After all, the fearsome Spinosaurus was not the scourge of the prehistoric seas

After all, the fearsome Spinosaurus was not the scourge of the prehistoric seas

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The largest predatory dinosaur to ever walk the Earth sported a massive sail that rose from its back, but it turns out this imposing creature would have been a very slow and clumsy swimmer, according to new research.

Spinosaurus was even larger than Tyrannosaurus rex at 45 feet (13.7 meters) long. The colossus had an unusual skull shape that made it look more like a toothed crocodile than a bird of prey, said Paul Sereno, a professor of biology and anatomy at The University of Chicago.

Spinosaurus mainly hunted very large fish, such as e.g sawfish, lungfish and coelacanths, and had long hair-shaped claws to grab and tear them. However, the dinosaur was more adapted to living 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 Nov. 30 in the journal eLife.

“Do I think this animal would regularly wade in water? Absolutely, but I don’t think he was a good swimmer nor capable of fully submerged behavior,” Sereno said.

“It’s just not an animal that in your wildest dreams would be as dynamic above water as a swimmer, much less underwater.”

The Spinosaurus has long intrigued scientists.

The German paleontologist Ernst Stromer called the prehistoric carnivore Spinosaurus aegyptiacus in 1915 after the first partial the skeleton was discovered by his fossil hunter Richard Markgraf in Egypt.

Stromer, who proposed that the dinosaur stood upright on its hind legs and ate fish, displayed the find at the Palaeontological Museum in Munich. The fossils were destroyed during Allied bombing in World War II, and only Stromer’s notes and drawings survive.

Many decades later, more fossils were discovered by miners in the sandstone cliffs of Southeastern Morocco. Sereno and his team studied the fossils, as well as museum specimens and Stromer’s original notes, and shared their findings in 2014.

A more complete image of the carnivorous dinosaur emerged as one with interlocking slanted teeth perfect for catching fish, a long neck and torso, short hind legs, and a towering sail made up of dorsal spines covered in skin.

The dinosaur’s small nostrils were placed further inside the skull, allowing it to breathe even when partially submerged in water. This anatomical trace suggests that Spinosaurus was “semi-aquatic” and waded in the shallow waters along river banks for its prey.

In recent years, other teams have published research as they studied new fossils suggesting that Spinosaurus was an entirely 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 was really like for the fearsome dinosaur.

Sereno started by encountering an error in the 2014 paper. When he and his team calculated the dinosaur’s center of gravity, the software didn’t subtract enough mass to account for its lungs. This made it appear that spinosaurs would have had to walk on all fours.

“I like to admit mistakes, especially when I can correct them myself,” Sereno said.

The team assembled CT scans of the Spinosaurus skeleton and added layers of musculature and body mass based on modern reptiles to construct a virtually new model. This time, Spinosaurus had its center of gravity above its hips and stood upright, much like T. rex and other towering carnivore dinosaurs.

“The stiff limbs are not there for ballast during swimming, but rather to support the great weight of the beast,” Sereno said.

The team then turned to Spinosaurus’ tail. Dr. Frank Fish, an expert on tail mechanics and professor of biology at West Chester University in Pennsylvania, took the initiative.

Fish compared the tail of Spinosaurus to that of alligators and other reptiles and found that the dinosaur would have been too stiff to function well underwater. While alligators retract their limbs while swimming and have the flexibility to twist and roll underwater in pursuit of prey, Spinosaurus’s massive body mass, tall sail, and dangling hind legs would have been a hindrance.

“The rear oars are an order of magnitude too small to produce any aftermotion or power in rowing,” Sereno said. “No fully aquatic animal, conversely, has such large forelimbs as Spinosaurus, because the forelimbs are very ineffective as paddles.”

Its bony, muscular tail would not have had the flexibility of a whale or fish, and the heavy sail might have been more of a hindrance than a useful tool.

If the Spinosaurus was submerged in deep water, the results would not be very pleasant.

“Its ribcage will be crushed and it will die in a minute,” Sereno said, not to mention the drag of its “super uncomfortable canvas and dangling limbs.” And he wouldn’t be able to catch a fish by swimming after them.

So what was the purpose of the canvas?

“A display, like a billboard,” Sereno said. Like some lizards today, which have spine-supported sails, Spinosaurus probably used its sail during competition and courtship, he said.

The fossil record also suggests that Spinosaurus was more adapted to rivers and lakes than oceans. Spinosaurus fossils have largely been found in the river banks of the Niger interior basins, which are remote from the prehistoric seashores.

Interestingly, the dinosaur probably lived in marine and freshwater habitats like other semi-aquatic reptiles, but this is not something that other extinct or extant large aquatic vertebrates such as ichthyosaurs or sea turtles. So Spinosaurus would have roamed coastal and inland waterways, stalking prey while wading in shallow waters.

“Non-marine dinosaurs dominated the world for 150 million years, but they never entered the water in a serious way,” Sereno said. “Sure, they can swim just like us, but that doesn’t mean we’re aquatic. We’re talking about whether they were really adapted to life in the water, and that’s the main question behind all this attention on Spinosaurus.

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