Therizinosaurus, Owner of the Biggest Claws in the Dinosaur World

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Let’s face it, there are some weird-looking dinosaurs out there, and one of the weirdest is Therizinosaurus. This dinosaur wins the record for longest claws ever with its 90-centimeter (three-foot) long, sickle-like appendages. They are part of a group of dinos called “maniraptorians,” which means “hand snatchers.” Other members of this group were thought to use their claws for hunting, defense, digging, and more. But Therizinosaurus and others in its family are a special case, and paleontologists are just now starting to discover clues as to what these immense claws were—and were not—used for.

Image: Matt Martyniuk, derived from Jaime Headden; Creative Commons 4.0 License

The first specimen of Therizinosaurus was discovered in Mongolia in 1948. With only a couple of ribs, its arms, and its sword-length claws, paleontologists weren’t quite sure what to make of it. It wasn’t until 1954 that Russian scientist Evgeny Maleev named the creature Therizinosaurus cheloniformis, or “tortoise-shaped scythe lizard.” Soon, however, scientists realized this wasn’t a lizard at all but a theropod dinosaur. Since the only theropods known up to that time were meat-eating predators, paleontologists assumed Therizinosaurus was also a carnivore and used its claws to hunt prey. Specifically, digging through termite mounds and anthills like an anteater would. Yet at 5000 kilograms (5.5 tons) and 10 meters (32.8 feet) long, how could this behemoth survive on such small food? Scientists were certain that such huge claws had to have been used to kill prey, yet how could this lumbering beast have hunted land prey efficiently? Perhaps, they thought, it used its claws to spear fish. With so few remains found over the years (mostly arms and claws), it proved immensely difficult for scientists to get a full picture.

In 1973, paleontologists made a discovery that changed everything. Scientists in Mongolia found Segnosaurus galbinensis, a smaller dinosaur with the same type of arms and claws as Therizinosaurus. The difference was that these fossil remains included part of a skull and some teeth. These teeth weren’t the point dagger-like teeth of a carnivore but the leaf-shaped teeth of an herbivore. Scientists were able to conclude that Therizinosaurus, Segnosaurus’s younger and larger cousin, was also actually a plant-eating theropod!

Okay, so that mystery was solved. But what about those claws? If Therizinosaurus didn’t use them to hunt, what did it use them for? What evolutionary purpose did they serve? Scientists have had to resort to speculation. Perhaps the dinosaur used them to tear down foliage or for self-defense. Mongolia was also home to the large meat-eating Tarbosaurus in the late Cretaceous, after all. While they thought some other maniraptorians used their claws in conjunction with their wing-like arms to climb trees, Therizinosaurus was the size of a T-Rex. If he tried that, the whole tree would just come down!

Image: Ghedoghedo; Creative Commons 3.0 License

Recently, scientists at the University of Bristol in the UK, as well as from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in Beijing, have worked to analyze the claws of several maniraptorians, including Therizinosaurus and its evolutionary cousins, to get a better picture of what their claws might be used for. They used a new biomechanics (a science that uses ideas from both science and engineering) method called functional space analysis to combine both two- and three-dimensional modeling in order to get a fuller context of how these long claws developed and how they were used across species.

They took data they collected from modeling dinosaur claws and compared it to data from existing creatures that we know a lot about in order to show how the forms and functions of similar claws might overlap. The structures of the claws showed that there were areas of high stress and areas of low stress, which allowed scientists to hypothesize the way those claws were used. If a certain claw showed high stress during digging, but low stress when piercing, then it can be assumed that the animal that owned that claw was a meat-piercing carnivore—if they tried digging with those claws, they would break, which would not make that creature very successful in evolutionary terms, if, say, they relied on eating ants to survive. Through this method, the scientists learned which creatures used their claws for digging, piercing, scratching, or a combination of things. But guess which dinosaur didn’t fit the pattern for any of these? You guessed it: our friend Therizinosaurus. The claws of this dinosaur showed a lot of stress doing, well, basically anything, which means that the claws weren’t used for digging, piercing, scratching, defense… you get the idea.

Image: Shuyang Zhou for the 3D modeling and functional scenario restoration of Therizinosaurus scratching. Red shows the most stress, blue the least.

“We conclude that the largest claws of any animal ever were actually useless in mechanical function, and so must have evolved under sexual selection to be used in display,” says Dr. Chun-Chi Liao, an expert on Therizinosaurs from IVPP who co-authored the paper based on this research. “The adult Therizinosaurus I guess could wave the claws at a competitor and effectively say, ‘look at me, back off’ or wave them around in some way like a peacock can use its tail in display to attract females for mating.”

So, basically, this study concludes that Therizinosaurus was the Johnny Bravo of the dinosaur world, a big lug who went around saying, “Man, I’m pretty.” But because of this study, scientists can look at the claws of other dinosaurs and pinpoint not only what their claws were used for, but in turn what those dinosaurs might have eaten and what their lives might have been like. This also contributes to our knowledge of what prehistoric ecosystems might have been like. When we combine different fields of study—in this case, biology and mechanical engineering—it can lead to interesting and eye-opening results.

Image: Johnson Mortimer, Creative Commons 3.0 License;

“Science and technology cannot bring dinosaurs back to life, but advanced computing and engineering techniques can show us how extinct animals lived,” says Dr. Emily Rayfield, who researches dinosaur biomechanics. “Especially for extinct animals like alvarezsaurs and therizinosaurs, they are so bizarre that we even can’t find any living animals like them. Luckily, advanced technology can help us to simulate, on a computer, the functioning of extinct animals using fundamental engineering and biomechanical principles. This study shows very well how selection for function can lead to the emergence of specific, sometimes very bizarre, forms.”

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