"The Therizinosaurs are still an enigmatic group of dinosaurs, " explains Stephan Lautenschlager from the University of Bristol. These lizards lived about 145 to 66 million years ago in what is now North America and Asia. Fossils show that her physique was more like the peaceful herbivore: they had a rather clumsy, broad-hulled body, a long neck and small head with small pointed teeth in the mouth. One feature, however, does not fit into the picture: all wore well-defined, sharp claws on their forelegs - actually something that is known from a predator like the Tyrannosaurus rex and others. In some early forms of therizinosaurs they were rather short, strong and strongly bent, in others longer and straighter. Most striking are the claws of the Therizinosaurus cheloniformes: they reached almost a meter in length. When explorers encountered some of these fossil giant claws in the 1950s, they first thought they were the ribs of extinct giant tortoises.
Digging, fighting, foraging?
What the Therizinosaurier their claws use, is still unclear. For carnivores they were according to current knowledge rather not. "They could have searched for food by digging nests of social insects - like anteaters or the extinct giant sloth Megatherium, " explains Lautenschlager. "But with their claws, they could have torn up the bark of trees or drawn down plant food." But it would be equally possible that the therizinosaurs used their claws only for display purposes or for rank or mating battles within their species. Even the defense could have served the fearsome giant claws - clear clues to their purpose were missing so far, according to Lautenschläger.
In order to solve the mystery of the Therizinosaurier claws, Lautenschlager analyzed and compared the claw shape and size of 65 dinosaurs of the close and further relationship of the Therizinosaurier. He also added the claws of 46 mammal species. In addition, he created computer models of the claws of five different Therizinosauriern and subjected them to a stress test. The claws had three different functions in a simulation: digging and scratching in the ground, pulling and drilling through an object. In each scenario, Lautenschlager determined the forces that worked on the claw and tested whether they could withstand it.
No robber or fighter
The result: Obviously, there was not the one, only purpose for the claws of Therizinosaurier, as Lautenschlager reported. Because only the variety of claw forms within this dinosaur group makes this extremely unlikely. Thus the Therizinosaurier Alxasaurus possessed rather short, strong claws, which withstood the forces with the ditch very well in the stress test. The longer claws of most other Therizinosaurier but were rather unsuitable for digging: they showed on their top clear traces of increased stress. The giant claws of the Therizinosaurus cheloniformes cut off relatively well during drilling and pulling. Theoretically, the dinosaur could have used it well for pulling leaves and branches. Strangely enough, however, that his neck was actually long enough to reach this food with his mouth, as the researchers explained. display
"None of the previously considered functions can be clearly confirmed or refuted, " says Lautenschlager. Instead, the results suggest that the therizinosaurs used their claws quite differently depending on their shape. However: "In contrast to the representation often found in popular culture, there is no evidence that Therizinosaurus or other members of this group used their claws in a defensive or aggressive context, " emphasizes Lautenschlager. A dangerous robber or fighter Therizinosaurus was therefore probably not despite his fearsome claws. But why he ever developed it and what he used it remains - still - his secret.
- Stephan Lautenschlager (University of Bristol), Proceedings of the Royal Society B: Biological Sciences, doi: 10.1098 / rspb.2014.0497