Baby faces are key to bird success

15 September 2017

Bird skulls and brains resemble those of young dinosaurs and may provide insight into their evolutionary origin and modern day success, according to researchers at Yale University and Imperial College London.

Birds are the only surviving members of a group of feathered dinosaurs, and are close cousins to modern crocodilians. However, modern birds look rather different from their reptilian relatives. As well as the ability to fly and having a toothless beak in place of a long snout, birds also have remarkably different skull shapes to their crocodilian cousins. Similar to non-avian dinosaurs, reptiles have flat skulls containing a relatively flat brain, in contrast, birds have large, domed skulls encasing a relatively large brain.

Arkhat Abzhanov, researcher at Imperial College London, is particularly interested in how this unique skull and brain structure can provide clues about the evolution of birds from their dinosaur ancestors.

Abzhanov and colleagues from Yale University used a 3D scanning technique to analyse the skull and brain shapes of a wide range of reptiles, including dinosaur fossils, as well as skulls of embryos and juveniles of modern species.

The results, published in Nature Ecology and Evolution, show that birds have a key developmental difference from their ancestors and modern-day relatives. In the early developmental stages of most vertebrates, certain parts of the skull correspond to specific regions of the brain. As the animal matures, this association is lost and the shape of the brain no longer mirrors the structure of the skull. However, in modern birds this association continues into adulthood, meaning that birds retain an infant-like brain and skull structure.

This is a striking example of an evolutionary phenomenon known as ‘paedomorphism’, where adults retain features that are usually only seen at juvenile stages in their ancestors.

So what does this tell us about the evolutionary origin of modern birds?

Were there advantages to being a baby-faced dinosaur in a Jurassic world?

The eye sockets and eyes in birds are relatively much bigger than those in dinosaurs and other reptiles. The midbrain is also much larger and is an important centre for tracking movement during flight. These features were present in the earliest birds, including the Archeopteryx, which evolved alongside the last dinosaurs.

Larger eyes and brains provide a recipe for evolutionary success as better eyesight and brainpower meant they could exploit resources unavailable to their more mature looking relatives, and birds are clearly still doing well today.

Abzhanov explains, ‘Birds are incredibly successful organisms and have diversified into thousands of species across the world. We think their paedomorphism contributed to this success’.

The team will now begin analysing the genetic and developmental processes underlying paedomorphism to better understand how this unique adaptation evolved.
 

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