Cretaceous bird from Brazil informs the evolution of the avian skull and brain
by: Luis M. Chiappel, Guillermo Navalón, Agustín G. Martinelli, Ismar de Souza Carvalho, Rodrigo Miloni Santucci, Yun-Hsin Wu1 & Daniel J. Field
Summarized by Jordyn Dutcher. Jordyn lives in Binghamton, NY, and attends Binghamton University seeking a B.S in Biology. Jordyn works for a local news station editing videos and is particularly interested in science communications. When she’s not out in the woods hiking, she can often be found attending local music shows.
What data was used? Researchers analyzed the skull of an avian (bird) dinosaur found in a bonebed within São Paulo State in southeastern Brazil and dated to be from the Mesozoic Era in the Cretaceous Period (85–75 mya). Additional skull data (i.e. the shapes and proportions of the skulls), from previous studies, of other birds both from the distant past and modern day was used.
What was the hypothesis being tested? Researchers used a newly discovered fossil, Navaornis hestiae gen. et sp. nov., to help fill the gap of evolution between the Archaeopteryx (one of the oldest known ancestors of modern birds) and closer known ancestors of modern day birds. More specifically, they were aiming to find the origin of derived skulls and brains which currently exist in birds of today but were not existent in Archaeopteryx.
Methods: Researchers used the newly discovered bird fossil skull and compared its features to those found both in modern and past avian skulls. This was done by scanning the fossil and then digitally reconstructing the skull using a 3D-imaging software (Blender and VGStudioMax). The researchers analyzed the reconstructed skull of the newly discovered fossilagainst other 3D reconstructions (obtained from CT scans and recreations done in previous studies) of 228 other avian skulls, including both those currently living and older ancestors such as the Archaeopteryx, and plotted them based on their differences and similarities using Principal Component Analysis (PCA) (Fig. 1) which is a commonly used analytical method which takes all the collected measurements and condenses them plot on a 2D graph for comparison amongst different specimens, with the further the plot points are the more they differ in their structure. Using an adapted method from previous studies, the scientists defined “landmark” points of significance in which these fossils either varied or shared similarities which were striking, such as marking various points within the brain cavity of the skull and parts of the beak/rostrum; essentially, all of the landmarks were placed on the same feature on each skull, to capture the variation between the studied specimens in those features. The analysis of the different shapes and features of these skulls allowed the researchers to draw connections about how evolution occurred between these different species and their ancestors based on the full range of variation in their skull shape.
Results: The discovered fossil plotted differently enough to be named a new genus and species by the researchers; Navaornis hestiae, was found to be a toothless bird belonging to the enantiornithine group, which is an extinct group of most-often toothed birds abundant during the Mesozoic Era which did not give rise to modern birds. This fossil represents one of the first well-preserved skulls found in the enantiornithine group. The skull shape of Navaornis is described by the researchers as an intermediate mix in structure between modern birds and their oldest known ancestor, the Archeopteryx, falling in the middle between the size and shape of the inner ear and brains of these two groups (Fig. 1). However, the enantiornithine group, the group the new fossil belongs to, aren’t genetic ancestors of modern birds and instead co-evolved alongside the group which would lead to modern birds before going extinct. This means that these similarities were likely convergent and evolved separately in these two groups. This intermediate mixing of prehistoric and modern birds skull structure seen in Navaornis supports the idea that the different parts of the skull were evolving separately and at different rates.
Why is this study important? This study is important as it defines the new genus and species Navaornis hestiae. which helps to detail the evolutionary history of the brain/skulls of the wildly diverse modern birds of today and their long distant ancestors/relatives. Additionally, this study supported “modular evolution” which is the idea that different anatomical regions of an individual can grow at different rates, which is exactly what was seen in the mixing of more prehistoric and modern features of the Navaornis skull.
Broader Implications beyond this study: Scientists hypothesize that this mixing of features is likely due to some form of evolutionary pressures shaped through their life habits (e.g. feeding, etc.). Prior to the splitting of the two groups, a disparity (difference) between them existed with the enantiornithine group having significantly narrower skulls compared to the wider and more stout skull seen in the group leading to modern birds. Further research into this topic may be able to help determine what sort of evolutionary pressure may have contributed to this mixing of structures within the enantiornithine group and the group leading to modern birds.