soon pushed back against the paper, disputing some of the team’s conclusions and arguing that their reconstruction was more speculation than certainty. Looking over the ensuing argument, only one thing is clear: The true form of Spinosaurus is still up for grabs.

* * *

Reconstructing fossil animals from limited bones is tough work, and few dinosaurs illustrate that as well as Spinosaurus. Stromer’s original vision for his beast was a blunt -headed, kangaroo­-shaped creature, with a long sail on its back. The discoveries of other members of the spinosaur genus in the 1980s and ‘90s rewrote that body plan considerably. Based on related species like Baryonyx and Suchomimus (also discovered by Sereno), experts guessed that Spinosaurus was an interesting, if subtle, variation on other carnivorous dinosaurs. Like its relatives, it was suspected to have heavy arms and long hind limbs. Scraps from North Africa suggested that it had a toothy, crocodilian snout. A seafood diet was likewise suspected but not confirmed. This was the version of the animal that appeared as a steroidal superpredator in Jurassic Park 3. But it was a version that, like Stromer’s kangaroo, was based mostly on conjecture.

Slowly, Sereno and Ibrahim’s team began trying to make sense of the fossils in the crate. The animal wasn’t fully grown. But the remains were good: both hind limbs, much of the pelvis, several neural spines, and a sizable portion of lower jaw. Using that material and other isolated specimens, the team found a convincing case for an aquatic lifestyle. The feet were wide, four­-toed and flat, Sereno said, useful for moving over soft, silty mud. A cross­-section of the bones revealed them to be solid, something unheard of in generally hollow­-boned theropods ( a type of carnivorous dinosaur) but useful for staying submerged. Skull elements from another Moroccan specimen showed nostrils that sat near the eyes, like those of a whale, while isotope data suggested the animal spent the majority of its time in water. And then there were those hind ­limbs, strangely small in comparison to those of related species. Looking at those, the team concluded that Spinosaurus likely couldn’t run like other predatory dinosaurs. “There’s no reason to have [small hind legs] except for paddling,” Sereno said. “The animal’s clearly not fast.”

The process of putting together a complete reconstruction was more complicated. “It was an extreme digital exercise,” Sereno said. “We had a teenage individual, we had bits of an adult, we had historical drawings, we had some other specimens, and we had to combine them all, overlap them, and justify that.” The team put together a composite skeleton using digital scaling, using lines of evidence like the proportions of the associated legs and neural spines, and comparing these to Stromer’s drawings of the original (and larger) specimen. Missing pieces, ­­like the arms and hands­­, were drawn from related species like Suchomimus and isolated remains. Putting all of this together, Ibrahim, the lead author on the 2014 paper, suggested that Spinosaurus moved on all fours while on land, perhaps walking on its knuckles to keep its claws sharp.

The new Spinosaurus got decidedly bumpy reviews from other researchers. While most agreed with conclusions like the four-­toed feet and isotopic data, the proposed body plan was less well received. In two blog posts, Scott Hartman, a scientist with experience in skeletal reconstructions, suggested that the team’s measurements of the pelvis did not match their proposed skeleton, noting that their reconstruction of the pelvis itself seemed off from other members of the spinosaur family. Playing with the data, Hartman came up with a different version of Spinosaurus: still short -limbed, but now with a bipedal body plan and center of gravity closer to that of other theropods. Researchers soon pointed out another issue with the knuckle­-walking hypothesis: The general wrist and shoulder anatomy of predatory dinosaur arms makes it tough for them to bear an animal’s weight. Without definitive remains of Spinosaurus arms or hands, they argued, knuckle-­walking was complete speculation.

The 2014 skeleton, critics argued, was likely a chimera, a reconstruction mixing elements from multiple species—in this case, the proposed Spinosaurus may have been composed partially from remains already assigned to other North African spinosaur species. This assertion was supported by a hefty 2015 paper describing new material from Sigilmassosaurus, another Kem Kem spinosaur, and suggested that some of its remains had found their way into the 2014 Spinosaurus.

According to the scientist and paleoartist Mark Witton, who himself wrote extensively about the 2014 Spinosaurus paper, the real issue was the lack of clarity around how, precisely, Ibrahim and Sereno’s team had reached their conclusions. “At the heart of the storm is a data vacuum about Spinosaurus,” Witton mused, “an odd state to be in seeing as we're now meant to have a good idea what it looked like.” Ibrahim and the rest of the team eventually jumped in to respond to the hind-­limb proportion controversy, arguing in responses to Hartman and Witton that the listed measurements and the reconstruction did, in fact, match up. They also promised that a longer, more detailed monograph on Spinosaurus was on the way.

But as Witton noted in an email, a lot of uncertainty remains, particularly around the missing elements of the skeleton and the possible chimeric nature of the reconstruction. “Many of the more famous components of the 2014 Spinosaurus paper have to be regarded as problematic,” he said. “As demonstrated by subsequent papers, there are multiple ways to interpret fossils of this animal. Some of these seem to explain current Spinosaurus data better than the hypothesis put forward by Ibrahim et al, and they do undermine the radical reinterpretation publicized in 2014.” Only the recovery of more fossil material, collected under well-documented conditions, will resolve the problem, he said.

* * *

All of this is business as usual in paleontology—­­claims are made and disputed all of the time, as various scientists attempt to interpret scanty evidence. What made the 2014 Spinosaurus reboot different was the sheer scale of the PR effort surrounding the paper: a 5,000 square foot exhibit, a science paper, a television special, and a cover story in National Geographic.

However, there’s a problem with this kind of overwhelming PR—­­it often implicitly represents a hypothesis as established certainty when there’s still intense debate. “The artwork, exhibitions, and models we've seen of this animal are really impressive,” Witton said, “so much so that many lay folk will take this interpretation of Spinosaurus as definitive. ... The reality is that scientists have already highlighted issues at the heart of the 2014 reconstruction, but the media campaign continues with the same confidence we saw in 2014.” Looking at the National Geographic presentation, a non­-specialist might easily conclude that the appearance of Spinosaurus as a knuckle-­walking swimmer has been decided for good.

These issues are complicated and unlikely to be resolved any time soon. Yet the new Spinosaurus material does offer some intriguing ideas about the appearance and lifestyle of the great sailed predator. To begin with, it seems clear that Spinosaurus and Sigilmassosaurus­­, if they are indeed different animals­­, shared a short-­legged body ­plan unique among predatory dinosaurs. These short hind limbs don’t necessarily preclude bipedal motion—­­a great deal will depend on the posture of the neck and arms, if that information ever comes to light. Animals that appear to be confined to all­ fours, like pangolins, sometimes are adept at moving on their hindlimbs.

Paradoxically, after all the fuss over the hind legs, spinosaurs may have been terrible swimmers. In recent analysis of his team’s findings, Sereno came to the conclusion that the animal’s center of gravity would have left it struggling to keep its head above water when floating. “It sucks!” Sereno said, laughing. “I get the sense that it’s in the subaquatic zone, not really adapted for deep water … the thing is very unstable.” Perhaps, Sereno said, Spinosaurus represents an unsuccessful experiment by dinosaurs attempting to move into aquatic niches.

It’s also possible that a more pelican-­like neck posture, as some have proposed, could have allowed Ibrahim and Sereno’s Spinosaurus to totter along on its tiny back­legs. While the North African spinosaurs likely weren’t as fleet-­footed as their relatives, they also probably didn’t need to be. Possibly they spent their time foraging in the deltas and waterways of Cretaceous North Africa, padding along mudflats and swimming in the rivers, snatching at the enormous bounties of coelacanths, freshwater sharks, and perhaps probing the muck for giant lungfish.

Viewed this way, the shape of Spinosaurus is an awkward compromise: ­­uncertain on land, uncertain in water. In a way, that’s appropriate: Paleontology is an uncertain science. To get to the bottom of the Egyptian giant, everybody is going to have to keep digging.