Diversity and evolutionary growth of biomineralized columns in early Cambrian phosphatic-shelled brachiopods

Reviewer #1 (Public Review):

This is a key paper examining the evolution of an important structure (pillars) in the shell architecture of organo-phosphatic brachiopods. The advantages of these structures are adequately discussed and the evolution of the pillars is described and illustrated. There is much that is of fundamental significance here in understanding the ecology and evolution of these groups as a whole.

1. In several places the biological control on the development of the pillars is noted. This is explained in terms of their relationship to the growth and evolution of epithelial cells. It would be useful and make the paper more understandable if this link was mentioned early on in the paper and developed during the narrative.

2. The Cambrian Explosion is mentioned a number of times. Are these changes driven by the Cambrian Explosion, i.e. the expansion of major new body plans, or are the changes merely coincident with the long duration of the 'Explosion'?

3. I have no doubt the process is one of adaptive innovation but it would be useful to expand on this. Why is it adaptive?

4. Are pillars present in living Lingula?

Reviewer #2 (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 are 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 the extent that information is cited wrongly.
The structure and language of the article need reworking in my opinion, the systematic part can be in the appendix but the main results and the results relevant for the discussion should be in the main article. A critical revision of the family Eoobolidae and Lingulellotretidae including a revision of the type species of Eoobolus and Lingulellotreta is needed.

Author Response

We appreciate very much your positive assessment and the comments of the two reviewers, all of which will greatly help us to improve our manuscript. In response, therefore, to these constructive comments we will take pleasure in submitting a revised manuscript during the next step of publication.

We take the opportunity to provide a provisional author response.

As for Reviewer #1.

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

We will add 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 early Cambrian was likely driven by the Cambrian Explosion. The columnar shell structures 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 in order to study the evolutionary trend of shell structures and compositions in brachiopods. Furthermore, the adaptive innovation of biomineralized columns in early brachiopod will be 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 most of them will be introduced into the revised version of the manuscript.

We agree that the knowledge is incomplete on the shell structures of early linguliform brachiopods and more research shall be helpful. We also express the idea in the first part of our manuscript that the shell structural complexity and diversity of linguliform brachiopods (especially their fossil representatives) require further studies. 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 morphologies, shapes and sizes of cylindrical columns are described in details in this research, and this work will be useful for further comparative studies. We are very sorry to miss the important reference paper on brachiopod shells by Butler et al. (2015), which will be added into the revised manuscript. The structure and language of the manuscript will be 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 will add 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.