Microstructural and crystallographic evolution of palaeognath (Aves) eggshells
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eLife assessment
This fundamental study represents a significant advance in our understanding of the complex evolutionary history of the eggshell features in one of the main leaving bird lineages, Palaeognathae, with compelling and thoughtfully presented results. The work will be of interest to many biologists, paleontologists, and archaeologists.
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Abstract
The avian palaeognath phylogeny has been recently revised significantly due to the advancement of genome-wide comparative analyses and provides the opportunity to trace the evolution of the microstructure and crystallography of modern dinosaur eggshells. Here, eggshells of all major clades of Palaeognathae (including extinct taxa) and selected eggshells of Neognathae and non-avian dinosaurs are analysed with electron backscatter diffraction. Our results show the detailed microstructures and crystallographies of (previously) loosely categorized ostrich-, rhea-, and tinamou-style morphotypes of palaeognath eggshells. All rhea-style eggshell appears homologous, while respective ostrich-style and tinamou-style morphotypes are best interpreted as homoplastic morphologies (independently acquired). Ancestral state reconstruction and parsimony analysis additionally show that rhea-style eggshell represents the ancestral state of palaeognath eggshells both in microstructure and crystallography. The ornithological and palaeontological implications of the current study are not only helpful for the understanding of evolution of modern and extinct dinosaur eggshells, but also aid other disciplines where palaeognath eggshells provide useful archive for comparative contrasts (e.g. palaeoenvironmental reconstructions, geochronology, and zooarchaeology).
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Author Response
Public Review:
- Despite I do not find negative arguments for any special section of the study, I have a question regarding Triprismatoolithu stephensis:
As mentioned in the text, Triprismatoolithus is analysed by the authors, and several pictures are provided in Fig.S12 alongside a brief description in de Supplementary Text 4. But it seems that it is not included in any of the phylogenetic analyses or figures. Why?
If the specimen has no implication for any of the main analyses, there is no need to be considered as "studied material".
We added more explanation for the purpose of Triprismatoolithus (Lines 803–806). We presented Triprismatoolithus to show the prismatic shell units of maniraptoran eggshell other than a famous case of Prismatoolithus levis. Thus, Triprismatoolithus was also presented in the Figure S1C …
Author Response
Public Review:
- Despite I do not find negative arguments for any special section of the study, I have a question regarding Triprismatoolithu stephensis:
As mentioned in the text, Triprismatoolithus is analysed by the authors, and several pictures are provided in Fig.S12 alongside a brief description in de Supplementary Text 4. But it seems that it is not included in any of the phylogenetic analyses or figures. Why?
If the specimen has no implication for any of the main analyses, there is no need to be considered as "studied material".
We added more explanation for the purpose of Triprismatoolithus (Lines 803–806). We presented Triprismatoolithus to show the prismatic shell units of maniraptoran eggshell other than a famous case of Prismatoolithus levis. Thus, Triprismatoolithus was also presented in the Figure S1C along with other eggshells with prismatic shell unit microstructure. Without this ootaxon, there are just three comparative pieces of material in Figure S1, and so we prefer maintaining this ootaxon. Admittedly, this eggshell was not used in our analysis in Figures 13–16 because the specific egg-laying taxon is unknown so its taxon-ootaxon relationship is not as solid as the cases of Elongatoolithus, Macroelongatoolithus, and Prismatoolithus levis. But please note that the role of this ootaxon in Figure S1 is not trivial because it supports the view that even prismatic shell units have rugged grain boundaries in the squamatic zone.
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eLife assessment
This fundamental study represents a significant advance in our understanding of the complex evolutionary history of the eggshell features in one of the main leaving bird lineages, Palaeognathae, with compelling and thoughtfully presented results. The work will be of interest to many biologists, paleontologists, and archaeologists.
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Public Review:
The study of Choi and collaborators provide novel information about the microstructural morphology and the crystallographic structure of palaeognathid eggshells.
In terms of format and structure, the work is well organized and the extinction of each section is appropriate. All figures, both those from the main text and Supplementary Information, are of good-quality, informative, and useful, facilitating the understanding of the text. The bibliography is very updated, and all essential references are mentioned.One of the strongest points of the work, in my opinion, is the designee of the study itself, which included specimens from all living palaeognathid birds and several extinct taxa from a large range of lineages.
The methodology used for analysing the crystallographic nature of the studied specimens (EBSD) is …
Public Review:
The study of Choi and collaborators provide novel information about the microstructural morphology and the crystallographic structure of palaeognathid eggshells.
In terms of format and structure, the work is well organized and the extinction of each section is appropriate. All figures, both those from the main text and Supplementary Information, are of good-quality, informative, and useful, facilitating the understanding of the text. The bibliography is very updated, and all essential references are mentioned.One of the strongest points of the work, in my opinion, is the designee of the study itself, which included specimens from all living palaeognathid birds and several extinct taxa from a large range of lineages.
The methodology used for analysing the crystallographic nature of the studied specimens (EBSD) is appropriate for the goals of the study. The phylogenetic approaches are also right, which are based on the most recent studies about the phylogenetic relationship of ratites.
Despite their complexity, the results are well presented, being relatively easy to understand for a person not versed in the subject. In fact, the ways in which they are described give them the potential to be used as a guideline to anyone interested in eggshell microstructure.
The discussion of the results seems consistent with the data obtained. Despite the phylogenetic relationship between some palaeognathid taxa remains partially instable, authors present different plausible scenario to explain the variability of the eggshell microstructure within a single monophyletic lineage (homology vs homoplasy). In fact, the homoplastic scenario is, perhaps, the most shocking one to me. In part, it is because it intrinsically suggests that all phylogenetic studies based on eggshell morphological features, and conducted during the last 20 years, are potentially artefacts, and they do not represent real phylogenetic relationships. Far from being a criticism, this interpretation has massive implications, especially for those studies where the taxonomic attribution of a fossil egg is based on phylogenetic results (i.e. Montanoolithus, Cairanoolithus).
Although I do not find negative arguments for any special section of the study, I have a question regarding Triprismatoolithu stephensis:
As mentioned in the text, Triprismatoolithu is analysed by the authors, and several pictures are provided in Fig.S12 alongside a brief description in de Supplementary Tex4. But it seems that it is not included in any of the phylogenetic analyses or figures. Why?
If the specimen has no implication for any of the main analyses, there is no need to be considered as "studied material".
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