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Sci Rep. 2016 Sep 19;6:33689. doi: 10.1038/srep33689.

Suture pattern formation in ammonites and the unknown rear mantle structure.

Author information

1
Graduate School of Frontier Bioscience, Osaka University, Yamada 1-3, Suita, Osaka 565-0871, Japan.
2
Hokkaido University Shuma-no-kai, 354 Clubs and Societies Building, Kita 17, Nishi 12, Kita-ku, Sapporo, Hokkaido 060-0808, Japan.

Abstract

Ammonite shells have complex patterns of suture lines that vary across species. The lines are formed at the intersection of the outer shell wall and the septa. The wavy septa can form if the rear mantle of the ammonite, which functions as the template, has a complex shape. Previous hypotheses assumed that the rear mantle is like a flexible membrane that can be folded by some physical force. The elucidation of the mechanism of septa formation requires that the detailed shape of the septa should be known. We developed a new protocol of X-ray micro-computed tomography (CT) and obtained high-resolution three-dimensional (3D) images of the septa of the Upper Cretaceous ammonite Damesites cf. damesi. The obtained image suggested that the wavy and branched structures of the rear mantle grew autonomously. We found that some extant sea slugs have branched structures and showed similar shape and growth sequence as those in fossils, suggesting that the mantle of molluscs basically has the potential to form branched projections. Based on the characteristics of the obtained 3D structure, we explain how ammonites might have formed the complex suture patterns.

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