Meeting Abstract

P3.36  Jan. 6  Disparity of planispiral ammonites in the Paleozoic SWIDERSKI, D. L.; Univ. of Michigan, Ann Arbor dlswider@umich.edu

Ammonites are cephalopod molluscs with coiled, chambered shells, similar to Nautilus. Many have approximately regular planispiral coiling patterns, meaning that their rate of whorl expansion (W), aperture shape (S), and relative distance of aperture from coiling axis (D) are nearly constant over ontogeny. Because of that regularity, ammonites are useful models of spiral forms, from the shells of other molluscs, to the cochleae of therian mammals. Previous quantitative studies of ammonite disparity have focused on analysis of the ratios describing the parameters W and D, and implicitly treat aperture shape as an ellipse that is adequately represented by the ratio of its major axes. This approach can bias analyses of morphological disparity because the scale of ratios is nonlinear, and because the simplification of aperture shape underestimates the disparity of shell shape. In this study, images of 40 genera of Paleozoic ammonites, sampled at 9 points around the aperture, were analyzed using geometric morphometrics. Two other points were included to enable estimation of W and D. These 11 points were used to compute aperture outlines at one half turn and one full turn back toward the embryonic shell. Disparity was not significantly different between the Devonian, when ammonites arose, and the Mississippian or the Permian, but was much lower in the Pennsylvanian. In all periods, mean nearest neighbor distances (NND) are significantly lower than expected, indicating genera are more tightly clustered than expected for a random distribution. Like disparity, mean NND was lowest in the Pennsylvanian. In contrast, the number of genera was highest in the Pennsylvanian. Thus, the range of ammonite morphologies apparently did not change greatly during the Paleozoic, but was more densely occupied in the Pennsylvanian than in other periods.