Researchers have uncovered the skull of a previously unknown starling-sized bird species called Navaornis hestiae. This skull was so well preserved that they were able to digitally reconstruct its brain and inner ear anatomy based on the shape of the brain body. This bird species lived in an arid environment approximately 80 million years ago, during the Cretaceous Period, the last part of the age of dinosaurs.
“This finding is one of a kind,” says University of Cambridge paleontologist Guillermo Navalón, lead author of the study published in the journal Nature.
Birds evolved from small feathered dinosaurs during the Jurassic Period. The Navaornis discovery filled a 70-million-year gap in the understanding of the evolution of avian neuroanatomy, dating back to the oldest known bird, Archeopteryx, which lived in Europe about 150 million years ago.
Featuring modern geometry in terms of its beak shape and large eye sockets, the Navaornis skull superficially resembles that of a small pigeon. His brain displayed a mosaic of modern and archaic features and some in between.
“This is long-sought evidence because well-preserved three-dimensional skulls of early birds flying over the heads of dinosaurs are extremely rare, and this is the best preserved yet,” said Luis Chiappe, a Los Angeles Museum of Natural History paleontologist and lead author of the study.
Daniel Field, a University of Cambridge paleontologist and senior author of the study, said: “Scientists have struggled to understand how and when birds’ unique brains and extraordinary intelligence evolved. “The field was waiting for a fossil exactly like this to be discovered,” he said.
The Navaornis brain—about four-tenths of an inch (10 mm) across—was smaller relative to skull size than modern birds, but larger and more complex than Archeopteryx.
The cerebellum, a brain structure in living birds that helps coordinate motor control during flight, was smaller than in modern-day bird species and more similar to that of Archeopteryx. But its brain, unlike Archeopteryx and the dinosaurs from which birds evolved, was connected to the spinal cord in a way similar to modern birds — and humans, for that matter.
It had some features intermediate between Archeopteryx and modern birds. The moderate size and shape of the cerebrum, a structure that contains areas involved in complex cognition in modern-day birds, suggests that it was more cognitively advanced than the earliest birds but less developed than modern-day birds.
GAP IN THE FOSSIL RECORD
Navaornis exhibits some unique features, such as the fact that its vestibular apparatus, the balance organ in the inner ear, is larger than in other known birds.
“There was a significant gap (in the fossil record) between birds that had a more dinosaur-like brain, such as Archeopteryx, and those that were very closely related to modern birds,” Chiappe said. “The new evidence documents an intermediate stage in the evolution of the brain, but with some unexpected specializations that may be related to functional traits such as flight.”
There are very few well-preserved fossils from the early stages of bird evolution. The fragility of bird bones makes their fossils rare. This skull was preserved three-dimensionally, rather than being crushed flat like many fossils. The fossil, which contains 80 percent of the bird’s skeleton, shows that it was a competent flyer.
Its name means “bird of Nava” and is named after scientist William Nava, who discovered the fossil in 2016 in the southeastern Brazilian state of Sao Paulo.
Navaornis belongs to a group of birds called enantiornithines that evolved during the Cretaceous period. However, it was wiped out 66 million years ago by the asteroid impact that put an end to the dinosaurs, but preserved the bird lineage that is still developing today. This means that Navaornis was not an ancestor of today’s birds and that its modern-looking features evolved separately from them.
The fact that its beak is thin and delicate shows that it feeds on insects and seeds that it can swallow whole. It lived alongside giant long-necked plant-eating dinosaurs and large meat-eating dinosaurs.
