Observations in thin and ultra-thin sections of the belemnite rostra
When examining longitudinal thin sections of well-preserved belemnite rostra under transmitted light microscopy (TL), the growth pattern of the belemnites is commonly observable, although the calcite in the apical region typically has cloudy appearance (Fig.
(2014), who compared the apical spine or prong of some Sepia cuttlebone with belemnite rostra, interpreting that the Sepia prong is a good analog for belemnite rostrum.
2, 3, 5E-F), the spherulitic part of the microstructure is no longer observable, and an equigranular mosaic of sub-euhedral and locally euhedral calcite crystals become recognizable in the middle sector of the rostra, which are the product of transversally cut prismatic calcite crystals (that is, perpendicular to the c-axis).
The internal zonation of the calcite crystals is also observable when the longitudinal thin sections cross-cut the belemnite rostra offset from the apical line (Fig.
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.
Identifications are based primarily on comparing rostral fragments with complete rostra from Recent fish where emphasis is placed on cross-sectional morphology at 0.25L and 0.5L, two areas that must be estimated in fossil fragments.
Table 1 Measurements and ratios of partial rostra of five specimens of the Family Istiophoridae from the San Jose del Cabo Basin, Trinidad Formation (upper Miocene to late Pliocene), Baja California Sur, Mexico.