One of the first examples of dinosaurs turning into birds was Jeholornis, a raven-sized bird that lived 120 million years ago in what is now China. The layers of sediment that have been deposited over the years have left the fossils that have been discovered perfectly preserved yet flat.
As a result, nobody has been able to acquire a clear view of Jeholornis’s head. However, in a recent study, scientists digitally recreated a Jeholornis skull, providing information about the creature’s eyes and brain that illuminated its vision and sense of smell.
“Jeholornis is my favorite Cretaceous bird, it has a lot of unusual, primitive traits, and it helps shed light on the bigger story of how different birds evolved,” says Jingmai O’Connor, associate curator of fossil reptiles at the Field Museum and one of the authors of the paper describing the discovery in the Zoological Journal of the Linnean Society. “This study is the first time we’re really getting at what this bird’s skull looked like, what its brain must have been like, which is really exciting.”
The study’s first author, Han Hu, went through roughly 100 fossils at China’s Shandong Tianyu Museum of Nature and selected the one with the best-preserved skull still a little flattened, but intact.
“It is very difficult to find the right skull among around 100 fossils, since we won’t know if one skull will provide us the information we want before the scanning, and due to the costs of high-quality scanning, we couldn’t scan all those specimens to choose the best one. However, I chose this one because at least from the exposed surface, it is relatively complete, and which is also important is that this skull is preserved to be isolated from other parts of its body,” says Hu, a researcher at the Department of Earth Sciences, University of Oxford, UK.
“This is very helpful since we usually won’t chop the skull off from the skeleton if they are articulated no one wants to hurt these previous fossils, but an isolated skull will reduce the size of the scanning area, which will increase the scanning quality a lot. Luckily, the specimen we chose here for this project is nearly a perfect one it provided us so much unknown information after the digital reconstruction.”
Jeholornis’s brain morphology is transitional, in-between what we see in non-avian dinosaurs and what we see in modern birds. If you look at the skulls of dinosaurs, what you see is a spot for a very reptile-like brain, meaning that they have very large olfactory bulbs, and the optic lobes that are in the midbrain are reduced. They probably had a very good sense of smell and not great sight, which is very reptilian. And on the other hand, if you look at modern birds, they do the reverse. They have small olfactory bulbs, and very large optic lobes. Jeholornis falls in the middle.
Matteo Fabbri
“These bones were kind of like the bottom of a bag of potato chips they weren’t completely crushed, but the pieces were compacted,” says O’Connor. “So we were able to CT scan them essentially taking a bunch of X-rays and stacking them together to form a 3D image and then digitally re-articulate them and reconstruct the skull from all these bones.”
“We were able to see different features of the skull that had never been seen before in Jeholornis, and we were even able to extrapolate what its brain looked like,” says co-author and Field Museum postdoctoral researcher Matteo Fabbri.
While soft parts like the brain are rarely preserved, the brains of birds and dinosaurs frequently fit snugly inside their skulls. Therefore, knowing a fossil bird’s skull’s proportions and shape can reveal a lot about its brain, similar to how a glove can roughly approximate how a hand is shaped.
Additionally, olfactory bulbs and the cerebellum are found in the same approximate locations whether you’re looking at the brain of a frog, a human, or a fossil bird. These brain components remain preserved through species and over time.
Thanks to the long-standing placements of these structures, the researchers were able to determine how Jeholornis’s brain compares with modern birds and dinosaurs (or, strictly speaking, non-avian dinosaurs all birds, including Jeholornis, are dinosaurs, but not all dinosaurs are birds).
“Jeholornis’s brain morphology is transitional, in-between what we see in non-avian dinosaurs and what we see in modern birds,” says Fabbri. “If you look at the skulls of dinosaurs, what you see is a spot for a very reptile-like brain, meaning that they have very large olfactory bulbs, and the optic lobes that are in the midbrain are reduced. They probably had a very good sense of smell and not great sight, which is very reptilian. And on the other hand, if you look at modern birds, they do the reverse. They have small olfactory bulbs, and very large optic lobes. Jeholornis falls in the middle.”
Since Jeholornis’ olfactory bulbs were larger than those of most modern birds, it probably relied more on smell than do birds today (with the exception of a few keen-smellers, like vultures). In light of a recent study by the team that demonstrates Jeholornis is the earliest-known fruit-eating animal, Jeholornis’ keen sense of smell makes sense.
“As fruits ripen, they release lots of chemicals,” says O’Connor. “We can’t prove it yet, but having a better sense of smell might have helped Jeholornis find fruit.”
The researchers discovered that Jeholornis presumably had a brain that was better attuned to smelling during the day than at night. Scleral rings, which are bones, enable birds control how much light enters their eyes.
Birds that are active during the day have narrower holes for light to pass through, similar to the aperture on a camera, whereas species that need to see at night, like owls, have broader scleral ring openings relative to their eye sockets. The scleral rings of Jeholornis appear to suggest that it was most active during the day.
All of these skull features ladder up to a better understanding of this early bird’s lifestyle and the role it played in its ecosystem.
“Reconstructing a skull is painstaking work, and as people are starting to put in the time to do it, It’s becoming more and more clear that the evolution of birds was more complicated than what we expected,” says Fabbri. “It’s not just different from dinosaurs and modern birds, it’s different from other early birds too. It’s not a straightforward evolutionary story.”
“The same as Jingmai, Jeholornis is also one of my favorite birds. Its special position as one of the most primitive birds during the dinosaur-bird transition determines that completing its story will reveal the true scenery of that critical evolutionary period, and also, tell us why and how the modern birds the only living dinosaurs evolved to be what we see now.” says Hu.
