Fig. 5 in The origin of ammonoid locomotion

Fig. 5. Changes in the orientation of the aperture of the adult conchs of ten representative Early and Middle Devonian ammonoids and Recent Nautilus through phylogeny (Erbenoceras, Mimosphinctes, Convoluticeras, Mimagoniatites, Agoniatites, Ponticeras, Cabrieroceras, Holzapfeloceras, Pharciceras). The two graphs on the left are based on diagrams figured by Saunders and Shapiro (1985) and Okamoto (1996). Comments on the modifications of these graphs are given in Klug (2001) and Korn and Klug (2003). Shell thickness is impossible to determine in most Early and Middle Devonian ammonoids and thus the lines of correlation between WER, BCL, and OA are printed as broad lines in the graphs. Note the shift of the orientation of the aperture from oblique to more or less horizontal in the agoniatitid, anarcestid, and tornoceratid lineages. In the agoniatitid lineage (E, F), the horizontal position was achieved by an increase in whorl expansion rate (relatively short body chambers) compared to A and B. In the anarcestid lineage (G, I, K), the body chamber lengths first increased in the progress of evolution and subsequently decreased again, leading to moderate body chamber lengths (K, H) and consequently more or less horizontal apertures. The positions of Erbenoceras (A) and Mimosphinctes (B) are shown in grey because in their cases, the orientation of the aperture does not correlate with the body chamber length and thus whorl expansion rate, as it is the case for advolute, evolute, and involute species.