![Artistic reconstruction of Jeholornis in life. Credit: Michael Rothman Reconstruction of Fossil Bird Skull Reveals Brain Made for Feeling and Eyes Made for Daylight](https://oponame.com/wp-content/uploads/2022/10/Reconstruction-of-Fossil-Bird-Skull-Reveals-Brain-Made-for-Feeling.jpg)
Artistic reconstruction of Jeholornis in life. Credit: Michael Rothman
Jeholornis was a crow-sized bird that lived 120 million years ago, among the earliest examples of dinosaurs evolving into birds, in what is now China. The fossils that have been found are finely preserved but crushed flat, the result of layers of sediment laid down over the years. This means that no one was able to see Jeholornis’s head well. But in a new study, researchers have digitally reconstructed a Jeholornis skull, revealing details about its eyes and brain that shed light on its vision and sense of smell.
“Jeholornis is my favorite Cretaceous bird, it has a lot of unusual and primitive traits, and it helps shed light on the larger evolutionary history of different birds,” says Jingmai O’Connor, associate curator of reptiles fossils at the Field Museum and one of the authors of the article describing the discovery in the Linnean Society Zoological Journal. “This study is the first time we’ve really found out what this bird’s skull looked like, what its brain must have looked like, which is really exciting.”
The study’s first author, Han Hu, examined about 100 fossils at the Shandong Tianyu Museum of Nature in China and selected the one with the best-preserved skull, still somewhat flattened, but intact. “It is very difficult to find the right skull among a hundred fossils, because we will not know if a skull will give us the information we want before scanning, and due to the costs of a high quality scan, we do not ‘ve been unable to scan all of these specimens to choose the best one. However, I chose this one because at least from the exposed surface it is relatively complete, and what is also important is that this skull is kept to be isolated from other parts of its body,” says Hu, a researcher in the Department of Earth Sciences at the University of Oxford, UK.
“This is very useful as we generally won’t cut the skull off the skeleton if they are jointed – no one wants to hurt those previous fossils, but an isolated skull will reduce the size of the scan area, which will increase the scan. Fortunately, the specimen we have chosen here for this project is near perfect – it has provided us with so much unknown information after digital reconstruction.
“These bones were a bit like the bottom of a bag of crisps – they weren’t completely crushed, but the pieces were compacted together,” O’Connor says. “So we were able to scan them, basically 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 those bones.”
![Reconstruction of the skull of Jeholornis, showing the bony rings around its eye. Credit: Han Hu et al Reconstruction of Fossil Bird Skull Reveals Brain Made for Feeling and Eyes Made for Daylight](https://oponame.com/wp-content/uploads/2022/10/1666648299_793_Reconstruction-of-Fossil-Bird-Skull-Reveals-Brain-Made-for-Feeling.jpg)
Reconstruction of the skull of Jeholornis, showing the bony rings around its eye. Credit: Han Hu et al
“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 his brain looked like,” says Matteo Fabbri, co-author and postdoctoral researcher at the Field Museum.
The brain itself is not preserved – soft tissues rarely are – but the brains of birds and dinosaurs tend to nest perfectly within their skulls. Knowing the shape and dimensions of a fossil bird’s skull therefore tells us a lot about its brain, much like how a glove gives a decent approximation of the shape of a hand. Additionally, brain structures are conserved across species and over time – things like the olfactory bulbs and the cerebellum in the same general locations whether you look at the brain of a frog, a human, or a fossil bird.
Thanks to the longstanding placements of these structures, the researchers were able to determine how Jeholornis’ brain compares to modern birds and dinosaurs (or, strictly speaking, non-avian dinosaurs; all birds, including Jeholornis, are dinosaurs , but not all dinosaurs are birds).
“The brain morphology of Jeholornis is transitional, between what we see in non-avian dinosaurs and what we see in modern birds,” says Fabbri. “If you look at dinosaur skulls, what you see is a stain for a very reptile-like brain, which means 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 eyesight, which is very reptilian. And on the other hand, if you look at modern birds, they do the opposite. They have small olfactory bulbs, and very large lobes optics. Jeholornis falls in the middle.”
![3D reconstruction of the brain of Jeholornis. Credit: Han Hu et al Reconstruction of Fossil Bird Skull Reveals Brain Made for Feeling and Eyes Made for Daylight](https://oponame.com/wp-content/uploads/2022/10/1666648299_627_Reconstruction-of-Fossil-Bird-Skull-Reveals-Brain-Made-for-Feeling.jpg)
3D reconstruction of the brain of Jeholornis. Credit: Han Hu et al
Jeholornis had larger olfactory bulbs than most modern birds, which means it probably relied more on its sense of smell than birds do today (apart from a few odorants, such as vultures). Jeholornis’ strong sense of smell comes into its own in the context of another recent study by the team, showing that Jeholornis is the oldest known fruit-eating animal. “As the fruit ripens, it releases a lot of chemicals,” says O’Connor. “We can’t prove it yet, but having a better sense of smell might have helped Jeholornis find fruit.”
In addition to a brain adapted to smell, the researchers found that Jeholornis was likely better able to see day than night. Birds have bones called scleral rings that help determine the amount of light that enters their eyes. Species that need to see at night, such as owls, have wider scleral ring openings relative to their eye sockets, to let in more light; birds that are active during the day have narrower apertures to let in light, like the aperture in a camera. Jeholornis’ scleral rings seem to indicate that he was most active during the day.
All of these features of the skull help to better understand the lifestyle of this early bird and the role it played in its ecosystem. “Rebuilding a skull is painstaking work, and as people start to spend time on it, it’s becoming increasingly clear that the evolution of birds has been more complicated than we expected,” says Fabbri. “It’s not just different from modern dinosaurs and birds, it’s also different from other early birds. It’s not a simple evolutionary story.”
“Like 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 the completion of its story will reveal the true landscape of this period of critical evolution, and also, tell us why and how modern birds – the only living dinosaurs – evolved into what we see now,” says Hu.
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Han Hu et al, Cranial osteology and paleobiology of the early Cretaceous bird Jeholornis prima (Aves: Jeholornithiformes), Linnean Society Zoological Journal (2022). DOI: 10.1093/zoolinnean/zlac089
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