The method by which Spinosaurus hunted has been a matter of controversy for decades despite being the largest predatory dinosaur known (it is over two meters longer than the longest Tyrannosaurus rex).
Spinosaurus, the longest predatory dinosaur known, is opening its elongate jaws, studded with conical teeth, to catch a sawskate. Contrary to previous suggestions, this animal was not a heron-like wader – it was a “river monster”, actively pursuing prey in a vast river system located in modern-day North Africa. Dense bones in the skeleton of Spinosaurus strongly suggest it spent a substantial amount of time submerged in the water. Credit: Davide Bonadonna
A team of paleontologists has used a novel method—analyzing the density of long-extinct species' bones—in a new publication that was published on March 23, 2022, in Nature.
The research team discovered that Spinosaurus and its close relative Baryonyx from the Cretaceous of the UK both had dense bones that would have allowed them to submerge themselves underwater to hunt by analyzing the density of spinosaurid bones and comparing them to other animals like penguins, hippos, and alligators.
Because spinosaurids' elongated jaws and cone-shaped teeth resemble those of fish-eating carnivores, and because the ribcage of Baryonyx from Surrey also included partially digested fish scales, scientists already knew that spinosaurids had certain connections with water.
Dr. Nizar Ibrahim, a paleontologist of the University of Portsmouth and a National Geographic Explorer, discovered pieces of a Spinosaurus skeleton in the Sahara Desert of North Africa a decade ago. Retracted nose, small hind legs, paddle-like feet, and a tail that resembled a fin were all characteristics of the skeleton Dr. Ibrahim and his team reported, all of which strongly suggested an aquatic lifestyle.
The skeleton truly has "water-loving dinosaur" written all over it, according to Dr. Ibrahim. "We battled sandstorms, flooding, snakes, scorpions, and more to excavate the most enigmatic dinosaur in the world," he continued.
Dr. Ibrahim and his team earlier hypothesized that Spinosaurus could swim and actively chase prey in the water based on its highly specialized anatomy, but others asserted that it was not much of a swimmer and instead trudged in the water like a big heron.
Baryonyx, from Surrey in England, swims through an ancient river with a fish in its jaws. Like its much larger African relative Spinosaurus, Baryonyx had dense bones, suggesting that it too spent much of its time submerged in water. It was previously thought to have been less aquatic than its Saharan relative. Credit: Davide Bonadonna
Researchers are still debating whether Spinosaurus stood in the shallows and dipped its jaws in to catch prey, or if it spent most of its time submerged, seeking animals in the water.
"In part, this is probably because we were challenging decade-old dogma, so even if you have a very strong case, you kind of expect a certain degree of pushback," Dr. Ibrahim said.
An international team of experts, including senior author Dr. Ibrahim and lead author Dr. Matteo Fabbri of Chicago's Field Museum, sought an alternative method of determining the lifestyle and ecology of long-extinct animals like Spinosaurus in response to this ongoing controversy.
The concept behind our work, according to Dr. Fabbri, was that there are many possible interpretations for the fossil data. What about the fundamental physical laws, though? Any organism on this planet must abide by a certain set of laws. One of these laws relates to density and water-submergence capabilities.
Bone density can reveal if an animal can swim and submerge itself across the animal kingdom.
An specialist on the internal structure of bone, Fabbri, stated that "previous studies have shown that mammals adapted to water have dense, compact bone in their postcranial (behind the skull) skeletons." Dense bone aids in buoyancy control and enables submersion of the animal.
The team compiled a massive dataset of femur and rib bone cross-sections from 250 species of extinct and living creatures, including both land- and water-dwellers, and covered animals weighing from a few grams to several tons, including seals, whales, elephants, mice, and even hummingbirds.
Additionally, they gathered information about extinct sea reptiles including plesiosaurs and mosasaurs. The researchers compared the cross sections of the bones from these creatures to those from the relatives of Spinosaurus, Baryonyx, and Suchomimus.
According to Dr. Ibrahim, "the scope of our study kept expanding because we kept thinking of more and more groups of vertebrates to include."
The researchers discovered a direct correlation between bone density and aquatic foraging behavior: species that dive underwater to collect food have bones that are almost entirely solid throughout, whereas the cross sections of bones in land animals more closely resemble doughnuts with hollow centers.
The scientists discovered that Spinosaurus and Baryonyx both had the kind of dense bone associated with complete submersion when they subjected spinosaurid dinosaur bones to this paradigm.
The bones of the distantly related African Suchomimus were hollower. Despite having a crocodile-like snout and conical teeth, it nevertheless resided near water and consumed fish, but due to its low bone density, it wasn't actively swimming. Ibrahim said, "That was a bit of a surprise, since Baryonyx and Suchomimus look rather similar." But the group quickly understood that it was not unusual and that other groups had also shown similar trends.
Dr. Nizar Ibrahim. Credit: Paolo Verzone
The thick bones in the limbs of other dinosaurs, such as the enormous long-necked sauropods, simply reflect the extreme stress placed on those limb bones.
Some of these animals would have weighed as much as several elephants, so giving the bones more load-bearing capacity makes a lot of sense, according to Dr. Ibrahim.
The future of paleontology, according to Dr. Jingmai O'Connor, a curator at the Field Museum and co-author of this study, lies in collaborative research projects like this one that draw on hundreds of fossils. Although they take a lot of time to complete, they enable researchers to identify significant trends.
Dr. Ibrahim is already considering the following inquiries. "With this new piece of evidence, I believe it is reasonable to rule out speculative ideas that Spinosaurus was a huge wader because they are not supported by the available data. The bones are unreliable, and now that we are aware of it, even their internal structure supports our understanding of the animal as a massive predator that hunted fish in huge rivers while using its paddle-like tail for propulsion. We are already attempting to reconstruct these river monsters' movements in great detail, which will be fascinating.
