The structure of some Middle Cambrian conodonts and the early evolution of conodont structure and function

Elements presumed to belong to conodontophorids from the lower Middle Cambrian of Scania, Sweden, are investigated with emphasis on histology. The new genus Amphigeisina is described, with A. danica (V. Poulsen) as type species. The material also includes Hertzina? bisulcata Müller and Proconodontus? sp. These forms, here termed proto-conodonts, appear to have grown by basal-internal accretion, unlike elements of the essentially post-Cambrian Conodontiformes, but partly similar to the Late Cambrian para-conodonts. A proposed model of the early evolution of conodonts involves the following points: (1) Conodonts were primarily external elements, secreted by an ectodermal epithelium. (2) An evolutionary trend during the Cambrian involved gradual retraction of the elements into pockets of the epithelium. (3) Elements of the Conodontiformes, herein termed euconodonts, were completely engulfed in epithelial pockets, which enabled them to grow holoperipherally and thus to assume more complex shapes that could be retained during growth. (4) The secreting epithelium adhered only to the basal body of the euconodont, the conodont proper being extruded from the pockets when the apparatus was in function.