Ecology of a Shell Convergence Between Subfamilies of Polygyrid Land Snails

Emberton, Kenneth

ABSTRACT: The shell convergence between Neohelix dentifera and Inflectarius ferrissi is an important key to understanding the unusual evolutionary pattern of polygyrid land snails in eastern North America. This group is characterized by conchologically distinct shell-static clades that have radiated ecologically to the extent that shell shape and habitat are not detectably correlated at the species level; furthermore, entire shell-static clades converge conchologically between anatomically distinct lineages (triodopsines and polygyrines). The triodopsine N. dentifera and the polygyrine I. ferrissi both represent unique, drastic shifts from their shell-static clades, and thus are possible founders of new shell-static clades. This study shows that, within a context of high intraspecific variance, N. dentifera and I. ferrissi exhibit ecological parallelism or convergence in addition to conchological convergence. In coarse-grained ecology, both are more associated with acidic, anionic, deep, low- density soils in talus in high-altitude, exposed non-oak-hickory forests than are their close relatives, even though the two are not very close to each other. In fine-grained ecology, N. dentifera converges on I. ferrissi by increased rock-association relative to outgroups, and I. ferrissi likewise converges on N. dentifera by decreased log association. Thus the shell shape unique to these two species appears to be adaptive, whereas their intraspecific ecological variances appear high enough to permit ecological radiation.