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.