The results of the study of the recently discovered skull of ichthyornis – the ancestor of modern birds that lived in the Late Cretaceous era – led scientists to a new hypothesis about how birds managed to survive the mass extinction at the turn of the Cretaceous and Paleogene periods, when all other representatives of the dinosaur group disappeared from the face of the Earth. Previously, it was believed that the small size of the birds ‘ bodies was the decisive factor. The authors suggest that the most important role was played by the evolution of the bird brain — an increase in its size and the visual system’s development. The article is published in the journal Science Advances.
American scientists led by Christopher Torres from the College of Osteopathic Medicine at Ohio University analyzed a recently found skull of ichthyornis aged 70 million years. This toothy seabird, which, judging by paleontological reconstructions was similar to modern gulls and petrels, lived in North America just before the mass extinction of the dinosaurs that occurred 66 million years ago.
It turned out that the ancestors of modern birds had a very different brain shape from other dinosaurs, including early birds. The location of the visual lobes of the individual strongly indicates that the brain of the extinct genus of seabirds resembled the brain of archaeopteryx, a feathered dinosaur similar to a bird, but probably differed greatly in function.
The authors analyzed the details of the skull structure of more than 2000 modern and extinct birds. They found that, compared with early birds such as archaeoptecrix, ichthyornis had an expanded brain with a displacement of the ventral-visual lobes. These features were inherited by modern birds, which they have developed even more.
Christopher Torres and his colleagues believe that the previously existing hypothesis linking the survival of birds during the mass extinction with the small size and mass of their bodies arose due to the lack of well-preserved skulls of small birds. In this regard, scientists have not yet been able to identify the features that distinguish modern birds from their ancestors, including those that may have benefited them during the cataclysm.
Based on the analysis of the body mass of birds before and after the Cretaceous-Paleogene boundary, the authors found no evidence that small size helped modern birds survive this event.
To study the phylogenetic relationships between modern and extinct birds and to assess the relative size of their body and brain, the authors compiled a matrix of 223 characteristics. In total, the sample included 2003 species of modern birds, archaeopteryx, and seven non-avian dinosaurs. To digitally reconstruct the facial skeleton and restore the shape of the ichthyornis brain, the researchers used modern methods of X-ray computed tomography.
The authors suggest that shortly before the Cretaceous-Paleogene catastrophe, the brain of birds took its current form. There was also an expansion of the functions of the visual system. This, according to scientists, was the decisive advantage that allowed the birds to outlive the rest of the representatives of the dinosaur group.