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spinosaurus swimming
Davide Bonadonna


Spinosaurus swam, but it wasn’t the first dinosaur to get its toes wet

You see, there are these things called ‘trace fossils’ ….

The end of April saw a big splash (pun intended) in the paleontology world, as a new paper on the ever-enigmatic Spinosaurus aegypticus revealed that the dinosaur’s tail had features that may have suited it for a semi-aquatic lifestyle. The study led by Nizar Ibrahim, which is something of a follow-up to one he published in 2014, reveals a dramatic new aspect of Spinosaurus’ anatomy — a tail fin that the authors propose was used for aquatic locomotion, allowing Spinosaurus to pursue prey in the water.

Study co-author Stephanie Pierce even said, “This discovery overturns decades-old ideas that non-bird dinosaurs were restricted to terrestrial environments.” Other paleontologists say that’s only true if you ignore trace fossils.

“Trace fossils reflect behavior of living plants or animals interacting with [their] environment, perhaps millions of years ago,” says Emory University paleontologist Anthony Martin. “So trace fossils act as ‘snapshots’ of life while it was happening, which makes them far different from most body fossils, such as shells or skeletons.”

While not as sexy to most people as body fossils, trace fossils like tracks, burrows, nests, or even feces can actually tell us more about what the animal did while it was alive. Martin says if you can determine when and what kind of environment the fossil was deposited in (like mudstone in an aquatic environment), you can make some pretty safe assumptions.

“Let’s say we find gigantic, three-toed theropod dinosaur tracks that show it was swimming, perhaps with a tail-drag trace between them,” Martin says. “These tracks in rocks the same age as Spinosaurus would be good evidence to support that at least one Spinosaurus went for a swim.”

This new Spinosaurus find comes from a geological formation in Morocco commonly referred to as the Kem Kem Group or Kem Kem fossil beds. While Ibrahim’s found theropod fossil tracks there before, they were of walking animals, and the number of large theropods in the formation (like Carcharodontosaurus, Spinosaurus, Deltadromeus) makes identifying the tracks difficult.

Though we may not have had evidence that any dinosaurs swam routinely, Andrew Milner, paleontologist and curator at the St. George Dinosaur Discovery Site, says there’s been no doubt that some dinosaurs swam for over 15 years.

Retired ophthalmologist Sheldon Johnson discovered dinosaur tracks from the early Jurassic period while developing his property in February of 2000. The unlikely location became “the best basal Jurassic track site in western North America,” according to Utah State paleontologist Jim Kirkland. Johnson was generous enough to allow his farm to be not only a dig site, but a museum open to the public, that has welcomed half a million visitors since its opening in 2005.

“I found the first swim track block on May 4, 2001, but at the time we didn’t realize what we had,” says Milner, and the real deluge began two years later. “We excavated approximately 150 blocks covered in natural cast swim tracks with at least 3,200 individual claw marks preserved,” Milner says. “This discovery ended all controversy on whether or not theropod dinosaurs could actually swim.”

The tracks are so well-preserved, Milner says you can even tell that some of the dinosaurs that left them were swimming against the current, or even cross-current, maybe across a river.

Of course, the cinematic masterpiece Jurassic Park III was ahead of everybody’s curve on this one.  

Spinosaurus swam, but it wasn't the first dinosaur to get its toes wet

Milner has written about the St. George tracks before, including a chapter co-written with Martin G. Lockley for Dinosaur Tracks: The Next Steps. The book highlights that theropod dinosaurs were moving in the water during the early Jurassic period, tens of millions of years before Spinosaurus popped up in the Cretaceous of North Africa.

For more information on trace fossils, you can also check out Martin’s new book, Tracking the Golden Isles: The Natural and Human Histories of the Georgia Coast.

Stephanie Pierce did not respond to AIPT’s requests for comments.

AIPT Science is co-presented by AIPT and the New York City Skeptics.


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