damesi (Ammonitina, Desmoceratidae) from Hokkaido, is shown in Fig. The 3D image for specimen NMA00802, Upper Cretaceous ammonite Damesites cf. We analysed such samples by using micro-computed tomography (CT) and obtained almost the complete sequence of septal structure with the highest resolution ( Supplementary Figs 1–6). In some types of ammonite fossils, the shell chambers were filled with calcite diagenesis then dissolved and eliminated the shell wall and septa, generating an empty space at their place. Various types of ammonite fossils that vary in mineral diagenesis are available. To determine how ammonites formed the complex suture patterns, we performed detailed 3D analysis of the ammonite shells.
Thus, we developed a method to obtain a super-high resolution 3D image of ammonite septa and then compared the previous models with the obtained image. However, because the suture lines are the intersection between the shell wall and septa, understanding how the three-dimensional (3D) structure of the septa is formed is necessary. Previous models mainly focused on the two-dimensional (2D) suture pattern and considered the behaviour of the marginal region of the ammonite mantle. His model does not involve any specific physical method, but simply assumes that the peripheral region of the rear mantle invaginates to form the folding of the septa. Hammer presented another idea in 1999 21.
Therefore, validating these models is difficult. These models are not based on the phenomena found in extant organisms. Although these models explain the formation of the suture pattern via a simple method, both have a problem. This model assumes that the Saffman–Taylor instability occurs on the surface of the rear mantle and in the fluid of the shell chamber. According to this model, when two fluids with different viscosity come in contact, the Saffman–Taylor instability is induced at the interface, generating finger-like shapes that are similar to the ammonite suture patterns. The other model is the viscous fingering model 17, 18, 19, 20. If hydrostatic pressure was applied from the apical side, the mantle would be pressed to move adorally, but the tie-point region would remain at the same position and eventually form an arch-like shape. The tie-point model, the best-known hypothesis, assumes that the rear mantle adheres to the shell wall at many small points known as tie-points 13, 14, 15, 16. In the two previously proposed hypotheses, the rear mantle of ammonites was assumed to be flexible and could be passively folded by some external force into complex folds. However, the formation of the complex suture pattern requires extensive deformation of the rear mantle. Since ammonites are related to Nautilus, the ammonite septa should also have been formed by using the rear mantle as a template. Living Nautilus also have septa they form these septa by using their round rear mantle as a template 12. However, very little is known about the mechanism of how this complex and diverged pattern was generated. The wavy line of intersection between the shell and septum is called the suture line, which is highly diverged among ammonite species it is a useful index for taxonomical identification 8, 9, 10, 11. Based on the characteristics of the obtained 3D structure, we explain how ammonites might have formed the complex suture patterns.Īmmonite conchs are separated into small chambers by septa, which might contribute to reinforce the shell wall 1, 2, 3, 4 and control buoyancy 5, 6, 7. 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. The obtained image suggested that the wavy and branched structures of the rear mantle grew autonomously. 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. The elucidation of the mechanism of septa formation requires that the detailed shape of the septa should be known. Previous hypotheses assumed that the rear mantle is like a flexible membrane that can be folded by some physical force. The wavy septa can form if the rear mantle of the ammonite, which functions as the template, has a complex shape. The lines are formed at the intersection of the outer shell wall and the septa. Ammonite shells have complex patterns of suture lines that vary across species.
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