Biological interaction between fossil species: character displacement in Bermudian land snails

Poecilozonites (Gastrelasmus) is an important component of the endemic land snail fauna of Pleistocene Bermuda. The type species P. circumfirmatus Redfield usually occurs in sympatry with its congener P. discrepans Pfeiffer, though each species is found alone at several localities. The species are less alike morphologically where they occur together than where they are allopatric. This allopatric convergence and sympatric divergence strongly suggests the biological interaction known as character displacement, often documented for living populations. The relatively complete fossil record of Bermuda offers advantages for studying this phenomenon. Collections can be made from a variety of microhabitats occupied through time. Statistical analysis of 1,600 individuals collected from more than 100 localities indicates that interspecific variation is primarily a function of the presence or absence of a congener and depends to a lesser degree on microhabitat. P. circumfirmatus undergoes a smaller morphological change between allopatry and sympatry than does P. discrepans. Study of relative abundances suggests that P. discrepans may have been competitively inferior, though no functional bases for differences between the species are known.