When considering how dinosaurs evolved into birds, many focus on how a body covered in scales could develop into a feather coating. But this overlooks another major feature of birds – their beak. How did a group of animals famous for their toothy grins evolve a toothless bill? Well, researchers think they might have the answer.
The scientists studied a series of dinosaur and early bird fossils to see if they could identify the changes to their gnashers that led to a loss of teeth and formation of a beak. Published this week in PNAS, the study discovered that initially, it was the older animals that lost their teeth and grew a small beak, before the process started occurring earlier and earlier and chicks hatched with no teeth.
It turns out that neither losing teeth nor growing a hard keratinous beak was an unusual thing for dinosaurs to do. At least seven separate lineages are known to have done away with their fangs, while many different dinosaurs had beaks, though perhaps not as we tend to think of them today. We imagine a beak to extend from the tip of the snout to the eyes, but many toothed dinosaurs had a far simpler beak sitting at the front of their snout.
The oldest recognizably bird-like creatures from the late Jurassic period, such as Archaeopteryx, and the early Cretaceous, had wings and feathers, but also a gob full of teeth. Yet others from the same period, such as Confuciusornis, had already lost them.
To see how a lack of teeth may have developed, the researchers looked at the internal structure of jaw bones from a theropod dinosaur called Limusaurus, which was closely related to the ancestors of birds, as well as Sapeornis, a bird from the early Cretaceous period. By studying tissues inside the jaws, the team could see if the structure supported teeth or not.
They found that in both species, the young hatched with teeth, but lost them as they then grew older, with the adult Limusaurus being completely toothless, and old Sapeornis only retaining teeth towards the back of its mouth. The authors argue that the mechanism that regulates beak growth, which initiated in many dinosaurs at the front of the snout, also inhibits the growth of teeth.
This, they suggest, means that over years of evolution, birds lost their teeth earlier and earlier until their young began hatching with fully formed beaks in place. Amazingly, this fits with modern animals, in which toothlessness is still linked to growing a keratinous beak.