By analyzing the density of bones of several spinosaurid dinosaurs and comparing them to other animals like penguins, hippos, and alligators, Field Museum of Natural History paleontologist Matteo Fabbri and his colleagues found that Spinosaurus and its close relative Baryonyx both had dense bones that would have allowed them to submerge themselves underwater to hunt.
Spinosaurus aegyptiacus. Image credit: Davide Bonadonna.
“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,” Dr. Fabbri said.
“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.”
All life initially came from the water, and most groups of terrestrial vertebrates contain members that have returned to it.
For instance, while most mammals are land-dwellers, we’ve got whales and seals that live in the ocean, and other mammals like otters, tapirs, and hippos that are semi-aquatic.
Birds have penguins and cormorants; reptiles have alligators, crocodiles, marine iguanas, and sea snakes.
For a long time, non-avian dinosaurs were the only group that didn’t have any water-dwellers. That changed in 2014, when a research team led by University of Portsmouth’s Dr. Nizar Ibrahim described a new skeleton of Spinosaurus aegyptiacus, a giant spinosaurid dinosaur that lived about 95 million years ago (Cretaceous period) in what is now North Africa.
The specimen had retracted nostrils, short hind legs, paddle-like feet, and a fin-like tail: all signs that firmly pointed to an aquatic lifestyle.
“We battled sandstorms, flooding, snakes, scorpions and more to excavate the most enigmatic dinosaur in the world and now we have multiple lines of evidence all pointing in the same direction — the skeleton really has ‘water-loving dinosaur’ written all over it!” Dr. Ibrahim said.
Dr. Ibrahim and colleagues previously suggested that Spinosaurus could swim and actively pursue prey in the water, but others claimed that it was not much of a swimmer and instead waded in the water like a giant heron.
Researchers have continued to debate whether Spinosaurus spent much of its time submerged, pursuing prey in the water, or if it just stood in the shallows and dipped its jaws in to snap up prey.
“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.
Baryonyx. Image credit: Davide Bonadonna.
This continuing debate led the researchers to try to find another way to infer the lifestyle and ecology of long-extinct creatures like Spinosaurus.
They assembled a very large dataset of femur and rib bone cross-sections from 250 species of extinct and living animals, including both land-dwellers and water-dwellers, and covering animals ranging in weight from a few grams to several tons including seals, whales, elephants, mice, and even hummingbirds.
They also collected data on extinct marine reptiles like mosasaurs and plesiosaurs.
They then compared bone cross sections of these animals to cross-sections of bone from Spinosaurus and its relatives Baryonyx and Suchomimus.
“The scope of our study kept expanding because we kept thinking of more and more groups of vertebrates to include,” Dr. Ibrahim said.
The scientists found a clear link between bone density and aquatic foraging behavior: 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 doughnuts, with hollow centres.
When the authors applied spinosaurid dinosaur bones to this paradigm, they found that Spinosaurus and Baryonyx both had the type of dense bone associated with full submersion.
Meanwhile, the closely related African Suchomimus had hollower bones. 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 much.
“That was a bit of a surprise, because Baryonyx and Suchomimus look rather similar,” Dr. Ibrahim said.
But the team soon realized that it was not out of the ordinary and similar patterns can be seen in other groups
Other dinosaurs, like the giant long-necked sauropods also had some dense bones in their limbs, but this simply reflects the huge amount of stress on those limb bones.
“Some of these animals would have weighed as much as several elephants so adding extra load-bearing capacity to the bones makes a lot of sense!” Dr. Ibrahim said.
The findings were published in the journal Nature.
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M. Fabbri et al. Subaqueous foraging among carnivorous dinosaurs. Nature, published online March 23, 2022; doi: 10.1038/s41586-022-04528-0
Source link: https://www.sci.news/paleontology/spinosaurid-underwater-hunting-10649.html
