Did the world’s biggest carnivorous dinosaur sink or float?

The fossil record can tell us a lot about what ancient life looked like. Modern imaging technology is allowing us to see long-lost body forms in incredible detail – reading intricate details of skin textures, tiny embryos preparing to hatch, and even prehistoric diseases, all from remains locked away in rocks for millions of years.

But how can we read behaviour from fossils?

A team of palaeontologists from the Field Museum in Chicago, US, are looking beyond basic anatomy to do just that.

Publishing in Nature, the team describe how they took a new approach to an old problem, using bone density to puzzle out the behavioural ecology of the biggest carnivorous dinosaur that ever lived – the Spinosaurus.

The way that Spinosaurus hunted has been the subject of debate for decades.

Based solely on the anatomy of its skeleton, some scientists have proposed that Spinosaurus could swim, but others believe that it just waded in the water like a heron.

With strong arguments made for both behaviours, anatomy alone clearly wasn’t going to be enough to solve the mystery.

“The fossil record is tricky – among spinosaurids, there are only a handful of partial skeletons, and we don’t have any complete skeletons for these dinosaurs,” says lead author Dr Matteo Fabbri, a postdoctoral researcher at the Field Museum. 

“Other studies have focused on interpretation of anatomy, but clearly if there are such opposite interpretations regarding the same bones, this is already a clear signal that maybe those are not the best proxies for us to infer the ecology of extinct animals.”

Palaeontologists have long known that spinosaurids spent some time by the water – their long, crocodile-like jaws and cone-shaped teeth are very similar to those of other aquatic predators, and some spinosaurid fossils have been found with tell-tale bellies full of fish.

But until recently, it has been widely believed that non-avian dinosaurs were the only group of terrestrial vertebrates that didn’t have any water-dwellers. Instead, it was thought that dinosaurs like the spinosaurids probably just stop in the shallows and dipped their heads in to snap up prey, much like a heron on the foreshore.

In 2014, the description of a new Spinosaurus specimen by Nazir Ibrahim from the University of Portsmouth, UK, threw the palaeontological world into a frenzy of debate. With retracted nostrils, short hind legs, paddle-like feet, and a fin-like tail, all signs pointed to an aquatic lifestyle for this beast.

But many researchers held firmly to the belief that spinosaurids were waders, not swimmers.

After much back-and-forth in the dinosaur community, Fabbri and his team decided it was high time to find a new way to look at the evidence.

“The idea for our study was, okay, clearly we can interpret the fossil data in different ways. But what about the general physical laws?” says Fabbri.

“There are certain laws that are applicable to any organism on this planet. One of these laws regards density and the capability of submerging into water.”

Right across the animal kingdom, bone density is a key determinant of whether an animal is able to sink beneath the surface and swim. Dense bone works as buoyancy control and allows the animal to submerge itself.

“Previous studies have shown that mammals adapted to water have dense, compact bone in their postcranial skeletons,” says Fabbri.

“We thought, okay, maybe this is the proxy we can use to determine if spinosaurids were actually aquatic.”

The team compiled a dataset of femur and rib bone cross sections from 250 species of both extinct and living animals, encompassing both land and water-dwellers.

“We were looking for extreme diversity,” says Fabbri. “We included seals, whales, elephants, mice, hummingbirds. We have dinosaurs of different sizes, extinct marine reptiles like mosasaurs and plesiosaurs. We have animals that weigh several tons, and animals that are just a few grams. The spread is very big.”

This menagerie of animals revealed a clear link between bone density and aquatic foraging behaviour: animals that submerge themselves underwater to find food have bones that are almost completely solid throughout, whereas cross-sections of land-dwellers’ bones look more like donuts, with hollow centres.

“There is a very strong correlation, and the best explanatory model that we found was in the correlation between bone density and sub-aqueous foraging. This means that all the animals that have the behaviour where they are fully submerged have these dense bones, and that was the great news,” says Fabbri.

With the correlation clearly established, the team could take cross-sections of bone from Spinosaurus and its close relatives, Baryonyx and Suchomimus, to see where these creatures fell on the bone-density scale – and extrapolating from this to their swimming capabilities. 

Spinosaurus and Baryonyx both had the sort of dense bone associated with full submersion. Meanwhile, the closely related Suchomimus had hollower bones. It still lived by water and ate fish, as evidenced by its crocodile-mimic snout and conical teeth, but based on its bone density, it wasn’t actually swimming.

Fabbri is excited by the potential for this study to tell us more about how dinosaurs lived.

“One of the big surprises from this study was how rare underwater foraging was for dinosaurs, and that even among spinosaurids, their behaviour was much more diverse that we’d thought,” says Fabbri.

Dr Jingmai O’Connor, a curator at the Field Museum and co-author of this study, says that collaborative studies like this one that draw from hundreds of specimens, are “the future of paleontology. They’re very time-consuming to do, but they let scientists shed light onto big patterns, rather than making qualitative observations based on one fossil.”