Exhibit Spotlight: Spinosaurus aegypticus
A unique physical profile gave Spinosaurus an early edge in the popular imagination despite the fact that for nearly a century we knew very little about it. Discoveries during the past 32 years have slowly clarified our understanding of this bizarre animal, and every little bit that we learn serves to deepen its mystique.
Ernst Stromer announced the discovery of Spinosaurus in 1915, ten years after Tyrannosaurus rex hit the scientific journals. He based the first description of this animal on a partial skeleton that may have belonged to a juvenile. The skeleton included a dentary (The tooth-bearing bone in the lower jaw) and several vertebrae with unusually long spines. The expedition uncovered several other dinosaurs, including a sauropod named Aegyptosaurus and two other theropods—Carcharodontosaurus and Bahariasaurus. Stromer had the bones transported from Egypt to Munich, where in 1944 they were destroyed during an Allied bombing raid. For 70 years our only information about this animal came from detailed drawings and descriptions of the bones.
Stromer also described another specimen from Egypt in 1934. He provisionally designated it “Spinosaurus B,” although he considered it likely to belong to at least a different species from the first specimen. Also destroyed during the Allied bombing raid, its identity remains unresolved. Possible identities forwarded by scientists include Carcharodontosaurus and Sigilmassasaurus (a mysterious cousin of Spinosaurus, if it existed at all). Unfortunately, the geology of many north African sites, the current geopolitical situation of the region, interference from black market dealers, and the difficulty of keeping the sort of precise records necessary for good scientific interpretation all combine against our ability to make sense of these taxa. Sorting out the relationships and ecology of north African dinosaurs, especially theropods, will likely take decades of perseverance before we sort things out.
With so little information, most early illustrations of this animal looked like a generic large carnivorous dinosaur with a Dimetrodon-like fin slapped on its back.
Our image of Spinosaurus began to change with the discovery of Baryonyx in 1986. Though scientists could not make the connection between these two species at first, they gradually began to recognize their similarities—a process hampered by the destruction of the original Spinosaurus specimens. Even today, most of what we know about Spinosaurus relies on its much more complete British predecessor.
In 2014, a team of scientists led by Nizar Ibrahim announced the discovery of a new, more complete Spinosaurus specimen which included a nearly complete hind limb. In order to create a full picture of what Spinosaurus looked like, they gathered bones from across northern Africa and reassigned them to Spinosaurus. They then re-scaled the bones in order to create a composite skeleton made up of elements from many individuals. Though composite skeletons are often an unfortunate necessity in depicting the form of the animal, they inevitably distort our idea of the complete animal. Sometimes those distortions only subtly affect the new, hypothetical whole; those incorporated into Spinosaurus have proven more controversial. Only further studies and new discoveries can clarify the accuracy of Ibrahim and company’s model.
Based on the leg’s proportions compared to associated backbones and on calculations of the reconstruction’s center of gravity, Ibrahim et al’s 2014 study controversially concluded that Spinosaurus walked on all fours. This would be unprecedented for a theropod dinosaur and requires extraordinary evidence to support it. Unfortunately, no forelimb material for this animal has been confirmed, and to date the forelimb material they assigned to Spinosaurus has not been analyzed for its mobility, strength, or fitness in supporting the body or a quadrupedal gait. So at this point we can’t know if it had the adaptations or even the proper joints to get around on all fours. As it stands, theropod arms don’t have the right joints to move in a striding motion—they’re mainly designed to pull things toward the chest regardless of the animal’s size.
The habitual posture of the head could also play a role in how the animal balanced on land. If it kept its nose pointed down and retracted its head up and back toward its sail, that would affect its balance significantly. We could test Spinosaurus’ habitual posture by examining its inner ear, but again, we don’t have confirmed parts from that region of the skull.
One significant discovery from this new skeleton deals with the construction of the limb bones. Unlike typical theropod bones, which were hollow like birds' even in large species, Spinosaurus bones were nearly solid. Measurements of their density compared favorably with whale bones, which suggests they acted like ballast to help the animal better control itself in water. This corroborates the findings of another study that measured isotopes in spinosaurid bones. These isotope patterns match those of crocodiles and water-dwelling turtles. Overall, we have a strong case based on several lines of evidence that spinosaurs spent much of their time in the water, snacking on sashimi. This also explains why Spinosaurus grew bigger than other theropod giants like Carcharodontosaurus, Giganotosaurus, and Tyrannosaurus rex: it cheated. Water buoyancy let it exceed the normal size ceiling for theropods, just as it helps whales grow bigger than any known land animal.
Evers, S. W., Rauhut, O. W., Milner, A. C., McFeeters, B., & Allain, R. (2015). A reappraisal of the morphology and systematic position of the theropod dinosaur Sigilmassasaurus from the “middle” Cretaceous of Morocco. PeerJ, 3, e1323.
Hendrickx, C., Mateus, O., & Buffetaut, E. (2016). Morphofunctional Analysis of the Quadrate of Spinosauridae (Dinosauria: Theropoda) and the Presence of Spinosaurus and a Second Spinosaurine Taxon in the Cenomanian of North Africa. PloS one, 11(1), e0144695.