Largest Meat-Eating Dino Was a True Swimmer and Had the Dense Bones to Prove It

Largest Meat-Eating Dino Was a True Swimmer and Had the Dense Bones to Prove It
Artist's conception of Spinosaurus hunting a large Onchopristis. (Illustration: Davide Bonadonna)

Spinosaurids — the largest carnivorous dinosaurs to have ever lived — hunted for prey while fully submerged in water, according to an innovative new study that considered the bone density of these fearsome creatures.

High bone density in spinosaurid fossils is a strong indicator that these Cretaceous dinosaurs were “aquatic specialists,” according to new research published today in Nature. Dense bones allow for better buoyancy control when animals are submerged in water, and at least two groups of spinosaurids — Spinosaurus and Baryonyx — featured bones that were discernibly solid. Meanwhile, Suchomimus, a related spinosaurid, was found to have hollow bones, which means it likely hunted by wading in the water like a heron.

But Spinosaurus and Baryonyx, they took the full plunge, using their powerful tails to swim through the water in search of prey. For non-avian dinosaurs, this sort of behaviour is a rarity, as only a small handful of species are believed to be partly or fully aquatic.

“Historically, spinosaurids have been this odd group of meat-eating dinosaurs with many peculiarities that implied some sort of ‘aquatic-curious’ lifestyle — unique within non-avian dinosaurs,” Eric Gorscak, an assistant professor of anatomy at Midwestern University who’s not affiliated with the new study, wrote in an email. “Although there have been a slew of qualitative observations to support this hypothesis, quantitative data have yet to test this hypothesis.”

The new research sought to overcome this limitation. The team, led by paleontologist Matteo Fabbri from the Field Museum of Natural History, provided this much-needed quantitative data by comparing the density of spinosaurid bones to a diverse range of living and extinct animals.

“The results of their tests strongly suggest that if it looks like a crocodile, floats like a cormorant, and submerges like a hippopotamus, then spinosaurids were most probably semi-aquatic dinosaurs,” Gorscak told me.

Spinosaurids lived during the early Cretaceous between 145 million and 100 million years ago. That they were semi-aquatic was largely suspected based on certain anatomical features, including long, crocodile-like jaws and cone-shaped teeth similar to those seen on aquatic predators. And as research from 2020 showed, ​​Spinosaurus featured a highly specialised tail that could propel it through water.

These animals were comfortable in the water — that much seemed clear — but the degree to which they had adopted an aquatic lifestyle wasn’t fully known. (As an aside, all dinosaurs had to lay eggs on land, requiring a certain level of terrestrial connection.) Anatomy can imply behaviour, but not always. It’s not immediately obvious that some creatures, such as the modern hippo, are aquatic. That’s where the study of bone density can help.

For the study, Fabbri and his colleagues made CT scans to analyse and compare the densities of hundreds of bones belonging to dinosaurs, crocodiles, birds, marine reptiles, and more. In particular, the team looked at femurs and ribs. Writing to me in an email, Fabbri said the “reason for using these bones is that they usually have a strong influence for motion, balance, and ecological adaptations.”

The CT scans allowed the team to gather digital cross sections of the bones. “In some cases, we cut the mid portion of the bone and made very thin slices of it, to better study the bone tissue,” Fabbri explained. “We imaged these sections, which were then transformed in black and white figures; black for bone tissue and white for empty spaces, such as vascular canals and medullary cavity.”

The scans were imported into specialised software that quantified bone tissue, in a process repeated for 480 bones and 291 species of extinct and extant species. In an emailed press release, Jingmai O’Connor, a curator at the Field Museum and co-author of the new paper, said studies like this represent “the future of paleontology.” While very time-consuming, “they let scientists shed light onto big patterns, rather than making qualitative observations based on one fossil,” she said, adding that it’s “really awesome that Matteo was able to pull this together, and it requires a lot of patience.”

Indeed, all this hard work resulted in an impressive payoff. The scientists were able to establish a clear link, showing a connection between bone density and aquatic foraging behaviour. Animals that submerged themselves in water tended to have bones that were nearly completely solid, while terrestrial animals tended to have bones with hollow centres. The high density of spinosaurid bones “tell us that they were spending a lot of time in water and swimming underwater,” said Fabbri.

Incredibly, the analysis allowed the team to infer behaviours in three different groups of spinosaurids: Spinosaurus, Baryonyx, and Suchomimus. The first two had dense bones suggestive of swimming, while the third had hollow bones suggestive of wading. Going into the study, paleontologists weren’t certain about Baryonyx and Suchomimus in terms of their aquatic lifestyles. Said Fabbri: “Our new study shifts the attention from Spinosaurus to other spinosaurids: underwater swimming was more widespread than we previously thought, opening a whole new narrative.”

I asked Fabbri if another explanation might account for the high bone density observed in spinosaurids.

“Based on our results, there is no alternative explanation for the high bone density observed throughout their entire skeleton,” he replied. “Some incredibly heavy dinosaurs and mammals have higher density in the limb bones, but this is not widespread throughout the skeleton, making it easy to discern between aquatic or simply very heavy.”

Further research is needed to better understand how these animals moved through the water, especially given their large size. Fully mature spinosaurids had bodies reaching 12.6 to 18 metres long, and no semi-aquatic animal living today can compare. Still, a picture is steadily emerging, showing how these soggy carnivores terrorised the rivers and lakes of the Cretaceous.