Experts disagree over whether this dinosaur could swim : Spinosaurus was once one of the largest carnivorous dinosaurs, and it ate fish. That much paleontologists agree on.

But did it just wade into rivers and snatch them out of the water like a grizzly bear? Or did it dive after its prey like a penguin or a sea lion? This has become a question of contention among dinosaur experts.

One group is convinced that Spinosaurus was a rarity among dinosaurs : one that stuck its head underwater and swam beneath the surface. Others say no way.

The latest salvo, published last month in the journal PLOS One, comes the Spinosaurus-couldn't - swim contingent to counter a pro-swimming paper published a couple of years ago.

The earlier work, published in the journal Nature, claimed that in general, animals that spend much of the time in the water, like penguins, have denser bones that provide ballast and make it easier to dive.

Spinosaurus also had dense bones, and therefore was most likely a swimmer, the Nature paper concluded.

But that bone density analysis was '' statistically absurd,'' said Nathan Myhrvold, a former chief technology officer at Microsoft and an amateur paleontologist who led the new research with Paul Sereno, a paleontologist at the University of Chicago.

Dr. Myhrvold and Dr. Sereno have also argued that the ungainly body shape of Spinosaurus would have made it a poor swimmer, if it could swim at all. The weight distribution of the dinosaur would have made it top-heavy and unstable. Dr. Myhrvold said. '' It's obvious as to why it can't swim,'' he said.

The giant sail on its back would make it difficult for a swimming Spinosaurus to stay upright, he said. '' If it tips even the slightest amount, it'll keep tipping.'' In other words, Spinosaurus would capsize and struggle to pull its sail out of the water.

In this dispute, there are points of agreement.

Spinosaurus was perhaps longer and heavier than Tyrannosaurus rex. It lived about 95 million years ago in what is now Western Sahara but was then a lush environment with deep rivers.

It was also an odd-looking dinosaur, with elongated vertebrae forming a huge sail on its back.

In the latest study, Dr. Myhrvold and colleagues argue that the paleontologists who made the bone density claims employed a sophisticated statistical technique without understanding its limitations.

Earlier work by other researchers had found that diving mammals tended to possess denser bones than mammals that stayed on land. But other mammals also have dense bones for other reasons.

In 2022, researchers led by Matteo Fabbri, now a post doctoral researcher at the University of Chicago, argued in their paper that bone density was a reliable predictor whether an animal lived in the water or on land for a much broader variety of creatures, including extinct species.

'' We thought, Oh, is this just mammals or is this also reptiles ?'' Dr. Fabbri said in an interview. '' And if this is true, can we infer ecology in extinct animals, including weird-looking dinosaurs like Spinosaurus?''  [ Kenneth Chang ]


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