Abstract:
In order to determine the timing of growth of belemnite rostra, we analyzed microgrowth patterns of seven, excellently preserved specimens of Megateuthis giganteus (v. Schloth.) from the Middle Jurassic of the Hannover area, Germany. Spectral analysis (single spectrum analysis, continuous wavelet transformation) of microgrowth curves suggests that the microgrowth increments and lines formed on a lunar daily basis. Microincrements are arranged in fortnight bundles of 15. Based on this interpretation, we estimated that the ontogenetic age of the studied specimens (at least the well-preserved visible portions) ranged between one and two years. Furthermore, chemical (energy dispersive spectrometry) and structural (X-ray diffractometry, scanning electron microscopy) analyses and monochromatic cathodoluminescence were employed to study the degree of diagenetic alteration, interpret the original mineralogical composition of the guards and explain the reason for the distinct alternation of dark and light laminae (microgrowth lines or rings). We found that the alternation of dark and light laminae in the rostrum is caused by regular changes in density of calcium carbonate rather variable organic content. Orthorostra were originally composed of low-Mg calcite rather than aragonite. The overall high carbon content (35 to 65% higher amounts than expected for pure calcite) indicates the presence of pristine intra-crystalline (and perhaps inter-crystalline) organic matrix. Despite the overall mint preservation, some portions of the rostra (stained blue by Mutvei's solution) have undergone diagenetic alterations such as cementation and/or recrystallization.
