Insights into mineral composition and ultrastructure of Jurassic sabellid tubes (Annelida, Polychaeta): the evolution of sabellid calcification and its palaeoecological implications

The mineral composition and tube ultrastructure of the Middle and Upper Jurassic calcareous sabellid Glomerula gordialis (Schlotheim, 1820) from the Polish Basin have been assessed. In all cases, the Jurassic sabellid tubes studied were found to consist of low-Mg calcite, as revealed by Raman spectroscopy and supplementary SEM-EDS analyses. The tube of Glomerula gordialis is single-layered and its ultrastructure has been identified as a spherulitic prismatic structure (oriented prismatic structure), supporting earlier ultra-structural observations of calcareous sabellids. The ambient seawater chemistry largely influenced the formation and evolution of sabellid skeletal mineralogy, and their mineral composition at the times of their first appearance presumably corresponded to the Permian aragonitic seas. The earliest skeletons of each calcareous polychaete group are considered to have autonomously evolved according to the chemistry of the seawater at the time of their origin. The relatively simple biomineralization system of sabellids is similar to that of calcareous cirratulids and remains less complex compared to serpulids. Physiologically less expensive formation of the calcareous tube in sabellids implies their palaeoecological avoidance strategy, allowing for a faster dispersal over the substrate during potentially unfavourable conditions.