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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 that this hulking creature would have made a very slow and clumsy swimmer, according to new research.
Spinosaurus was even larger than Tyrannosaurus rex and measured 13.7 meters in length. The colossus had an unusual skull shape that made it look more like a toothy crocodile than a raptor, said Paul Sereno, professor of biology and anatomy of organisms at The University of Chicago.
Spinosaurus mainly hunted very large fish, such as sawfish, lungfish and coelacanths, and had long scythe-like claws to grab and tear them apart. However, the dinosaur was more suited to living on land and hunting from shore rather than filling the niche of an aquatic and underwater predator, said Sereno, lead author of a new paper published Nov. 30. in eLife magazine.
“Do I think this animal would have waded through the water regularly?” Absolutely, but I don’t think he was a good swimmer or capable of full immersion behavior,” Sereno said.
“It’s just not an animal that in your wildest dreams would be dynamic above water as a swimmer let alone underwater.”
Spinosaurus has long puzzled scientists.
German paleontologist Ernst Stromer named the prehistoric predator Spinosaurus aegyptiacus in 1915 after the first part skeleton was discovered by his fossil hunter Richard Markgraf in Egypt.
Stromer, who suggested the dinosaur stood on its hind legs and munched on fish, displayed the find at the Paleontological Museum in Munich. The fossils were destroyed during Allied bombings in World War II, and only Stromer’s notes and drawings have survived.
Several decades later, other fossils were discovered by miners in the sandstone rocks of southeastern Morocco. Sereno and his team studied the fossils, along with museum specimens and Stromer’s original notes, and shared their findings in 2014.
A fuller representation of the predatory dinosaur emerged with interlocking angled teeth perfect for catching fish, a long neck and trunk, short hind legs, and a towering sail made up of skin-covered spines.
The dinosaur’s small nostrils were recessed further into the skull, allowing it to breathe even when partially submerged in water. This anatomical clue suggested that Spinosaurus was “semi-aquatic” and waded through shallow waters along the banks for its prey.
In recent years, other teams have published research as they study new fossils suggesting that Spinosaurus was an all-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 resumed their work with Spinosaurus in search of answers about what life was really like for the fearsome dinosaur.
Sereno first faced an error in the 2014 paper. When he and his team calculated the dinosaur’s center of gravity, the software didn’t infer enough mass to account for its lungs. This gave the impression that the spinosaurs would need to walk on all fours.
“I like to admit my 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 above its hips and stood upright, much like T. rex and other hulking predatory dinosaurs.
“Strong limbs are not there to weight down while swimming, but rather to support the great weight of the beast,” Sereno said.
Next, the team turned to the tail of Spinosaurus. Dr. Frank Fish, a tail mechanics expert and professor of biology at West Chester University in Pennsylvania, took the lead.
Fish compared the tail of Spinosaurus to those of alligators and other reptiles and found that the dinosaur would have been too rigid to function well underwater. While alligators flex their limbs as they swim and have the flexibility to turn and roll underwater in pursuit of prey, Spinosaurus’ enormous body mass, large sail and drooping hind legs would have been a obstacle.
“The hind paddles are an order of magnitude too small to produce any consistent movement or paddle power,” Sereno said. “Conversely, no fully aquatic animal has forelimbs as large as Spinosaurus, because the forelimbs are very inefficient as paddles.”
Its bony, muscular tail would not have had the flexibility of a whale or a fish, and the heavy sail might have been more of a hindrance than a useful tool.
Had Spinosaurus been plunged into deep water, the results would not have been very pleasing.
“His chest would be crushed and he would be dead in a minute,” Sereno said, not to mention the drag of his “super unsightly veil and hanging limbs.” And he couldn’t have caught fish swimming after them.
What was the purpose of sailing?
“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 that Spinosaurus was more suited to rivers and lakes than oceans. Spinosaurus fossils have largely been found in riverbank deposits of the interior basins of the Niger, which are distant from prehistoric sea coasts.
Oddly, the dinosaur probably lived along marine and freshwater habitats like other semi-aquatic reptiles, but this is not something that other large extinct or extant aquatic vertebrates like ichthyosaurs or turtles of sea did. Thus, Spinosaurus would have prowled along coastal and inland waterways, ambushing its prey as it waded through shallow waters.
“Non-avian dinosaurs ruled the world for 150 million years, but they never got into the water in any serious way,” Sereno said. “Of course they can swim like us, but that doesn’t mean we’re aquatic. We talk about whether they were really adapted to life in water, and that’s the central question behind all this attention on Spinosaurus.
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