Fig. 3. Reconstructions and a in The origin of ammonoid locomotion

Fig. 3. Reconstructions and a simplified cladogram of one bactritid and nine primitive ammonoids from Early and Middle Devonian (from left to right: Lobobactrites, Cyrtobactrites, Kokenia, Metabactrites, Anetoceras, Erbenoceras, Chebbites, Talenticeras, Mimagoniatites, Agoniatites). Note the change in the orientation of the aperture and the increase of soft body volume in relation to the conch diameter. The morphology of the soft body is largely speculative. The number and proportion of arms, however, is here supposed to have been similar to coleoids, because of similarities in embryonic shell, radula and beak morphology between ammonoids and coleoids (Landman et al. 1997; Tanabe and Fukuda 1999). Additionally, the presence of a hood as in Recent Nautilus is presumed based on the absence of jaw apparatuses in early ammonoids which were suitable as a lid for the aperture. In the cladogram (modified after Korn 2001, see this article also for the character matrix) with the most important evolutionary steps among Devonian ammonoids, those taxa not discussed in detail are marked with an asterisk.