This Fossil Friday features the bird Navaornis hestiae from the Late Cretaceous Adamantina Formation of southeastern Brazil, which is dated to an age of about 80 million years. This fossil was attributed to an extinct group called Enantiornithes, which thrived in many species around the globe in the Cretaceous period. The new taxon was just described this month by Chiappe et al. (2024) in the journal Nature. The discovery of this perfectly preserved fossil bird turned out be a kind of puzzle for evolutionary biologists, who study the history of bird origins.

The beautiful fossil even preserved detailed structures of the brain that could be studied with high-resolution CT scanning. A morphometrical analysis of the geometry of the brain placed Navaornis about midway between Archaeopteryx and modern birds. This is certainly interesting and arguably fits with the common evolutionary scenario. The authors of the new study say that "Navaornis exhibits a brain morphology intermediate between Archaeopteryx and crown birds along the main axis of endocranial shape variation" and thus "the morphology of the endocast of Navaornis shows an intermediate stage in the evolutionary history of the unique avian brain." A news report in SciNews (News Staff 2024) quotes one of the authors with a comment that "the brain structure of Navaornis hestiae is almost exactly intermediate between Archaeopteryx and modern birds -- it was one of these moments in which the missing piece fits absolutely perfectly." Such a gem of course directly made it into the headline of the article that is titled "80-Million-Year-Old Enantiornithine Fossil Fills Gap between Archaeopteryx and Modern Birds".

But there is a little complication. Even though entantiornithine birds are considered as stem birds because of several relatively "primitive" traits in their anatomy, the new species shows a remarkable similarity to modern birds that was quite unexpected for the scientists. The authors describe that the "cranial geometry of Navaornis shows an unprecedented degree of similarity between crown birds and enantiornithines" and note that "despite an overall geometry quantitatively indistinguishable from crown birds, the skull of Navaornis retains numerous plesiomorphies." They admit this "implies that the origins of these 'advanced' traits often associated with crown birds either predated the origin of Ornithothoraces or evolved convergently among both Enantiornithes and crownward Euornithes." They conclude as follows:

This degree of geometric convergence between Enantiornithes and crown birds suggests that developmental constraints responsible for canalizing the general shape of the bird skull may have been present throughout much of avian evolutionary history, predating both the phylogenetic divergence between Enantiornithes and Euornithes more than 130 million years ago as well as the evolutionary acquisition of several apomorphic characteristics of crown bird skull and brain morphology. The exceptionally well-preserved skull of Navaornis emphasizes the necessity of hitherto elusive undistorted Mesozoic bird skulls for illuminating the complex sequence by which the unique brains and skulls of modern birds arose.

Convergence means similarity not based on common descent, "may have been" means they have no clue, and "complex sequence" means that the data are not what they expected to find. Prior to this discovery none of the experts would have predicted such a modern skull in a "primitive" stem bird, but after the fact evolutionary biologists are always quick to offer a fancy just-so-story that reconciles the evidence with the theory.

It is quite remarkable that popular science reports such as an article in New Scientist (Woodford 2024) only emphasize that this "exquisite bird fossil provides clues to the evolution of avian brains", and quotes one of the authors as saying that "Navaornis fills a roughly 70-million-year-long gap in our understanding of how the distinctive brains of modern birds evolved." Reuters reports that this "'One-of-a-kind' skull fossil from Brazil reveals bird brain evolution" (Dunham 2024). Is it really just a happy accident that there is no mention of the unexpected similarity of this alleged primitive bird to modern birds and the implied problems for bird evolution? Why do we mostly hear only one side of the story in the media? I suppose we must be protected from dangerous questions that could come up.