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A peer-reviewed article of this Preprint also exists.
I wish to commend the authors on a really interesting manuscript that holds the potential to enhance our overall picture of the theropod-avian transition and open up a whole new avenue of research on non-avian dinosaur eggs. I'm very glad to see that the authors made zonal porosity measurements across the egg. As they correctly note, this method allows for more detailed inferences about nest structure than sampling only single eggshell fragments.
I have two main suggestions for the authors:
1) Though I am not qualified to comment in detail on the experimental procedures, I suggest obtaining a chromatogram from extant avian eggshell that is known to lack or contain very low amounts of the pigments of interest. This would provide a further control for comparison with their chromatograms in Figure 2, to help rule out the possibility (however unlikely) that there is some other effect of the eggshell itself (whether pigmented or not) that drives the observed results. It would also be interesting to compare the results to chromatograms from eggs that appear white (but still contain pigment), given some of the complexities discussed by Cassey et al. (2012) in correlating pigment concentration and observed color.
2) When discussing their observations of a cuticle in this eggshell, I suggest that the authors cite Jackson and Varricchio (2010), who earlier noted the possible presence of a cuticle in the theropod ootaxon Triprismatoolithus stephensi. A figure of the cuticle microstructure of the Heyuannia eggshell would also enhance the reader's understanding.
Again, this is a very interesting work and I look forward to seeing the progress of this manuscript!
Daniel E. Barta
Ph.D. Student, Richard Gilder Graduate School and Division of Paleontology, American Museum of Natural History, email@example.com
Cassey, P., Thomas, G. H., Portugal, S. J., Maurer, G., Hauber, M. E., Grim, T., Lovell, P. G. and Mikšík, I. 2012. Why are birds’ eggs colourful? eggshell pigments co-vary with life-history and nesting ecology among british breeding non-passerine birds. Biological Journal of the Linnean Society 106: 657–672.
Jackson, F.D. and Varricchio, D.J. 2010. Fossil eggs and eggshell from the lowermost Two Medicine Formation of western Montana, Sevenmile Hill Locality. Journal of Vertebrate Paleontology 30: 1142–1156.
I'd only like to note that the referral of any or all Macroolithus yaotunensis to Heyuannia has no published support. Cheng et al. (2008) referred their eggs to "Heyuannia huangi or an oviraptorosaurian of similar kind" based purely on the narrow proximal metatarsal III, but this is true in most oviraptorids (e.g. Yulong, Wulatelong, Citipati, IGM 100/42, Khaan) except "Ingenia". Heyuannia isn't known from the formations these shells derive from, but e.g. NXMV (from Lu's 2004 thesis) is known from the Pingling Formation, and Banji, Jiangxisaurus and Nankangia are known from the Nanxiong Formation.
This is a very interesting work providing rare chemical data on extinct organisms. To my knowledge it is the first report on the pigments of dinosaur eggs. However, it should be mentioned that biliverdin and protoporphyrin have previously been detected in subfossil eggshells from extinct birds and I suggest to cite the following paper:
Igic, B., Greenwood, D., Palmer, D., Cassey, P., Gill, B., Grim, T., Brennan, P.R., Bassett, S., Battley, P., Hauber, M., 2010. Detecting pigments from colourful eggshells of extinct birds. Chemoecology 20, 43-48.
It is interesting that the fossil pigments are preserved in a carbonate biomineral matrix that protected these compounds from degradation over millions of years. Similar observations have been made in the case of fossil pigments from crinoids and putative red algae:
Wolkenstein, K., 2015. Persistent and widespread occurrence of bioactive quinone pigments during post-Paleozoic crinoid diversification. Proc. Natl. Acad. Sci. USA 112, 2794-2799.
Wolkenstein, K., Gross, J.H., Falk, H., 2010. Boron-containing organic pigments from a Jurassic red alga. Proc. Natl. Acad. Sci. USA 107, 19374-19378.
I’m looking forward to seeing the final article.
Dr. Klaus Wolkenstein, Geoscience Centre, University of Göttingen, firstname.lastname@example.org