During the last half million years, pulses of gigantism in the anagenetic lineage of land snails of the subgenus on Bermuda were correlated with glacial periods when lower sea level resulted in an island nearly an order of magnitude larger than at present. During those periods, the island was colonized by large vertebrate predators that created selection pressure for large size and rapid growth in the snails. Extreme reduction in land area from rising seas, along with changes in ecological conditions at the onset of interglacial episodes, marked extinction events for large predators, after which snails reverted to much smaller size. The giant snails were identical in morphology during the last two glacials when the predators included a large flightless rail (marine isotope stages (MIS) 4-2) and a crane and a duck (MIS 6). In a preceding glacial period (MIS 10), when the fauna also included the tortoise , the snails were not only large, but the shells were much thicker, presumably to prevent crushing by tortoises. Evolution of provides an outstanding example of dramatic morphological change in response to environmental pressures in the absence of cladogenesis.
