Comparison of the Calcareous Shells of Belemnitida and Sepiida: Is the Cuttlebone Prong an Analogue of the Belemnite Rostrum Solidum?

• Main Authors: M. Isabel Benito, Matías Reolid
• Format: Article
• Language: English
• Published: MDPI AG 2020-08-01
• Series: Minerals
• Subjects: crystal growth; organic matter; coleoids; microstructure; non-classical mineralization
• Online Access: https://www.mdpi.com/2075-163X/10/8/713

The microstructure of the rostrum solidum of Jurassic belemnites is compared with that of <i>Sepia</i> cuttlebones, in order to examine possible convergences in their style of growth. For this study, transmitted and polarized light, cathodoluminescence, epifluorescence, scanning electron and backscattered electron microscopy have been employed. Despite differences in the primary mineralogy of the studied belemnites and sepiids, calcite and aragonite, respectively, many similarities have been observed between the microstructure of the belemnite rostra and the prong of <i>Sepia</i> cuttlebone: (1) In both, crystals start growing from successive spherulites, from which crystals emerge radially towards the apex and the external walls, displaying internally micro-fibrous texture. (2) Both display concentric growth layering, comprising an alternation of organic-rich and organic-poor layers, which, in turn, is traverse by the radially-arranged micro-fibrous crystals. (3) The highest organic matter content and porosity have been observed along the apical area of the <i>Sepia</i> prong, similarly to that interpreted for belemnite rostra. The strong convergences observed suggest that the growth of belemnites occurred similarly to that of the prong of sepiids and that the <i>Sepia</i> prong is the analog of the belemnite rostrum. Additionally, non-classical crystallization processes are proposed to be involved in the formation <i>Sepia </i>endoskeleton.