Researchers at the University of Bonn have found evidence indicating that some Sauropod dinosaurs, typically known for their enormous size, were island dwellers and evolved into dwarfs.

By studying the structure of their fossils, researchers confirmed that the sauropod dinosaur Magyarosaurus dacus never grew any larger than a horse. The results appear this week in PNAS.

The M. dacus bones were originally discovered in 1895 in Transylvania and interpreted as the remains of dwarfed animals that had once lived on an island. But over the years, palaeontologists debated the question of whether or not the Magyarosaurus was a dwarf. Martin Sander, spokesperson of the Research Group on Sauropod Biology notes, "an animal the size of a horse may not seem like a dwarf to most people but, in sauropod terms, it's tiny!"

M. dacus

(Photo credit: University of Bonn)

When Magyarosaurus was discovered in Transylvania (then part of the Austro-Hungarian Empire), the palaeontologist Franz Baron Nopcsa advanced the idea that Magyarosaurus was an island dwarf, but he could not prove it back then, at the beginning of the 20th century. Many discoveries have since indicated that his theory might be correct, especially the fossils of dwarf elephants and hippopotamuses found on Mediterranean islands like Sicily, Malta and Cyprus.

However, scientists first pursued a different theory. For in the subsequent decades, other researchers found big sauropod bones on the Transylvanian site. They therefore concluded that Magyarosaurus was simply a youngster, while the larger bones came from fully grown adults.

But the Bonn Team decided to cut up the fossil bones of the dwarfed dinosaur and study their microstructure. "It's astonishing that the microanatomy of these bones has been preserved for us to study after 70 million years," says Koen Stein, who carried out the research as part of his PhD studies.

"Bone is a living tissue, and throughout an animal's life it is constantly dissipating and building up again." Humans, for example, have completely resorbed and rebuilt their skeleton by the time they are fully grown. This also occurred in sauropod dinosaurs. "We were able to distinguish these rebuilding features in Magyarosaurus, which prove that the little dinosaur was fully grown," Stein explains.

The new study provides conclusive evidence that Nopcsa's hunch had been right all along. "Our study shows that dinosaurs on islands were subject to the same ecological and evolutionary processes that shape modern mammals," explains Sander. "We were also able to demonstrate that the bigger bones found in that area belong to a different dinosaur species." Whether they come from stray animals who swam to the island from the mainland, or from large ancestors of the dwarf Magyarosaurus, remains unknown.

Citation: Stein et al., 'Small body size and extreme cortical bone remodeling indicate phyletic dwarfism in Magyarosaurus dacus (Sauropoda: Titanosauria)', PNAS, May 2010; doi: 10.1073/pnas.1000781107