Giant Sail-Backed Dinosaur Might Have Been 'Heron From Hell'

A huge dinosaur with a "sail" on its back was a "heron from hell", researchers have argued.

The dinosaur in question, Spinosaurus aegyptiacus, was one of the largest predators ever to prowl the Earth. It lived in what is now Africa, during the Cretaceous.

This was the final period in the time of the dinosaurs, and it spanned from around 145 million to 66 million years ago.

Spinosaurus, known from only one specimen and a few scattered parts of others, is one of the largest carnivorous dinosaurs that has ever been discovered.

"We believe it was about 7 tons—or one ton more than a bull African elephant," Nathan Myhrvold, one of the authors of the latest PLOS ONE study with Intellectual Ventures, told Newsweek.

He added: "It's a very unique looking dinosaur because it has a huge 'sail' on its back."

While paleontologists generally agree that Spinosaurus was a fish-eater, exactly how this dinosaur lived and caught its prey has been the subject of lively debate among experts.

For example, some research has suggested that these dinosaurs waded or swam in the shallows close to shore in search of prey, while other studies hint that they were aquatic pursuit predators that dove deep into the water to hunt.

"Unlike other meat eating dinosaurs, there are strong clues that it lived near water and ate fish or other aquatic fare," the authors said in a press release.

"This has fueled a lot of controversy about how Spinosaurus lived—was it a fast swimming predator that chased fish like a sea lion?

"Or was it an ambush predator at the water's edge, grasping with its clawed hands like a giant version of a brown bear chasing salmon, or plunging its head into water like a 7-ton heron from hell?"

In order to shed light on this issue, the authors of the new study conducted a new analysis in which they re-examined the density of the dinosaur's bones as a way of determining its life habits in water.

In the paper, the authors discussed an earlier study that supported the aquatic pursuit predator hypothesis. This paper, published in the journal Nature in 2020, was authored by a different team led by Matteo Fabbri of the Field Museum of Natural History and colleagues.

The PLOS ONE authors examined the arguments put forward in the 2022 paper by Fabbri et al., coming the conclusion that it may have had significant flaws.

"The gist of that paper is that they basically wanted to use bone density—sampled from the thigh bone and ribs—and a novel statistical analysis to show that Spinosaurus was an underwater forager," Myhrvold told Newsweek.

"We showed that the data set they used for the method was not appropriate—a lot of statistical methods only work if the data has particular characteristics. The data in Fabbri et al did not have the necessary characteristics," he said.

In addition, Myhrvold and colleagues said the aquatic pursuit predator hypothesis does not seem feasible on a biological level.

While some modern aquatic mammals, like manatees or sea otters, have evolved denser than normal bones to act as a ballast and help them stay underwater, many dinosaurs, including Spinosaurus, had air sacs in the bones of their spine (modern birds have them too).

"So the density of thigh and rib bones doesn't tell you whether a dinosaur could dive," Myhrvold said.

In fact, in a 2022 paper that Myhrvold and colleagues published, they argued that the air sacs in Spinosaurus provided so much buoyancy that it could not dive—a bit like a person wearing a life vest.

While the latest PLOS ONE study does not settle the debate over the lifestyle of Spinosaurus, Myhrvold said their key finding was that the Fabbri et al. paper is not correct in stating that their analysis proves Spinosaurus hunted underwater.

The findings suggest that Spinosaurus was not a swimming or diving predator that chased fish like a sea lion, the authors said.

"We find no evidence from bone density to suggest that Spinosaurus was anything other than what we have judged from many other lines of evidence in previous research—a semiaquatic, fish-eating predator that haunted shorelines and shallow water ambushing prey of all sizes," Paul Sereno, another author of the PLOS ONE study with the University of Chicago, told Newsweek.

"It may have waited in shallow water to ambush fish the way a heron does," Myhrvold said. "But unlike a heron it weighed 7 tons and had both a full set of nasty teeth and arms with sharp claws. So it was a heron from hell."

The findings of the latest study, however, have been disputed by Fabbri and colleagues.

"We don't think their data support their conclusions," Fabbri told Newsweek. For one, the researcher said that high bone density is found all over the skeleton of Spinosaurus.

"Myrvhold et al. suggest that high bone density is present only in the legs, but this is incorrect: we recovered high density in the spines of the sail and tail, in the ribs, and even in the bones forming the hand," Fabbri said.

Animals adapted to underwater diving show high bone density across the whole skeleton, he explained.

Secondly, the researcher said the model his team used to support the claim that Spinosaurus was deep diver—using bone density as a variable—is highly accurate.

'I want to remind that our model is based on a training dataset of modern species for which we know the bone density and ecology," Fabbri said.

"Considering the strongly significant correlation between the two among species that we can observe today, we inferred that the ecology of Spinosaurus is consistent with diving animals."

Finally, the researcher said even the most complete specimen ever found of Spinosaurus is lacking bones for the portion of the body where the air sacs would be—the neck. This could cause problems in interpretation.

The model published in Myrvhold et al.'s 2022 paper would have required several assumptions that might have led to speculation, Fabbri said.

"The skeleton can easily suggest where air sacs were located, but there is no trace left regarding their actual volume. In other words, the quantification of air sacs is a process that can be easily overstated," Fabbri said.

"As a result, the estimation of the body density of Spinosaurus published by the authors is lower than the known density of modern chickens, which are characterized by hollow bones; this is impossible, considering the high bone density found all over the skeleton of Spinosaurus."

"I also want to add that numerous semi-aquatic and aquatic extant birds have extensive air sacs across the whole vertebral skeleton and, yet, these species dive under water to hunt. This just to say that the presence of a morphological feature doesn't exclude an ecological behavior."

Do you have a tip on a science story that Newsweek should be covering? Do you have a question about paleontology? Let us know via science@newsweek.com.