Apatitic connecting rings in moulds of Baculites sp. from the middle part of the Smoky Hill Member, Niobrara Chalk (Santonian), of western Kansas

Moulds of Baculites sp. are common in the Smoky Hill Member but only five known specimens contain connecting rings that have been preserved because of mineralisation by carbonate apatite. Analysis of four of these specimens suggests that the connecting rings were originally composed of organic material and were mineralised during early diagenesis. Thin sections and scanning electron microscopy demonstrate that the connecting rings had a two-layered structure consisting of a thick siphuncular wall and a thin pellicle.