Flight Lost, Ear Barely Changed: Did the Back of the Skull Shape the Avian Ear?

A team led by Guillermo Navalón of the University of Cambridge reports the findings in the Proceedings of the National Academy of Sciences. The researchers examined modern birds and species that became extinct only recently. For comparison, they included early bird relatives from the age of dinosaurs, other dinosaurs and pterosaurs.

Flight Lost 24 Times, Yet the Ear Barely Flinched

Bird lineages independently abandoned flight around 24 times. Losing flight transforms a bird’s life, yet its semicircular canals record the change only faintly. According to the study, the mode of locomotion has little effect on the shape of these tubes.

The loss of flight therefore does leave a trace. But the major remodelling comes from somewhere else.

For years, the equation seemed simple: if the tubes changed, locomotion had changed. Researchers often used precisely this argument to explain the origin of avian flight. That equation is now beginning to crack.

The Back of the Skull Tilted — and the Inner Ear Tilted with It

At the rear of the skull sits a single joint knob. It connects the head to the first neck vertebra and allows it to nod and turn. Dinosaurs, including birds, and crocodiles have only one such knob; mammals such as humans have two.

In most dinosaurs, this joint points backwards. When the back of the skull tilts downwards, the semicircular canals change markedly. The same pattern appears during the transition from dinosaur to bird.

The team interprets this as follows: the realigned head–neck joint reshaped the tubes. Under this interpretation, the balance system did not need fine-tuning for flight. The ear followed the head.

A Cretaceous Bird with a Giant Balance Organ

A discovery outside this study shows how difficult inner ears can be to interpret. Around 80 million years ago, Navaornis hestiae lived in what is now south-eastern Brazil. In a reconstruction of its world, the bird whips its head round as something rustles in the undergrowth, then freezes. Its skull looks modern but still carries ancient features, revealing an intermediate stage on the road towards the avian brain.

Navalón co-authored the 2024 description of the species in Nature. Its labyrinth, the balance organ inside the inner ear, is unusually large. It meant something, Navalón said at the time; researchers simply did not yet know what.

Amateur palaeontologist William Nava discovered the site in 2004. Researchers did not extract the crucial skull from the rock until 2015. That discovery alone showed how hard it is to read meaning from the shape of an inner ear.

Fossil Ears Reveal Less Than We Thought

A Yale University study had already shown in 2017 that the skull roof and brain changed in step along the road to birds. The bone plate above the forehead covers the front of the brain, while the plate behind it covers the middle. Evidence came from embryos of lizards, alligators and birds.

The new work fits this picture because the head was once again remodelled as a whole. At the same time, it challenges an old habit in palaeontology. Until now, the shape of fossilised semicircular canals was regarded as a reliable clue to how extinct animals moved.

This does not make every reconstruction based on the inner ear worthless. But anyone trying to read from fossil ears whether an animal flew must also read the architecture of the entire skull.

The birth of birds was also a remodelling of the head and neck. The balance organ sat at the heart of that transformation and changed shape as the back of the skull tilted downwards.

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