Long-necked, gigantic-clawed, feathered, herbivorous T. rex cousins, therizinosaurs were some of the weirdest creatures ever to walk the earth. A study published in PNAS earlier this week offers an explanation for how and why these beasts developed beaks.
A sketch of E. andrewsi courtesy of ArthurWeasley via wikimedia commons.
At the ends of their usually long necks, therizinosaurs had rather small heads. One particularly well-preserved skull of the therizinosaur Erlikosaurus andrewsi lacked teeth at the front of its jaw. This toothless portion seemed to suggest that Erlikosaurus likely featured a beak. (And a beak would not be surprising given that other dinosaur species from the same geologic era probably had beaks too — ceratopsians, for instance.) More specifically, Erlikosaurus‘s partial lack of teeth suggests that the toothless portion of the jaw was likely covered with a rhamphotheca, the keratinous sheath that makes up the exterior of most beaks. But what benefit would a beak bring to a bizarre creature like a therizinosaur?
University of Bristol paleontologist Emily Rayfield and some of her colleagues used the Erlikosaurus skull to answer that question. They scanned the skull to create a very precise digital model, then modified that model in different ways — adding a modest beak in one, adding a large beak in another, and adding teeth to the front of the jaw in a third. They then used the model in its original and modified forms to simulate the stresses caused by biting. They found that a beak would have helped Erlikosaurus harvest tough plants:
Evidence … suggests that the tip of the snout (i.e., edentulous
premaxilla plus overlying rhamphotheca) was used as the main
device to procure and process food. The presence of a keratinous
rhamphotheca in this region would have helped to dissipate and
absorb stress and strain while making the snout less susceptible
to bending and displacement. As an ever-growing material,
keratin has the advantage over bone that it is able to rapidly
repair fractures and has a slower crack propagation rate, thus
reducing the risk of constant damage.
So there’s the answer: beaks are useful for eating.
This might not seem surprising, but given some context it is — at least somewhat. Previously, it had been thought that beaks evolved as lighter alternatives to teeth, a feature valuable to flying animals or those in the process of developing flight. This new paper calls that view into question. (In fact, the paper notes that “a more extensive rhamphotheca would actually have increased the weight (mass) of the cranium” in Erlikosaurus.) A press-release quote from the paper’s lead author Stephan Lautenschlager sums the issue up nicely:
“It has classically been assumed that beaks evolved to replace teeth and thus save weight, as a requirement for the evolution of flight. Our results, however, indicate that keratin beaks were in fact beneficial to enhance the stability of the skull during biting and feeding.”
Of course that doesn’t mean beak necessarily evolved in birds for the same purpose, but it does add some complexity to the question of why beaks exist.