Progressively towards the external walls of the rostra, the calcite crystals in the mosaic become elongated because they are cut tangentially (Figs.
Examination of thin-sectioned rostra under the BSEM reveals a similar pattern to that observed under TL and FL: the fluorescent sectors of the crystals are dark grey in color under BSEM, and the non-fluorescent sectors of the crystals has a light-grey color (compare Figs.
When the longitudinal section cross-cut the belemnite rostra along the apical line (Figs 5C-D, 10A-C), a concentric pattern of alternating layers with an overall dark-grey or light-grey color is observed, which is comparable to what was reported under FL, with intensely fluorescent layers being dark grey under BSEM and weakly fluorescent layers being light grey in color.
A similar pattern to what was observed under TL and FL is evident when observing longitudinal sections that do not cross-cut the belemnite rostra along the apical line.
Towards the external walls of the rostra, the dark-grey triangles become more elongated, creating a micro-fibrous appearance, similar to what was observed under FL (compare Figs.
Radial and outwardly thickening prismatic crystals that display an internal micro-fibrous texture, undulose extinction and concentric growth layering are observable under TL in transversal thin sections of the well-preserved and non-luminescent rostra (Fig.
Towards the external wall of the rostra, both the fluorescent, dark-grey radial sectors and the non-fluorescent, lightgrey radial sectors become more elongated or even fibrous in appearance (Fig.
This low Fe and Mn contents are coherent with the non-luminescent appearance of the analyzed belemnite rostra and with the elemental (ICP-MS) analyses performed by Benito and Reolid (2012), which recorded Fe and Mn values below 50 and 10 ppm, respectively, in non-luminescent portions of belemnite rostra.
Original microstructure andporosity of belemnite rostra
2003) studied Jurassic belemnite rostra under CL and noted that "recent CL-investigations have shown that locally the skeleton (of the belemnite rostra) and the former intraskeletal porosity can be distinguished" and that "further investigations of the reconstruction of the primary porosity would be important because belemnite and brachiopod skeletons are used for the reconstruction of the chemical and isotopic composition of seawater during Earth history".
Our observations of longitudinal fresh-cut and ultra-thin sections of the rostra showed that the calcite crystals in the rostrum solidum began growing from spherulithic sectors or spherulites, which successively originated along the apical line during the progressive growth of the belemnite rostra.
These sectors or zones display different shapes depending on whether the section is longitudinal, whether the section cross-cut the belemnite rostra along the apical line, or whether the section is transversal (Fig.