A new paper published this week in the journal Evolution details how a group of scientists led by Arhat Abzhanov of Harvard and Bhart-Anjan Bhullar of the University of Chicago have unraveled the genetic changes that transformed the skull of a theropod dinosaur into the unique beak of modern birds.
The study integrated elements of paleontology, comparative anatomy and developmental biology. They began by making an extensive comparative study of how the bones in the skull develop via both the fossil record and examination of modern birds, crocodilians, lizards and turtles.
The upper beak is formed from the premaxillary bones of the jaw. In most vertebrates, these bones make up the central part of the snout. In humans, they carry the upper incisors. The earliest known bird with a full fledged beak is Confuciusornis from the early Cretaceous of China, roughly 125-120 Mya. The premaxilla bones make up roughly one-third of it’s skull and it has no teeth. By comparison, in Archaeopteryx, which is around 150 Mya, the premaxilla makes up one-fifth of the skull and retains it’s teeth. See this paper for more on the comparison of Archaeopteryx and Confuciusornis.
The researchers then examined prior studies to identify genes which control the development in these bones. They found two proteins, Fgf8 and WNT, which showed significant differences in their activity in birds versus reptiles so these were chosen for the next phase of their research.
Inhibitors were introduced into developing chicken eggs to block the activity of these two proteins. The skulls were then examined just prior to the embryos hatching stage. No dino-chickens were actually born. They obtained a range of results. Some showed virtually no difference compared to normal chickens, while in those with the heaviest doses of inhibitors, the premaxillae never fused into a beak and looked somewhat similar to those found in dinosaurs. The illustration above is a digital model with the chicken at the left, a crocodile at the right and the result of the experiment in the middle.
The team was surprised to find that the palate had also changed significantly. In a chicken, like most birds, the palate has been greatly reduced and disconnected from the neighboring bones to save weight and allow more movement of the beak. In some of the embryos, the palate was more like typical vertebrates, flat and connected the other bones.
Since the experiments focused on the proteins, the actual genetics involved are still unknown. Earlier studies have showed that the sonic hedgehog gene is involved in beak development, but, really sonic hedgehog is heavily involved in development generally.
The paleontologist Jack Horner has famously been working on creating a “devolved” chickenosaurus through genetic engineering. This finding may give him another step towards that goal. I’m not as enthusiastic about this idea as I am about de-extincting creatures like mammoths since the result will be something brand new and be exactly the kind of “mad scientists create a monster” situation that feeds right into the worst stereotypes of genetic research.