Did Spinosaurus spend most of its time in the water?
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Art by Davide Bonadonna.
Did Spinosaurus spend most of its time in the water?

The New Spinosaurus

Spinosaurus has changed dramatically since I was a kid. The model I used to terrorize my other toys with looked like an overgrown Allosaurus with a giant sail on its back. As paleontologists rearranged the dinosaur family tree and found new species, however, they realized that Spinosaurus was a very different sort of animal, allied with croc-snouted, heavy-clawed dinosaurs like Baryonyx. When Spinosaurus finally tore up the celluloid in 2001’s Jurassic Park III, it was as a monstrous carnivore with giant claws, an elongated snout filled with conical teeth, and a flashy fin atop its back. And the evolution of Spinosaurus imagery has not stopped. A paper out in Sciencexpress today proposes that Spinosaurus was far stranger than paleontologists expected.

[The turn-of-the-century Spinosaurus. Today’s Sciencexpress paper suggests that the dinosaur was even more aquatic.]

The core of the new study, led by University of Chicago paleontologist Nizar Ibrahim, is a partial skeleton of Spinosaurus found in the 97 million year old rock of Morocco. The importance of the new specimen is in revealing parts of Spinosaurus never seen before. The skeleton includes parts of the skull and some vertebrae, but the real keys to the new Spinosaurus are the hips and hindlimbs.

The turn-of-the-century Spinosaurus popularized in Jurassic Park III and numerous pieces of paleoart stood tall over Cretaceous floodplains. But the hips and legs described by Ibrahim and coauthors look quite small and relatively weak. The femur – or upper leg bone – is short, yet has a robust flange of bone for a major leg-tail muscle retractor. And the foot bones of Spinosaurus are flat, with broad toe claws. This was not a dinosaur suited to running down prey. Spinosaurus, as envisioned in the new research, would have propelled itself through the water with strokes of its feet and sinuous flicks of its tail.

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A new reconstruction of Spinosaurus. The red bones represent the parts of the new skeleton. Model by Tyler Keillor, Lauren Conroy, and Erin Fitzgerald.

Whether the new Spinosaurus best represents the real animal remains to be seen. The fantastic images of Spinosaurus that have come out of today’s media blitz are based upon a hodgepodge reconstruction that draws from many different dinosaurs. There’s the new subadult skeleton, digital representations of the original and long-lost Spinosaurus bones, vertebrae and hands that may or may not belong to Spinosaurus, as well as replacement parts from an assortment of spinosaurs, all scaled to fit together. The dramatic departure of Spinosaurus from the previous release is a hypothesis that will be tweaked as additional specimens are discovered.

Putting bones together is one thing, though. Understanding what those bones can tell us about vanished lives is another.

Spinosaurs stalked the shores of Cretaceous lakes and rivers. Paleontologists have known this for decades. The remains of fish and juvenile dinosaur bones found in the gut of Baryonyx – the dinosaur responsible for highlighting spinosaurs as oddballs – showed that these dinosaurs probably used their conical teeth and huge claws to nab prey in and around waterways. And geochemical evidence from spinosaur teeth further supported the notion that these carnivores stuck close to freshwater habitats. As paleontologist Thomas Holtz, Jr. has pointed out, spinosaurs were likely the dinosaurian equivalent of grizzly bears.

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A reconstruction of the skull of Spinosaurus, with known elements in blue. Art by Davide Bonadonna.

Spinosaurus itself may have done more than just wade into the shallows, though. Ibrahim and colleagues point out several lines of evidence for swimming Spinosaurus. Some are relatively weak. The nasal opening of Spinosaurus, Ibrahim and colleagues note, is retracted up the snout, possibly to stop the dinosaur from inhaling water. But the dinosaur’s fleshy nasal opening was probably far forward of the bony aperture, and some semi-aquatic animals – such as crocodiles and hippopotamus – have their noses at the tips of their snouts. A retracted nasal opening isn’t a dead giveaway.

Better evidence for swimming Spinosaurus comes from the structure of the dinosaur’s bones. The dinosaur’s long bones are solid. This is odd.

Spinosaurus was a theropod – a member of the major lineage that includes dinosaurs such as Tyrannosaurus and Allosaurus – and these dinosaurs typically had relatively light long bones with cavities inside. The dense Spinosaurus bones, by contrast, seem similar to those of some early whales and other semi-aquatic mammals that evolved heavy bones to act as a kind of internal ballast. Based upon what paleontologists have previously discovered about animals that transition from land to the water, the solid leg bones of Spinosaurus are the best evidence that this was a dinosaur that preferred to swim. Trackways have shown that other theropods swam from time to time, but Spinosaurus may have been unique in its dedication to a semi-aquatic lifestyle.

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David Martill, Nizar Ibrahim, Paul Sereno and Cristiano Dal Sasso at the site of the new Spinosaurus. Photo by Cristiano Dal Sasso.

And then there’s that fin. Ibrahim and colleagues hint that the sail of Spinosaurus might have been a way to add some extra weight for staying submerged. That could be true, but it doesn’t explain how such a sail evolved in the first place.

Smaller spinosaurs, such as Suchomimus and Ichthyovenator, had sails, too, and they don’t share the proposed aquatic adaptations that Spinosaurus did. Spinosaurus inherited that sail rather than evolving it anew. The sail of Spinosaurus could have been an aid in adding weight, a prominent display structure, or even a way to store energy to help propel itself as it swung its tail from side-to-side, or even all three. The point is that if we’re going to understand why Spinosaurus was so gaudy, we need a deeper perspective on spinosaur evolution.

The function and evolution of spinosaur sails will continue to be debated. But one thing’s for sure – Spinosaurus surely wasn’t able to sneak up on any dinosaurs coming down to the water for a drink. The sail of Spinosaurus would have jutted above the surface as it swam along, advertising its presence.

Spinosaurus couldn’t have spent all of its time in the water, though. All dinosaurs laid eggs, and that, at the very least, required journeys into the terrestrial realm. Based on the proportions of their new reconstruction, Ibrahim and coauthors suggest that Spinosaurus must have walked on all fours while on land. This looks intuitive given the heavy arms and comparatively puny legs of the new Spinosaurus, although future reconstructions and biomechanical analyses could contradict the image of Spinosaurus as a paddling, four-on-the-floor dinosaur.

Spinosaurus isn’t done changing. The new subadult described by Ibrahim and colleagues fills in parts of the dinosaur’s skeleton that have been a total mystery until now, but the fact remains that most of the new reconstruction is assembled from bits and pieces of various dinosaurs. It’s a hypothesis. Spinosaurus was undoubtedly a spectacular dinosaur, but just how strange this prehistoric piscivore truly was is a secret still held tight by the fossil record.

Reference:

Ibrahim, N., Sereno, P., Dal Sasso, C., Maganuco, M., Martill, D., Zouhri, S., Myhrvold, N., Iurino, D. 2014. Semiaquatic adaptations in a giant predatory dinosaur. Sciencexpress. doi: 10.1126/science.1258750