The evolutionary line from theropod dinosaurs is absolute. There is no question that this is one of the greatest stories that life on Earth has ever told us, But evolution is not linear; it’s chaotic. It’s bizarre. Along this theropod line, dinosaurs were experimenting – they were the evolutionary scientists of their time.
One of the weirdest things that theropods did was become herbivores again – this is exceptionally odd when we consider that they’re the cousins of epic carnivores like Tyrannosaurus and Deinonychus.
Therizinosaurs were one of these groups of hipster dinosaurs. Not content with a life dining on raw steak, they actually estranged themselves and became ecological innovators, eventually turning herbivorous.
But how did this happen? Was morphological change a driver of dietary change, or vice versa?
One of the most notable aspects of change is the reduction in teeth in these dinosaurs, coincident with the development of a horny beak covering the tip of the snout, called a rhamphotheca. This is quite similar to modern birds, which have beaks instead of teeth, with serrated edges giving what is known as an ‘edentulous’ appearance. Sort of like pseudo-teeth.
Modern palaeo-techniques provide cool opportunities to explore the evolutionary significance of these bizarre adaptations in therizinosaurs. A new study by the Bristol palaeo research team used a method called CT-scanning, usually associated with medicine, to digitally reconstruct the skull of the therizinosaur Erlikosaurus andrewsi. Then, using a pretty ace technique called finite-element analysis, they were able to investigate the biomechanical functions of the skull, even using restored soft tissues.
A series of these digital models were examined, using different reconstructions of the rhamphotheca and different hypothetical biting scenarios. The most significant find was that mechanical stress increases the further bites are simulated towards the back of the mouth.
This, combined with other lines of evidence suggests that the rhampotheca may have developed in a stress-limiting role. For example, there are no wear facets on Erlikosaurus’ teeth, usually indicative of food processing; low tooth replacement rates suggesting that they were not used as much; and that the upper and lower jaw did not occlude (like we have to increase chewing efficiency).
In a broader evolutionary perspective, the development of the rhampotheca and herbivory has some interesting implications. Birds are thought to have developed their keratinous beaks in a weight-saving role to aid flight. Therizinosaurs are within the same group, Maniraptoriformes, as birds, or more specifically the group Avialae. As such, it appears that dinosaurs were highly experimental with beaks and edentulism around this evolutionary period, and diversified into a range of functions.
As to whether this morphological adaptation was the driver of a herbivorous lifestyle, or vice versa, well, the jury is still out on that one.
Lautenschlager, S. et al. (2014) Edentulism, beaks and biomechanical innovations in the evolution of theropod dinosaurs, Proceedings of the National Academy of Sciences (open access link)