Perhaps he could smell his ripe food from afar: The Cretaceous bird Jeholornis, which is considered the earliest known fruit eater, had a comparatively large olfactory bulb in its brain. This is reflected in the three-dimensional reconstruction of its crushed skull, report paleontologists. The characteristics of the bone structures around the eyes also indicate that Jeholornis was probably not traveling in the twilight. This confirms the assumption that the early representatives of the birds were diurnal.
They are considered the only surviving descendants of the dinosaurs and gave birth to thousands of species: the evolutionary history of birds is one of the most exciting topics in paleontology. One of the earliest examples of a representative of the transition from dinosaurs to birds is a creature about the size of a raven that fluttered through what is now China about 120 million years ago. Only recently has Jeholornis been in the spotlight: the research team led by Han Hu from the University of Oxford presented it as the oldest known fruit-eating animal. This diet was reflected in the fossil stomach contents and features of the beak structures.
In their new study, the scientists now shed more light on the question of how the skull and thus also the brain and the eyes of the animal were made. One problem so far, they explain, has been that while numerous well-preserved Jeholornis fossils have been found, the skull has always consisted of flattened fragments, leaving the original 3D structures of the head unclear. "These bones form something like what you'd find at the bottom of a bag of potato chips," explains co-author Jingmai O'Connor of the Field Museum in Chicago.
Reconstructed from flattened pieces
However, as the paleontologists report, they have succeeded in creating a detailed digital reconstruction of a Jeholornis skull from such remains. To do this, they performed elaborate CT scans on a fossil that were able to reveal hidden structures. "It was difficult to choose the right skull for the expensive procedure from among the 100 or so fossils because we didn't know before scanning whether a specimen would give us the information we wanted," says Hu. "In addition to completeness, it is important that the skull is preserved in such a way that it can be isolated from other body parts. Fortunately, the selected specimen turned out to be almost perfect,” says the researcher. O'Connor continues, "It made it possible to capture the pieces with the CT scanner and stitch them together into a 3D image."
Eventually, a detailed computer model of the skull as it once appeared uncrushed appeared on the computer screen. "It enabled us to discern various features of Jeholornis that were previously unknown. Among other things, this allowed us to extrapolate what his brain was like,” says co-author Matteo Fabbri from the Field Museum. Because, as the researchers explain, the forms of the long-gone nerve structure are reflected in the characteristics of the brain capsule. Many basic parts of the brain have remained structured in the same way across species and time: functional units are located in the same places, regardless of whether you look at the brain of a frog, a human or a fossil bird. The researchers were now also able to make comparisons between the structures in Jeholornis and known characteristics in other animals.
Olfactory bulbs and scleral rings in sight
Finally, the researchers focused on a special part of the brain: the so-called olfactory bulb, which is responsible for processing olfactory information. It became apparent that Jeholornis possessed a larger olfactory bulb than most modern birds. It is only slightly larger in extremely sensitive birds such as vultures, which can smell carrion from afar. The expression in Jeholornis suggests that the sense of smell also played a comparatively important role in the life of this animal. Coming full circle with previous study results showing that Jeholornis ate fruit, the researchers say: “When fruit ripens, it releases a lot of volatile substances. So a good sense of smell could have helped Jeholornis find fruit,” says O'Connor.
Another conclusion that emerges from the skull reconstruction concerns the vision of the primeval bird, the paleontologists report. Birds have specific bone structures, they explain, called scleral rings, which, like the aperture on a camera, determine how much light gets into their eyes. Species adapted to see in dim light, such as owls, have very wide scleral ring openings in relation to their orbits to let more light through. Diurnal birds, on the other hand, have narrower openings. In the case of Jeholornis, it now appears: "The digitally reconstructed scleral ring indicates a highly diurnal lifestyle of this animal, which also supports the hypothesis that the early representatives of the birds were predominantly diurnal," the researchers write.
Hu concludes, “Jeholornis is fascinating. His unique position as one of the most primitive known transitional species allows us to gain insight into why and how modern birds - the only living descendants of dinosaurs - evolved into what we see today." Paleontologist.
Source: Field Museum, Article: Zoological Journal of the Linnean Society, doi: 10.1093/zoolinnean/zlac089