Reviewer #2 (Public Review):
Wang et al. present a detailed description and analysis of the previously reported cranial remains of enantiornithine bird Yuanchuavis. The authors use X-ray CT scan data to reconstruct the cranial elements and retro-deform the facial and palatal skeleton. The authors also use principle component analysis with geometric morphometrics data to investigate where Yuanchuavis falls in palatine phylomorphospace. The authors use these data to make inferences about the kinetics of the Yuanchuavis skull as well as the evolution of cranial kinesis across birds.
Generally, I find the authors' direct interpretation of their anatomical and PCA data to be convincing and compelling. The anatomical description is thorough and accurate. The methods used for the geometrics morphometrics and PC analyses are appropriate. I find compelling the authors' interpretations that Yuanchuavis largely retained the ancestral non-avialan akinetic skull.
One of the greatest strengths of this paper are the extremely attractive figures. In particular, I find figure 4 to be exceptionally useful - this is easily the most effective illustration I have yet seen of avian cranial kinesis and the shifts in cranial morphology that underlie its evolution. I applaud whoever designed this figure.
My one major concern with this paper's methodology is that the palatine used for Ichthyornis is incorrect. Torres et al. (2021) published the correct palatines, which were very different from those incorrectly (but understandably) identified in Field et al. (2018) and used here. I strongly urge the authors to rerun their GMM analysis with corrected data.
The remaining weaknesses I find in this paper are not major but are worth addressing, and generally pertain to the broader discussion of significance of the authors' more direct interpretations of their data. The authors' suggestion that reduction/loss of the jugal process of the palatine was an early step towards the modern kinetic avian skull is logical, but I don't think the GMM analysis presented here demonstrates that (contrary to lines 390-392). The GMM analysis can only help identify such morphological shifts, not connect them to functional shifts. Rather, I think this analysis helps refine when this shift occurred - indicating that, if there is such a functional link, the earliest steps towards the modern kinetic skull occurred early in avialan evolution.
I find the discussion of the evolution of cranial kinesis as exaptation (lines 427-441) confusing, distracting and largely unnecessary. Has anyone previously suggested that avian cranial kinesis is an example of preadaptation?
I am similarly confused by the connection made by the authors of evolutionary modularity, the akinetic skull of enantiornithines and patterns of avialan diversification (lines 442-464). Specifically, I do not understand how the dominance of enantiornithine clade in the Cretaceous is "counterintuitive" (line 452), nor do I understand how this pattern is explained by evolutionary modularity.