Spinosaurus reveals a link between bone density and hunting underwater

Spinosaurus was among the largest terrestrial dinosaurs ever to exist, possibly even larger than the other two contenders for this crown, Tyrannosaurus and Giganotosaurus. Its length has been estimated at 15 to 16 meters (49 to 52 ft), while its weight was somewhere between 6 and 7 metric tons. This formidable dinosaur had a skull that measured 1.5 meters in length and a massive “sail” structure on its back, supported by enormous bony spines that originated on its vertebrae.

The dinosaur is known from only six partial skeletons, all excavated in North Africa, and its anatomy has led to intense debate about the kind of lifestyle this creature had. 

Since fossilized fish scales were present with some of the Spinosaurus bones, it would appear that it was a piscivore (preyed on fish). However, some scientists proposed that, although it had some features of modern crocodiles, its enormous body, bony spines and large sail would have made it unstable and slow while swimming in the water. They thought, instead, that it was probably a terrestrial predator like T. rex.

Others proposed that its excessive length, rather short back legs and unusually small pelvis would have made it an ineffective hunter on land, and that its great mass would have made walking in muddy lagoon or estuarine environments impossible. 

A new study, published today in Nature, has taken a different approach to the problem, since anatomical clues left in the fossilized bones have given no obvious answers. Scientists from the Field Museum decided to examine the density of the fossilized bones instead. Bone density is a function of several different factors but, by comparing Spinosaurus bones with the bones of other animals, such as penguins, hippos and alligators, the team hoped to gain new understanding. 

“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,” said study lead author Matteo Fabbri. “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.”

Fabbri and his colleagues put together a dataset of femur and rib bone cross-sections from 250 species of extinct and living animal species, both land-dwellers and water-dwellers. The researchers then compared these cross-sections to cross-sections of bone from Spinosaurus and its relatives Baryonyx and Suchomimus.

“We had to divide this study into successive steps,” said Fabbri. “The first one was to understand if there is actually a universal correlation between bone density and ecology. And the second one was to infer ecological adaptations in extinct taxa.” 

Essentially, the researchers had first to show that a relationship exists, in living species, between bone density and choice of aquatic or terrestrial habitat. After that, they could apply this criterion to extinct animals for which habitat choices can no longer be observed. 

When selecting animals to include in the study, the researchers had to consider as wide a selection of different species as possible. 

“We were looking for extreme diversity,” said 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 wide array of different species showed a clear link between bone density and underwater foraging behavior. All the animals that forage underwater have bones that are almost completely solid. This suggests that dense bones have a buoyancy function and help the animal to stay submerged while moving around and hunting. In contrast, the bones of species that spend their lives on land are less dense and contain spongy tissue and a hollow center. 

“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 behavior where they are fully submerged have these dense bones, and that was the great news,” explained Fabbri.

When the bones of Spinosaurus and Baryonyx were tested for density, they both had the dense bone structure associated with being able to submerge underwater and hunt for prey. The closely related Suchomimus, however, had bones that were more hollow. It still lived by water and ate fish, as evidenced by its crocodile-like snout and conical teeth, but based on its bone density, it wasn’t actually swimming, stated the authors. 

Animals that are heavy also have dense bones, even when they spend their lives on land. “Very heavy animals like elephants and rhinos, and like the sauropod dinosaurs, have very dense limb bones, because there’s so much stress on the limbs,” explained Fabbri. “That being said, the other bones are pretty lightweight. That’s why it was important for us to look at a variety of bones from each of the animals in the study.” Fabbri is excited by the potential for this new approach to reveal 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 behavior was much more diverse that we’d thought.”

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. It’s really awesome that Matteo was able to pull this together, as it requires a lot of patience.”

Fabbri also notes that the study shows how much information can be gleaned from incomplete specimens. “The good news with this study is that now we can move on from the paradigm where you need to know as much as you can about the anatomy of a dinosaur to know about its ecology, because we show that there are other reliable proxies that you can use. If you have a new species of dinosaur and you just have only a few bones of it, you can create a dataset to calculate bone density, and at least you can infer if it was aquatic or not.”

Image Credit: Davide Bonadonna

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By Alison Bosman, Earth.com Staff Writer