Article: Internal mould markings in a Cretaceous ammonite from Nigeria

Linear and concentric markings are described from steinkerns of the Upper Cretaceous ammonite Paravascoceras from Nigeria. The markings are intimately associated with the lobules of the suture lines. They probably record adapical projections of a preseptal prismatic zone of shell material which were secreted by those parts of the mantle corresponding to the lobules during mantle translocation. The posterior mantle margin was under muscular attachment at these points during translocation and subsequent septal secretion. Translocation itself probably took place in two stages: an initial phase of rapid mantle movement during which liquid entering the new chamber space was derived from the main body tissues; and a second, longer, phase of gradual translocation during which existing cameral liquid was transferred to the new chamber space.