Fig. 9 in The dorsal shell wall structure of Mesozoic ammonoids

Fig. 9. Occurrences of dorsal shell wall types in Mesozoic ammonoid superfamilies (after Rouget et al. 2004; cf. Tables 1, 2, SOM: table A). The wide distribution of reduced dorsal shell walls in Mesocoic taxa suggests a plesiomorphy. In general, nacreous reduced dorsal shell walls or complete dorsal shell walls follow a stage of a prismatic reduced dorsal shell wall. The wide distribution of nacreous reduced dorsal shell walls and complete dorsal shell walls in Mesozoic ammonoid taxa suggests that the ability to form dorsal nacre is also a plesiomorph feature. Note: The dorsal nacreous layer of Eoderoceratoidea (Amaltheidae) has a prismatic appearance (*). The complete dorsal shell walls in Stephanocertatoidea, Haploceratoidea, and Hoplitoidea lack a dorsal nacreous layer, i.e., seemingly complete dorsal shell wall (**). The complete dorsal shell walls of Anclyceratoidea and Douvilleiceratoidea can be reinforced by additional pair of nacreous and prismatic layers, i.e., reinforced complete dorsal shell wall (***).