Did Spinosaurus hunt its prey deep underwater or from shore? Analysis continues

For years, controversy has swirled around how a Cretaceous-era, sail-backed dinosaur—the giant Spinosaurus aegyptiacus—hunted its prey. Spinosaurus was among the largest predators ever to prowl the Earth and one of the most adapted to water, but was it an aquatic denizen of the seas, diving deep to chase down its meals, or a semiaquatic wader that snatched prey from the shallows close to shore?

Dueling studies, each led by paleontologists from the University of Chicago, contribute new entries to this ongoing debate by reexamining the density of Spinosaurus bones.

Deep water swimmer or shoreline predator?

When detailed descriptions of a nearly complete specimen of Spinosaurus were first published in Science in 2014, a UChicago research team, co-led by Paul Sereno, PhD, Professor of Organismal Biology and Anatomy, pitched it as the first truly semiaquatic dinosaur, with muscular hind legs and webbed feet used for paddling and a flexible, undulating tail (read more about the history of Spinosaurus and its initial discovery).

Later in 2020, an international group of researchers countered that description with a study in Nature, using its newly discovered, tall-spinedtail bones to further support the theory that it propelled itself like an eel to hunt underwater.

A 2022 Nature study by many of those same researchers, including lead author Matteo Fabbri, PhD, who is currently a Chicago Fellows postdoctoral scholar at UChicago, backed their 2020 assessment showing that Spinosaurus also had dense bones to use as ballast for diving like a penguin. They also argued that some other spinosaurids, such as its older African cousin Suchomimus, had less dense bones and were likely waders.

About the same time in 2022, Sereno again teamed up with colleagues elsewhere to test these ideas by creating digital skeletons and flesh models of the Spinosaurus and Suchomimus. Their results, published in eLife, claimed that both species would have been unstable when swimming at the surface and far too buoyant to dive and fully submerge.

Now Sereno and that same team, including lead author Nathan Myhrvold, PhD, Founder and CEO of Intellectual Ventures, has also taken on the question of bone density. Their study, “Diving dinosaurs? Caveats on the use of bone compactness and pFDA for inferring lifestyle,” appeared on March 6, 2024, in the journal PLOS ONE.

“We had made the thin sections of these species that were used for [Fabbri’s] bone density calculations, and so we thought we would start by trying to replicate their measurements,” Sereno said. He argues that his team’s new calculations confirm that Spinosaurus wasn’t well-suited to dive deep and hunt underwater.

Meanwhile, Fabbri and his colleagues contend that their own new analysis, currently published as a preprint, still supports their original findings that the beast was suited for the seas. And the argument continues.

New calculations for ancient fossils

Sereno’s team began their latest study by asking new technical questions about measuring bone density, such as how to digitize thin sections, where to slice through the thigh and rib bones to best expose the interior, and whether to include bones from more than one individual.

Some modern aquatic mammals like manatees have swollen, dense bones to help them stay underwater, like a scuba diver’s weight belt. Large land animals, like elephants and dinosaurs, also have dense bones to support their increased body mass. Most modern birds and many dinosaurs, including Spinosaurus, have air sacs attached to their lungs or inside bones that act like a life vest preventing submergence. Assessing the aquatic abilities of an extinct species like Spinosaurus needs to take account of all these possibilities.

Sereno and team worked with Myhrvold to reevaluate how the statistical technique used in the 2022 Nature study was applied to support the claim that Spinosaurus was a deep diver. The approach, known as phylogenetic flexible discriminant analysis (pFDA), is like machine-learning: the pFDA technique trains a classification algorithm on a group of species whose lifestyle is well understood.

In principle, researchers could then use the algorithm to estimate the likelihood that a poorly understood species such as Spinosaurus falls into one behavioral group or another. In practice, however, Myhrvold said he believes there are problems that must be overcome.

“Unfortunately, that technique does not work properly unless you have lots of data, you make apples-to-apples comparisons, and you check that the data meet certain statistical prerequisites,” he said. “None of those requirements were met in that earlier study, so the results just didn’t hold up under reexamination.”

Before this latest salvo was published in PLOS One, Fabbri’s team had already begun responding to a preprint version of their arguments. They reran their density models, adjusting for the concerns raised by Sereno and Myhrvold, and say their results support the same conclusion as before—that Spinosaurus bones were dense enough to allow it to fully submerge underwater. Moreover, Fabbri said, bones throughout the skeleton have the same density according to their calculations, higher than any non-avian dinosaur studied to date, and higher than any existing large animals that do not spend most of their time in water, like elephants or rhinos.

“Obviously, the degree of support for our conclusions is different, because we are using different algorithms that work in different ways,” Fabbri said. “But we still have strong statistical support, even when we take into consideration all their major points.“

Competing definitions of aquatic lifestyle

The sticking point between the two groups may be a concept known as “ecological fallacy.” This happens when someone tries to make conclusions about individuals based on data collected at the group level. Fabbri’s argument rests on data showing that Spinosaurus bones were dense like existing animals that can fully submerge, like penguins and manatees. Hence, they conclude, along with other findings about the dinosaur’s tail bones and mechanics, that it could swim underwater too. Sereno and Myhrvold, on the other hand, contend that this is an ecological fallacy, and just because Spinosaurus had the dense bones of modern swimmers doesn’t mean it had other characteristics that allowed it to dive deep.

At the end of the day though, the argument may be more about semantics, and what terms like “aquatic” and “underwater” really mean.

“We think Spinosaurus, one of the largest predatory animals ever to have evolved, needed extra bone strength to support its weight on its relatively short hind limbs,” Sereno said. “Spinosaurus was able to wade into waterways more than six feet deep without floating, where it could ambush fish of any size with its claws and jaws—but all while keeping its toes firmly anchored in the mud.”

Fabbri says it’s more helpful to consider both possibilities. Large, modern-day animals like crocodiles and hippos spend lots of time both on land and underwater, and Spinosaurus probably did the same.

“I think this argument of opposite ecologies actually doesn't exist, because an animal that is fishing along shorelines is not arguing against the possibility of going farther into the water,” he said. “This dinosaur was not a dolphin living only in water, it was simply capable of going underwater sometimes to fish. Both hypotheses are putting Spinosaurus in the water. Our point is that we can argue, based on bone density, that this animal was probably capable of going underwater too.”

—Adapted from an article first published by the Biological Sciences Division.