837. ONTOGENETIC SHELL VARIATION AND STREAMLINING OF SOME CRETACEOUS AMMONITES

Abstract

Comparative shell shape analysis of some Cretaceous ammonites has provided significant differences in the ontogenetic pattern and variation of geometric coiling parameters. The differences are especially conspicuous in the distance of venter from coiling axis (D), relative whorl thickness (S) and flank position (F), all of which are key factors for considering hydrodynamic efficiency of the shells. In every species, the ranges of variation of the geometric parameters tend to decrease with age. At the same growth stage, intraspecific variation of parameters S and D is largest in the heavily ornamented morphotypes (Acanthocerataceae) and heteromorphs (Scaphitaceae), imermediate in fine-ribbed platycones (Lytocerataceae), and smallest in the weakly ribbed, highly streamlined involute-compressed morphotypes (Hypophylloceras, Placenticeras and most Desmocerataceae). This evidence strongly suggests a wide variety of adaptive designs of the shells related to the mode of life of ammonites.