ECOLOGICAL, CONSTRUCTIONAL, AND BIOTIC CONTROLS ON LONG-TERM TRENDS IN AMMONOID SHELL DISPARITY

Ammonoids were common members of the plankton over their 350 million-year history, evolving a vast range of shell shapes that reflect, at least in part, aspects of their ecology. The shape of non-heteromorph ammonoid shells can be described by simple geometric parameters, enabling quantification of trends in disparity over the evolutionary history of the group. To assess disparity, I performed principal components analysis on shell dimensions and on elliptical Fourier coefficients of digitized aperture outlines. Shell-shape disparity peaked in the Early and Middle Permian, dropping precipitously with the shift to ceratite-dominated faunas in the Late Permian and again with the evolution of ammonites in the Jurassic. Despite the high diversity of Mesozoic ammonites, disparity never recovered to Paleozoic values. Disparity was low in the Mesozoic because non-heteromorph ammonites rarely re-occupied areas of the morphospace characterized by high width:diameter ratio shells, areas that were routinely occupied by goniatites in the Paleozoic. Changes in ammonoid disparity are consistent with decreasing disparity observed in numerous other groups, but potential causes remain elusive. Decreasing disparity may reflect ecological constraints, perhaps because ecological niches were instead filled by heteromorph ammonites or by non-ammonoid competitors. Disparity trends may also respond to constructional limitations, due to the larger size of ammonites or other factors, or intrinsic biotic controls on shell growth in different lineages.