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Stevens, Kevin; Griesshaber, Erika; Schmahl, Wolfgang W.; Casella, Laura A.; Iba, Yasuhiro; Mutterlose, Jörg (2017): Belemnite biomineralization, development, and geochemistry: The complex rostrum of Neohibolites minimus. In: Palaeogeography Palaeoclimatology Palaeoecology, Vol. 468: pp. 388-402
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Abstract

Belemnites, a group of extinct coleoid cephalopods, were important components of Mesozoic marine ecosystems. Their calcitic rostra are extensively used in biostratigraphy and geochemistry. The preservation state of the rostra is a crucial factor for the interpretation of their geochemical data. However, their preservation is difficult to assess, since no modern cephalopod forms calcitic rostra. The biomineralization process of the rostrum, which functioned as an internal skeleton, is poorly understood for most species. This also applies for the epirostrum, a late ontogenetic extension of the rostrum which is only present in some belemnite species. The primary structure and function of this character are still being debated controversially. We have investigated the microstructure, stable isotope (delta C-13, delta O-18), and elemental composition (Mg/Ca, Sr/Ca, Fe/Ca, Mn/Ca) of both, ortho-and epirostra of the belemnite species Neohibolites minimus (Early Cretaceous, Albian). For comparison, we studied argonauts (Argonauta argo). These recent octopuses build a calcitic shell. Ortho-and epirostra of N. minimus show microstructural and geochemical differences. Cathodoluminescencemicroscopy and the microstructures of belemnites and argonauts indicate that blue/orange luminescent growth lines in the studied belemnites are largely a primary biomineralization phenomenon and not solely due to diagenesis. Based in part on analogy to argonaut shell, we suggest that the inner part of the belemnite epirostrum is a continuation of an originally organic apical line. The development and evolution of the epirostrum as a potential sexually selected character is discussed. Well-preserved epirostrum calcite has higher element/Ca ratios than the calcite of the orthorostrum, suggesting a vital effect affecting these values. We further found evidence for vital effects on the stable isotope signatures due to metabolic and kinetic fractionation. Our findings suggest a complex mineralization process of the studied rostra. Different microstructures with distinctive geochemical signatures occur in a single belemnite species. Vital effects played an important role in the rostrum geochemistry, complicating the application of the rostra as paleoclimatic archives.