Fig. 4

The caecum of Cyrtobelus hornbyense, Groenlandibelidae, Upper Cretaceous (upper Campanian), Canada. a Apical view of the open protoconch showing the caecum; scale bar = 10 µm. b Longitudinal section of the protoconch/teleoconch transition showing the caecum in lateral view; scale bar = 10 µm. The caeca (ca) of an ammonoid, Upper Cretaceous, Hkkaido, Japan (c) and Nautilus, recent (d); scale bars: d = 1 mm; c = 100 µm. e The caecum-less protoconch of Holcobelus munieri, Belemnitida, Middle Jurassic, NW France. Note a closing membrane is absent, although the likewise organic pellicula (“protoconch pockets” after Doguzhaeva et al. 2014) is preserved; pe Pellicula, ps proseptum; scale bar = 500 µm