Spinosaurus aegyptiacus, a sail-backed theropod dinosaur that lived in what is now North Africa during the Cretaceous period, about 95 million years ago, was unstable in deeper water with little ability to right itself, swim, or maneuver underwater.
Spinosaurus aegyptiacus was adapted for hunting along shorelines instead of venturing deep underwater. Image credit: James Gurney.
In 1915, the German paleontologist Ernst von Stromer announced the discovery in Egypt’s Western Desert of the elongate jaws and partial skeleton of Spinosaurus aegyptiacus.
Other bones found nearby contributed to his initial reconstruction of the dinosaur as a sail-backed, fish-eating biped, shortly before all of these bones were destroyed in World War II.
Over the last 30 years, additional skull and skeletal bones came to light in western Morocco in beds of similar age to those in Egypt.
Central among these finds was a partial skeleton that allowed a more complete reconstruction of Spinosaurus aegyptiacus, confirming its interpretation as a semiaquatic fish-eater.
Since the initial findings, paleontologists collected the nearly complete tail of the rediscovered skeleton, which was hidden in sediment to the side of the original bone quarry.
The tail had tall, slender spines that that would have been covered with skin, a version of the sail along its back.
In 2020, they proposed that Spinosaurus aegyptiacus would have used this fleshy tail to propel itself like an eel through the water column as a fully aquatic predator.
This year, the researchers published another paper that compared the density of Spinosaurus aegyptiacus bones to a wide variety of living and extinct animals.
They concluded that the dinosaur had very dense bone walls like penguins, suggesting it actually spent most of its time in water, using the heavier bones for ballast to submerge itself regularly in underwater pursuits.
In the new research, University of Chicago’s Professor Paul Sereno and colleagues constructed a fresh model of Spinosaurus aegyptiacus from CT scans of its skeleton and fleshed it out with its musculature and body mass, based on modern reptiles.
“The solid limbs are not there for ballast while swimming, but rather to support the great weight of the beast,” Professor Sereno said.
“It turns out a lot of large animals, including the largest dinosaurs, fill in their hollow cores.”
The scientists also studied the biomechanics of the more complete tail structure and analyzed how useful it was for swimming.
Using formulas often applied to calculate eel-like swimming in water, they extrapolated the swimming power a Spinosaurus aegyptiacus could produce by flexing its tail and paddling with its feet.
They found that it would have been an order of magnitude less than an alligator, which tucks its limbs away as ineffective when swimming.
Spinosaurus aegyptiacus, with such a huge body mass, large sail, and hind legs dangling behind, would have been resistant to forces underwater and far too rigid to power itself via aquatic undulation.
“We based our calculations on accurate renderings of the tail and foot and how those should scale up if it were built like crocodiles,” Professor Sereno said.
“It’s pretty much isometric, which means that as the animal gets larger, the hind foot and tail become less and less effective as paddles to push its much greater mass.”
“The hind paddles are an order of magnitude too small to produce any consequential paddling motion or power.”
“No fully aquatic animals, conversely, has forelimbs as proportionately large as Spinosaurus aegyptiacus, as the front limbs are very inefficient as paddles.”
The authors also calculated that Spinosaurus aegyptiacus would have been too buoyant to fully submerge itself regularly, needing 15 to 25 times the estimated power of its tail.
The bone and muscle structure of the tail wouldn’t have been flexible enough to propel it smoothly through water, unlike the fleshy tail flukes on whales or light, springy fish fins.
The heavy, bony sail on its back also would have made it an awkward swimmer that struggled to right itself, unlike alligators and crocodiles that can easily spin and roll to pursue their prey.
“Spinosaurus aegyptiacus is best understood as a semiaquatic bipedal ambush piscivore that frequented the margins of coastal and inland waterways,” the paleontologists concluded.
The findings were published in the journal eLife.
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Paul C. Sereno et al. 2022. Spinosaurus is not an aquatic dinosaur. eLife 11: e80092; doi: 10.7554/eLife.80092
