Combined Electron Backscattered Diffraction (EBSD) and SEM-EDX Investigation of the Calcitic Annelid Tube of Ditrupa (Annelida, Serpulidae)

The calcareous tubes of the annelid worm Ditrupa are known from the Eocene to present-day marine settings. It has a distinctive and unique outer layer comprising regularly ridged prismatic fabric (RRP), and a more chaotic inner layer of semi-oriented, needle-to-thread-like calcite, simple preferentially orientated prismatic (SPOP) fabric. We use electron backscattered diffraction (EBSD) analysis in conjunction with scanning electron microscopy (SEM) backscattered (BSE) imaging and energy dispersive X-ray (EDX) mapping in the study of the tube structure of (?)Eocene Ditrupa sp. The fabric of both layers can often be observed even within the older fossil materials. EBSD Inverse Pole Figures and pole figure plots show that the crystallographic c-axis is orientated parallel to both of the elongate calcite crystals of the two fabric layers, although orientation between the two layers is perpendicular to each other. The crystals and c-axis of the RRP fabric is approximately perpendicular to the outer tube margin, while those in the SPOP fabric are more distributed with an orientation parallel to the tube circumference. The current work confirms the single crystal origin of RRP fabric crystals and indicates that these grew incrementally and were likely controlled by an organic framework. The inner layer fabric is also shown to be unique to Ditrupa, and although it has elements similar to lamello-fibrillar (LF) and irregular orientated prismatic (IOP) fabrics, should be assigned to simple preferentially orientated prismatic (SPOP) fabric previously noted from Ditrupa bartonensis. Given the range of unusual features associated with the latter fabric, it may also at least in part be controlled through an in situ organic framework. Therefore, both layers of the tube of Ditrupa may be unique amongst the annelids, as well as being organic framework controlled or influenced.