Carl Zeiss Nanotom scanner was used to obtain data on specimens contained in sediments with slices 0.5 mm thick. On simple samples and specific areas, slices of 17 μm and 70 μm were analyzed with a Carl Zeiss Metrotom and a Carl Zeiss Metrotom. The best image quality and inter-operability were ensured by exporting all CT and micro-CT data using the DICOM export file format. Processing of 2D and 3D images from the CT and Micro-CT DICOM stacks was performed on a Dell Precision T7400 Dual Quad Core Intel Xeon 3.2 GHz workstation, using 32 GB of DDR RAM and a Quadro FX 4600 graphics board running VG Studio Max 2.0 and 2.2 × 64 from Volume Graphics GmbH. The Micro-XCT-400 system at Tohoku University was used to perform completementary micro-CT analyzes for X-strata; and the photographs of all strata were made using a 3D MOLCER image viewer.

Neutron imaging is an alternative method to X-ray imaging that provides new insights in the sample composition. The techniques are similar but the radiation type is changed to neutrons. Neutrons are sensitive to different elements than X-rays and in particular light elements deliver higher contrast in the recorded images to reconstruct a three-dimensional image of a sample [10] . Examples of high contrast elements are hydrogen and lithium, while many metals exhibit low attenuation. This is in particular interesting for the investigation of fossils that often contain iron while the parts of organic origin contains lighter elements [11] [12] [13] . Neutron imaging requires high neutron flux to obtain a good image quality. This can only be obtained at large scale facilities like research reactors or spallation neutron sources. In this investigation computed tomography with neutron radiographs provided 3D information about the sample composition. The experiments were performed at Paul Scherrer Institut using the cold neutron imaging beam line ICON [14] . The data was acquired using the micro-tomography setup. It provides a 13.5 µm pixel size and a field of view of 27 × 27 mm². A Gadox scintillator with 20 µm thickness was used to convert the neutrons to visible light, the light image was captured using a cooled CCD camera (Andor DW436). The CT scan was made over 360 degrees with 625 projections that were reconstructed using the Octopus CT reconstruction software. The reconstruction was made using parallel beam geometry which is a valid approximation due to the high collimation of the neutron beam at ICON. This resulted in a volume data set with a 13.5 µm voxel size. The signal to noise ratio of the data was relatively low. Therefore, a denoising filter based on the ROC model in inverse scale space setting [15] was applied using KipTool [refQuoVadis] to improve the image quality without destroying relevant features. Finally, the volume data was visualized and animated using the Drishti 2.5.1 volume rendering software.

SEM-EDS (electron microscopy-energy dispersion X-ray spectrometry) analyses were performed on Os-coated samples to investigate the distribution of elements such as potassium, calcium and iron using a Hitachi S-3400N SEM with a Bruker Xflash 5010 at the Tokyo Institute of Technology. Measurements were carried out at an acceleration voltage of 15 kV.

PIXE (particle-induced X-ray emission) by nuclear microprode allowed quantitative imaging of the distribution of chemical elements in various types of materials, including fossil samples. In addition, the both PIXE and RBS (Rutherford backscattering spectrometry) combination allowed complete quantification of the concentrations of these chemical elements [16] . A polished section of Akouemma hemisphaeria sample (AKOU16) was analyzed using a PIXE microprobe at CENBG (Bordeaux Gradignan). The 1.775-MeV proton beam used provided information on the distribution of the chemical elements with the atomic number > 11 and with an improvement of the carbon signal RBS by carbon resonance at 1.75 MeV. The proton beam concentrated to 2 micrometers at the surface of the sample provided an intensity of 500 pA. The raster scanner was used to allow chemical imaging of 1.2 × 1.2 mm² of the sample. The chemical maps recorded in a matrix of 256 × 256 pixels have a spatial resolution of 4.6 μm/pixel. In the Akouemma hemisphere (AKOU 16), 393 square areas of 1.2 × 1.2 mm² were each analyzed to cover the entire section of the sample (approximately 576 mm²). For specific areas such as carbonate rich regions, higher spatial resolution maps have been generated with a smaller beam (0.8 μm) and a lower intensity (180 μA) for digitization. A matrix of 256 × 256 pixels was used in regions with a maximum area of 0.2 × 0.2 mm², resulting in a larger spatial resolution of the configuration (0.8 μm). PIXE analysis made it possible to obtain distribution maps for Al, Si, S, K, Ca, Ti, Mn and Fe. The carbon and oxygen concentrations were determined simultaneously by RBS analysis. The geochemical reference materials of the United States BCR2 (Basalt, Columbia River) and BIR1 (Icelandic basalt) were used for the validation of this quantification procedure.

Shapes, edges and discontinuities of Akouemma hemisphaeria specimen consists of spheroidal and elongated shapes with either a “lateral budding”, (Figure 4), or a “homogeneous lateral elongation” (Figures 5-7).

The spheroidal shapes, of diameter about 2 cm on average are characterized, whatever their size, by two hemispheres with their central nipple-like feature of the upper hemisphere and corresponding central depression of the lower hemisphere which likely correspond to a communication point between the two hemispheres separated by the median disc with its central orifice and its various rods or lamellae; the internal radial fabric is upwardly centrifugal in the upper hemisphere and downwardly centrifugal in the lower hemisphere (Figure 3, Movie 1, Movie 2) [8] . These spheroidal shapes have vertical axial symmetry; and the median zone usually has bi-concave surfaces on the hemispheres (Figure 1 (D0), 3) [7] [8] .

The elongated shapes have a polar diameter similar to spheroidal shapes, the maximum length of the measured elongation axis exceeds just 6 cm (Figure 1 (D6, D7), 4, 6). These elongated specimens consist to several forms: elongated forms in single “piece”; elongated forms with “bud” to several pieces, and elongated double and triple specimens (Figure 1 (D1-D6), 4, 5, 6) [7] [8] . They have a bipartite structure with two hemispheres separated by an internal medial surface, and a median disc often visible at one or both ends of the specimens in vertical section (Figure 4(B), Figure 4(C), Figure 4(M), Figure 4(D), Figure 4(E), Figure 4(F), Figure 7); this median disc is visible in each “new specimen” in the horizontal section of the median zone (Figure 4(I), Figure 4(L), Figure

5(H), Figure 5(L)).

The spheroidal forms gradually pass to bud elongated specimens which may consist of two to four bonded “pieces”, separated by hardly scored furrows (Figure 2 (n₂, n₅), Figure 4, Figure 5(G), Figure 5(I)), or consisting of two to three contiguous “new specimens” separated by “constriction furrows” of varying degrees of depth according to their degree of division (Figure 1 (D2-D6),

Figure 2 (n₁-n₅), Figure 5, Figure 6)).

Four types of “boundaries” and discontinuity are observed on the specimens: border, median surface, transverse constriction furrow (constriction zone), and vertical division plane in the elongated forms.

The border (outer) of the specimens is always net; it is net on contact with

sediments and on contact with other specimens (Figure 1(D), 2-8)). Two or more contiguous “independent” specimens are generally deformed at their contact surfaces (Figure 1 (D8), 8)) [7] [8] . An alignment of six specimens in chain, in the sedimentary plate P01, presents six joined and deformed specimens and a well-developed, undeformed specimen located in the middle of the chain (Figure 8(I)). The deformed specimens located on either side of the well-developed specimen have varying degrees of deformation. The deformations between specimens are essentially deformations by crushing due to lateral mutual compression between them; which is sometimes very accentuated and lead to significant volume reduction, matter spreading, flexion, micro-folding and sometimes rotation of hemispheres and median disc (Figure 8). Some specimens show deformations by more or less pronounced flattening of the upper hemisphere without deformation of the lower hemisphere (Figure 5(A), Figure 5(B), Figure 5(E), Figure 5(F)). Flattened shapes, due to vertical compression, are sometimes observed on some specimens (Figure 10(F)) [7] [8] .

The internal medial surfaces are located in the median zone, and laterally separate the two hemispheres from the median disc. Spheroidal forms generally have bi-concave surfaces on the median disc, or biconvex on the internal surfaces of the hemispheres (Figure 1 (D0), Figure 3(B) & Figure 3(C)), and usually horizontal between the two hemispheres in elongated form (Figure 4(C), Figure 4(G), Figure 4(J), Figure 4(M)), Figure 5(A), Figure 5(B), Figures 5(D)-(F), Figure 5(J), Figure 5(K)), Figure 6) [7] [8] .

The external furrows of “constriction” and the internal vertical division planes generally separate two contiguous “pieces” of the same elongated specimen containing several “pieces”. These two structures are always associated and form a partition between two contiguous “pieces” in a bud elongate specimens (Figure 4), the external furrows are very little marked, and are joined to the internal vertical division planes (Figure 4(C), Figures 4(E)-(G), Figure 4(J), Figure 4(M)). In elongated double and triple specimens, the external furrows and the “division planes” are more or less well developed depending on the degree of separation of the new specimens (Figure 5, Figure 6). The external furrow (which would correspond to a constriction) is growing deeper and the increasingly net separation between new specimens when passing from elongated shapes into a single “piece” to the double and triple specimens (Figure 5, Figure 6). The junction of the median discs of two new specimens in these double and triple specimens form an internal polygonal structure whose “division plane” is the symmetry plane (Figure 5(B), Figures 5(J)-(L)), Figure 6 (b, b₁, b₂, d, f, g)), (Movie 3). The triple elongated specimen (Figure 6) illustrate perfectly the different degrees of separation of the new specimens, with one side, an incomplete separation of the specimen and on the other, a distinct separation with formation of an ovoid new specimen with two hemispheres and a median disc. The case of Akouemma hemisphaeria OKO 3 in single “piece” is particular with two polygonal structures and two internal division planes which develop simultaneously (Figure 7(A), Figure 7(B)).