Exquisite bird fossil provides clues to the evolution of avian brains

An 80-million-year-old fossil bird has been discovered with a skull so exquisitely preserved that scientists have been able to study the detailed structure of its brain.

In both age and evolutionary development, the new species, named Navaornis hestiae, is almost midway between the earliest known bird-like dinosaur, Archaeopteryx, which lived 150 million years ago, and modern birds. It lived in the Cretaceous Period alongside dinosaurs such as Tyrannosaurus and Triceratops.

The fossil, which bears a superficial resemblance to a pigeon, was found near Presidente Prudente, Brazil, in 2016 and was immediately recognised as significant because of the rarity of a full bird skeleton, particularly one of that age.

But Daniel Field at the University of Cambridge says it wasn’t until 2022 that he and his colleagues realised the skull was so intact that they could possibly scan it and create a 3D model of its brain.

High-resolution CT scanning allows palaeontologists to peer inside fossils. “This involves careful ‘digital dissection’: separating out each individual component of the skull and then reassembling them into a complete, undeformed three-dimensional reconstruction,” says Field.

“The new fossil provides unprecedented insight into the pattern and timing by which the specialised features of the brain of living birds evolved.”

Based on the team’s reconstruction of the brain, Field says the cognitive abilities and flying capacity of Navaornis were probably inferior to those of most living birds.

The portions of the brain responsible for complex cognition and spatial orientation aren’t as enlarged as those of modern birds, he says.

“Although the cerebrum of Navaornis is greatly expanded relative to the condition in a more archaic bird relative like Archaeopteryx, it is not as expanded as what we see in living birds.”

The enlarged brains of modern birds support a huge range of complex behaviours, says Field, but understanding how their brains evolved has been challenging due to a lack of adequately complete and well-preserved fossil bird skulls from early bird relatives.

“Navaornis fills a roughly 70-million-year-long gap in our understanding of how the distinctive brains of modern birds evolved.”