Evolution and diversity of biomineralized columnar architecture in early Cambrian phosphatic-shelled brachiopods

Joint Public Review:

Summary: Two early Cambrian taxa of linguliform brachiopods are assigned to the family Eoobolidae. The taxa exhibit a columnar shell structure and the phylogenetic implications of this shell structure in relation to other early Cambrian families is discussed.

Strengths: Interesting idea regarding the evolution of shell structure.

Weaknesses: The early record of shell structures of linguliform brachiopods is incomplete and partly contradictory. The authors maintain silence regarding contradictory information throughout the article to an extent that information is cited wrongly. The article is written under the assumption that all eoobolids have a columnar shell structure. Thus, the previously claimed columnar structure of Eoobolus incipiens which has been re-illustrated in the paper is not convincing and could be interpreted in other ways.

The article needs a proper results section. The Discussion is mainly a review of published data. Other potential results are hidden in this "discussion". In addition, a more elaborate Methods section is needed in which it is explained how the data for shell thicknesses and numbers of laminae was obtained.

A critical revision of the family Eoobolidae and Lingulellotretidae including a revision of the type species of Eoobolus and Lingulellotreta is needed.

The potential evolutionary patterns that are discussed towards the end (summarized in Fig 6) are interesting but rather unconvincing as the way the data has been obtained has never been clarified. Shell thicknesses and numbers of laminae that built up the shell of several taxa are compared, but at no point it is stated where these measurements were taken. Shell thicknesses vary within a shell and also the presence of the never mentioned tertiary layer is modifying shell thicknesses. Hence, the presented data appears random and is not comparable. The obtained evolutionary patterns must be considered as dubious.

Author Response

The following is the authors’ response to the original reviews.

We greatly appreciate your positive assessment and the comments by the two reviewers on the previous version of our manuscript, all of which are very helpful and greatly improved our manuscript. We have incorporated all changes and corrections requested by these reviewers and we believe their suggestions have enhanced the overall quality of our manuscript.

As for Reviewer #1.

We thank Reviewer 1 very much for her/his very positive and detailed remarks, all of which have been introduced into the revised version of our manuscript.

We have added the information about the biological control on the development of phosphatic-shelled brachiopod columns in the introduction, so that our late narrative can be more understandable. The Cambrian Explosion is the innovation of metazoan body plans and radiation of animals during a relatively short geological time. The expansion of new body plans in different groups of brachiopods in the early Cambrian was likely driven by the Cambrian Explosion. The columnar architectures are not developed in living lingulate brachiopods, and thus it is important to get a better understanding of this extinct shell architecture from the fossil records on a global scale in order to study the evolutionary trend of shell architectures and compositions in brachiopods. We hope the current comparison study of columnar shell architectures from some of the oldest known brachiopods will help to pursue this goal. Furthermore, the adaptive innovation of biomineralized columnar architecture in early brachiopods is discussed in the revised manuscript.

As for Reviewer #2.

We thank Reviewer 2 very much for her/his very constructive and detailed remarks. All the comments have been thoroughly considered, and introduced into the revised version of the manuscript.

The current information on the shell structures of early linguliform brachiopods is unclear, which has been introduced in the revised manuscript and the supplementary Appendix 1. We also state that more detailed studies of the complexity and diversity of linguliform brachiopod architectures (especially their early fossil representatives) require further investigations. As the shell structure and biomineralization process are crucial to unravel the poorly resolved phylogeny and early evolution of Brachiopoda, in this paper, we undertake a primary study of exquisitely well-preserved brachiopods from the Cambrian Series 2. The shapes and sizes of microscopic cylindrical columns are described in detail in this research, and this work will be useful for further comparative studies on brachiopod shell architecture. The important reference paper on brachiopod shells by Butler et al. (2015) has been added to the revised manuscript. The structure and language of the manuscript are revised based on the very helpful suggestions.

Concerning the families Eoobolidae and Lingulellotretidae, we are aware of the current problematic situation of these families, and we have added more discussion about the detailed characters of Eoobolidae in the Systematic Palaeontology part of the manuscript. However, the revision of the families Eoobolidae and Lingulellotretidae falls outside the scope of this paper. We prefer to leave it now as it will be part of an upcoming publication based on more global materials from China, Australia, Sweden and Estonia that we are currently working on.

On behalf of my co-authors, I thank you for taking the time to consider our manuscript for publication in eLife and I hope that with the changes we have made to our paper, it is now suitable for publication. If you have any further questions about our revised manuscript, please do not hesitate to get in contact. Thank you very much for your time and consideration.