Taxonomic recognition of species ofNeogastroplites(Ammonoidea,Cenomanian) by geometric morphometric methods

Species ofNeogastroplitesdisplay a wide range of morphological variability such that withouta prioriknowledge of the stratigraphical location of some individuals, it is not possible to make an unequivocal specific determination on familiar traditional grounds. Using new techniques of geometric morphometric analysis it is possible to narrow down the margin for error in determining isolated specimens. We have also demonstrated that there are slight though significant differences in average lateral shape of the three species studied in this note:N. americanus,N. muelleriandN. cornutus. If counts of rib-frequencies are admitted into the analysis, the discriminatory effect is heightened. Unexpectedly, the addition of maximum whorl-breadth, a standard distance measure, to the analysis has a relatively slight positive effect on the results. A canonical analysis of the full data-set provides evidence of a gradual shift in shape and ornament over time.