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The largest predatory dinosaur to ever walk on Earth sported a massive sail that rose from its back, but it turns out this imposing creature would have made for a very slow and awkward swimmer, according to new research.
Spinosaurus was even larger than Tyrannosaurus rex and measured 45 feet (13.7 meters) long. The colossus had an unusual skull shape that made it look more like a toothy crocodile than a raptor, said Paul Sereno, professor of organismal biology and anatomy at The University of Chicago.
Spinosaurus primarily hunted very large fish, such as sawfish, lungfish and coelacanths, and had long, scythe-shaped hand claws to catch and rip them apart. However, the dinosaur was more adapted to life on land and hunting from shorelines rather than filling the niche of an aquatic, underwater predator, said Sereno, lead author of a new paper published November 30 in the journal eLife.
“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,” Sereno said.
“This is simply not an animal that in your wildest dreams would be dynamic above water as a swimmer much less underwater.”
A tricky path
Spinosaurus has long intrigued scientists.
German paleontologist Ernst Stromer named the prehistoric predator Spinosaurus aegyptiacus in 1915 after the first partial skeleton was discovered by his fossil hunter Richard Markgraf in Egypt.
Stromer, who suggested the dinosaur stood upright on its back legs and snacked on fish, displayed the find in Munich’s Paleontological Museum. The fossils were destroyed during Allied bombing in World War II, and only Stromer’s notes and drawings survived.
Many decades later, more fossils were unearthed by miners from the sandstone rocks of southeastern Morocco. Sereno and his team studied the fossils, as well as specimens from museums and Stromer’s original notes, and shared their findings in 2014.
A more complete depiction of the predatory dinosaur emerged as one with interlocking slanted teeth perfect for catching fish, a long neck and trunk, short back legs and a towering sail made up of dorsal spines covered in skin.
The dinosaur’s small nostrils were set back farther in the skull, enabling it to breathe even when partially submerged in the water. This anatomical clue suggested Spinosaurus was “semiaquatic” and waded into the shallow waters along river banks for its prey.
In recent years, other teams have published research as they studied new fossils that suggested Spinosaurus was a fully aquatic predator with a fleshy paddle-like tail that would have allowed it to move like an eel, and dense bones that acted as ballast, allowing it to dive deep into the water column.
Sereno and his team returned to their work with Spinosaurus in search of answers about what life had really been like for the fearsome dinosaur.
Sereno began by confronting a mistake in the 2014 paper. When he and his team calculated the dinosaur’s center of gravity, the software didn’t deduct enough mass to account for its lungs. This made it seem like Spinosaurs would need to walk on all fours.
“I love to admit mistakes, especially when I can correct them myself,” Sereno said.
The team collected CT scans of the Spinosaurus skeleton and added layers of musculature and body mass, based on modern reptiles, to virtually construct a new model. This time, Spinosaurus had a center of gravity over its hips and stood upright, much like T. rex and other towering dinosaur predators.
“The solid limbs are not there for ballast while swimming, but rather to support the great weight of the beast,” Sereno said.
Next, the team turned to Spinosaurus’ tail. Dr. Frank Fish, tail mechanics expert and professor of biology at West Chester University of Pennsylvania, took the lead.
Fish compared the Spinosaurus tail with those of alligators and other reptiles and found the dinosaur would have been too rigid to function well underwater. While alligators tuck their limbs in as they swim and have the flexibility to spin and roll underwater in pursuit of prey, Spinosaurus’ huge body mass, tall sail and dangling hind legs would have been a hindrance.
“The hind paddles are an order of magnitude too small to produce any consequential paddling motion or power,” Sereno said. “No fully aquatic animals, conversely, has forelimbs as proportionately large as Spinosaurus, as the front limbs are very inefficient as paddles.”
Its bony, muscular tail wouldn’t have had the same flexibility of a whale or fish, and the heavy sail might have been more of an obstacle than a useful tool.
If Spinosaurus was plunked down in deep water, the results wouldn’t have been very pleasant.
“Its thorax would be crushed, and it would die within a minute,” Sereno said, not to mention the drag of its “super ungainly stiff sail and hanging limbs.” And it wouldn’t have been able to catch fish by swimming after them.
So what was the purpose of the sail?
“Display, like a billboard,” Sereno said. Similar to some lizards today that have spine-supported sails, Spinosaurus likely used its sail during competition and courtship, he said.
The fossil record also suggests Spinosaurus was more adapted to rivers and lakes than oceans. Spinosaurus fossils have largely been found in the riverbank deposits of Niger’s inland basins, which are distant from prehistoric marine coastlines.
Intriguingly, the dinosaur likely lived along marine and freshwater habitats like other semiaquatic reptiles, but it’s not something that any other extinct or extant large aquatic vertebrates like ichthyosaurs or sea turtles did. So Spinosaurus would have prowled along coastal and inland waterways, ambushing prey as it waded into shallow waters.
“Nonavian dinosaurs dominated the world for 150 million years, but they never went into the water in a serious way,” Sereno said. “Of course, they could swim just like we do, but that doesn’t mean we’re aquatic. We’re talking about whether they were truly adapted to life in the water, and that’s the central question behind all this attention on Spinosaurus.”