A TALE OF TWO SNAILS: TESTING LIMITING SIMILARITY IN QUATERNARY THEBA

The hypothesis of limiting similarity, which postulates that morphologically similar sympatric species will differ enough in shape/size to minimize competition, has been controversial among ecologists and paleoecologists. Whereas the hypothesis has been indirectly supported by snapshot studies of modern populations and individual fossil sites, empirical time series demonstrating the persistence of limiting similarities over longer time scales are lacking. We have integrated radiocarbon-calibrated amino acid dating techniques, stable isotope estimates, and morphometric data to test the hypothesis of limiting similarity in late Quaternary land snails from the Canary Islands over a period of 27,600 yrs. Multiple proxies of body size consistently show that two endemic congeneric pulmonate gastropod species (Theba geminata and T. arinagae) maintained a difference in size from ~42,500 BP through the last occurrence of T. arinagae 14,900 BP, with a concomitant trend of a decreasing body size, possibly related to climate change. Theba geminata displayed volatile fluctuation in body size after T. arinagae went extinct. The morphology of T. geminata did not converge on that of T. arinagae following its extinction, therefore character release did not occur and the hypothesis of limiting similarity is not supported.