MORPHOLOGICAL DISPARITY AND ONTOGENETIC VARIABILITY IN AMMONOIDS

Ammonoids evolved a great variety of shell morphologies over their 350 million-year history, reflecting the ecological and evolutionary success of the group. Despite the abundance and diversity of ammonites in the Jurassic and Cretaceous, morphological disparity actually among planispirally-coiled forms actually peaked during the Paleozoic. Reduced disparity in younger ammonoids could arise from functional or ecological constraints, but also from developmental canalization potentially related to evolutionary bottlenecks suffered by the group. I investigated changes in the variability of ammonoid ontogeny using more than 11,000 digitized whorl sections, including ontogenetic series for more than 600 specimens, analyzed with elliptical Fourier analysis. Planispiral ammonoids preferentially become more compressed with narrower umbilical diameters during ontogeny, but ontogenetic trajectories do not cluster into discrete modes. Similar to trends in morphological disparity, ontogenetic variability was also greatest in Paleozoic ammonoids. Goniatites underwent more ontogenetic change in whorl section shape and had a greater variety of ontogenetic trajectories, whereas ammonites had short ontogenetic trajectories with less variability. Furthermore, goniatites had significantly more interspecies disparity even at juvenile stages (3-5 mm shell diameter) than ammonites, consistent with controls other than ecological or functional constraints at the adult stage. Given the correspondence between interspecific disparity and ontogenetic variability, developmental constraints may have been important in the evolution of ammonoid morphology.