Spinosaurus hunted underwater thanks to the density of its bones

Writing Science, 23 Mar For decades, one of the most debated topics by paleontologists has been how Spinosaurus (one of the largest dinosaurs known) hunted. Now, an article in Nature approaches the subject from a totally different approach: bone density. After analyzing the bones of these carnivorous dinosaurs and comparing them with other animals such as penguins, hippos and alligators, scientists conclude that Spinosaurus, which lived in the mid-Cretaceous (112 to 93 million years ago) in North Africa and its close relative Baryonyx had sufficient bone density to dive under water and hunt. Scientists knew that spinosaurids had optimal characteristics for water, such as elongated jaws and cone-shaped teeth similar to those of predators that feed on fish. Over the past decade, paleontologist at the University of Portsmouth and National Geographic explorer Nizar Ibrahim has excavated fragments of a Spinosaurus skeleton in the Sahara desert. The skeleton had retracted nostrils, short hind legs, paddle-shaped feet and a fin-like tail: all signs that firmly pointed to an aquatic or “water-loving” lifestyle, as Ibrahim defined him. However, while it was clear to this paleontologist that this specialized anatomy allowed Spinosaurus to swim and chase its prey in the water, other scientists argued that this dinosaur was not a great swimmer, but rather hunted in the water like a giant heron. The debate led Matteo Fabbri, from the Field Museum in Chicago, and Ibrahim to lead an international team of researchers to try to clarify what the Spinosaurus lifestyle was. The team gathered a large data set of femur and rib cross sections of 250 species of live and extinct animals, terrestrial and aquatic, and 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 such as mosasaurs and plesiosaurs and compared cross-sections of their bones with cross-sections of bones of Spinosaurus and their relatives Baryonyx and Suchomimus. The researchers found a clear link between bone density and aquatic behavior: animals that dive underwater to find food have bones that are almost completely solid, while the cross-sections of the bones of those who hunt on land have hollow centers. Afterwards, they analyzed the bones of Spinosaurus and Baryonyx and saw that they had the type of dense bone associated with total immersion, while the African Suchomimus had hollower bones, lived in water and ate fish, but did not swim. Although for Ibrahim this finding was “a bit surprising” because Baryonyx and Suchomimus “look quite alike”, the team realized that it was not out of the ordinary and similar patterns can be seen in other groups. Other dinosaurs, such as giant long-necked sauropods, also had some dense bones in their limbs, but that only reflects the high amount of stress on those bones, according to the authors For Jingmai O'Connor of the Field Museum, collaborative studies like this one, based on hundreds of specimens, are “the future of paleontology. They are time-consuming, but they allow scientists to shed light on big patterns.” According to Ibrahim, this study shows that considering Spinosaurus “as a kind of giant mosquito” can be completely ruled out. “Bones don't lie, and we now know that even the internal architecture of bones is entirely consistent with our interpretation of this animal as a giant predator that hunts fish in vast rivers, using its paddle-shaped tail to propel itself. It will be interesting to reconstruct in much more detail how these river monsters moved, something we are already working on”, he concludes.