Tube structure and ultrastructure of serpulids from the Jurassic of France and Switzerland, its evolutionary implications

Four different tube ultrastructures were identified in the eight studied Jurassic serpulid species. The simple prismatic structure of single layered Serpula aff. convoluta and Tetraserpula planorbiformis, and the spherulitic prismatic structure of outer tube layer in S. aff. trigona are interpreted as the original tube ultrastructures. The fine homogeneous granular structure in Mucroserpula quinquangularis, Serpula conformis, S. limata, S. aff. subcrispa, S. aff. trigona and coarse angular crystal homogeneous structure in Filogranula tricristata are interpreted as diagenetically altered structures. The diagenetically altered tube ultrastructure is common in the Jurassic serpulids, which can be explained by their original aragonitic or dominantly aragonitic composition. There is no evidence of complex oriented tube ultrastructures in the Jurassic, such as calcitic lamello-fibrillar structure, which may have evolved later in the Cretaceous or early Cenozoic.