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1 Dept. of Geology, Field Museum of Natural History, Chicago, Illinois 60605, pwagner{at}fmnh.org
Twentieth century fossil gastropod systematics relied extensively on neontological paradigms. However, recent appreciation of the extant gastropod diversity suggests that those early paradigms provided very unsound models. This likely is a greater problem for Paleozoic taxa than for Meso-Cenozoic gastropods because Meso-Cenozoic taxa frequently have easily recognized extant relatives whereas Paleozoic taxa frequently do not. Also, many of the taxa that apparently diverged in the Paleozoic now are limpets and retain little information about the morphologies of their coiled ancestors.
Snails could be a model taxon for investigating macroevolutionary patterns because of the clade's dense fossil record. However, paleontologists usually study only adult shells (teleoconchs), and many malacologists maintain that teleoconch characters reflect phylogeny poorly if at all. This is important because many macroevolutionary hypotheses make their most specific predictions given phylogeny. Studies evaluating species- or genus-level relationships typically use more shell characters and states than do studies evaluating suprageneric relationships, as expected if shells evolve rapidly. Monte Carlo tests reject a null hypothesis that rates of homoplasy are equal among shell and soft-anatomy characters for two neogastropod clades, but suggest that these rates differ by less than an order of magnitude. Finally, teleoconch characters fail to unite bellerophontiform species with gastropod muscle scars but successfully unites clusters bellerophontiform species with tergomyan muscle scars. These results corroborate the conventional wisdom that teleoconch character distributions reflect abundant homoplasy, but the results also suggest that these distributions reflect phylogeny, too.
If we can control the effects of homoplasy, then gastropods are an excellent "model" group for testing macroevolutionary hypotheses such as changing rates of evolution. Two obvious candidates are rates of morphologic evolution among basal neogastropods, and rates of molecular evolution within clades radiating after the K/T mass extinction.
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