“Bird brain” insults be damned. The noggins of our flying friends are packed with neurons, and recent studies have shown birds can develop complex tools and even discriminate between paintings by Claude Monet and Pablo Picasso. But is this avian acumen a recent development, evolutionarily speaking, or does it trace back tens of millions of years?
A remarkably preserved fossil unearthed in Brazil may hold some answers. The 80-million-year-old bird skull contains impressions of advanced brain structures, suggesting early birds were bright like modern ones.
The preserved braincase, from a now-extinct bird lineage, is “exceptional … a big step forward,” says Matteo Fabbri, an evolutionary biologist at the Field Museum of Natural History who was not involved with the work. “This is the first time we have really good information regarding the brain of [this] group.”
Birds began to evolve about 165 million to 150 million years ago from dinosaurs. Some of the earliest—whose ancestors were carnivorous icons such as Velociraptor—were the famous feathered Archaeopteryx. Over time, avians branched into a group called the enantiornithines and close cousins who became modern birds.
Ranging from the size of hummingbirds to turkeys, enantiornithines took to the skies in the Mesozoic era beginning 130 million years ago. The creatures eventually spanned the globe before going extinct 66 million years ago from the same asteroid impact that killed off the dinosaurs. Their position between Archaeopteryx and living birds gives them a “magical place on the dino-bird family tree,” says Daniel Field, a paleontologist at the University of Cambridge and co-author of the new study.
To reconstruct the brains of ancient birds, researchers need fossils that preserve the hollow space where a brain would sit: the braincase. But no enantiornithine skeletons have preserved that space—until the new find.
In 2004, co-author William Nava, a paleontologist at the Marília Museum of Paleontology, unearthed an unusual 80-million-year-old fossil deposit near the city of Presidente Prudente in southeastern Brazil. From this urban bone bed, researchers dug up a 1-centimeter-wide partial skull embedded with other bones in a thick chunk of rock. They chiseled out a smaller piece containing the braincase and compared its shape with those of other known ancient birds, concluding it belonged to an enantiornithine.
When Field and fellow Cambridge paleobiologist Guillermo Navalón first saw the fossil, “we were almost in tears,” Navalón says. “This was a dream come true.”
In the lab, the researchers imaged the braincase—still embedded in rock—with a micro-CT scanner, which beamed x-rays at the fossil in thin slices. Software then virtually reconstructed the skull in the computer, so the scientists could view the interior of the braincase and model what the organ within would have looked like.
From the outside, the partial skull looked primitive, “like a little dinosaur,” says study author Luis Chiappe, a paleontologist at the Natural History Museum of Los Angeles County. But the reconstructed brain caught the researchers by surprise.
It was flexed: Rather than lying flat on one plane, it was rotated and curved like a C with the brainstem pointing downward. The large hole in the back of the skull connecting the head and brain to the neck and spinal cord pointed downward like in modern hummingbirds and hawks, rather than perpendicularly out of the back of the brain like in ancient Archaeopteryx and closely related dinosaurs.
Flexing would have allowed the enantiornithine to smush more relative brain volume into its tiny skull than did its dino cousins. The enantiornithine also had a large and twisted inner ear structure, which helps maintain balance, the team reports today in the Proceedings of the Royal Society B. Some research in live birds suggests this inner ear arrangement boosts flying skills, but Chiappe cautions that direct links between shape and function are not yet clear. The fossil is now housed in the Marília Museum of Paleontology.
The find shows a flexed brain and large inner ear evolved earlier than thought, Fabbri says. But that’s not the end of the story. Some modern birds lack those features, so the question remains whether the newfangled noggins evolved once a long time ago and then were later lost in some groups, or evolved twice: once in enantiornithines and again in some modern birds.
The best way to find out is to discover more 3D braincases, Fabbri says. The team says it’s on it. “We’ve found more incredible cranial remains,” at the Presidente Prudente site, Navalón says. “This is an appetizer.”
Source: Science Mag