Comparative shell shape distributions in Jurassic-Cretaceous ammonites and Jurassic-Tertiary nautilids

Contours of whorl expansion rate (W), distance of the generating curve from the axis of coiling (D) and shape of the generating curve (S) were plotted from data on 988 Jurassic through Cretaceous ammonite species and 205 Jurassic through Tertiary nautilid species. The areas occupied by the ammonites and nautilids are largely nonoverlapping when plotted as WD and DS graphs.

The almost complete absence of overlap in Juro-Cretaceous ammonite and nautilid shell shape distributions is consistent with the view that ammonites and nautilids were dissimilar ecologically, and evolved different shell shapes in response to different adaptational needs. The terminal Cretaceous extinction of the ammonites may have opened up new opportunities for nautilid evolution during the Tertiary, because Tertiary nautilids are dominanted by moderately compressed, hydrodynamically efficient shell shapes which were rarely present among Jurassic and Cretacepus nautilids, but common among ammonites. The restriction of nautilids to a far more limited portion (compared to ammonites) of the theoretically available spectrum of planispiral shell shapes may be due in part to fabricational constraints imposed by the simple nautilid septum and septal suture. Nautilid shell shapes were limited to globular or slightly compressed to depressed shells, which are inherently strong due to pronounced shell wall curvature, and would be strengthened to only a small extent by buttressing from internal septa. Strongly depressed or compressed shell shapes, commonly evolved among ammonoids, had flattened shell regions which are inherently weak, and would be strengthened to a considerable extent by numerous thick, or strongly folded and fluted septa and septal sutures.