Lakes as laboratories of evolution; endemic fishes and environmental cyclicity

Lacustrine sedimentary sequences have tremendous potential for evolutionary studies because 1) the sedimentary record in lakes provides greater resolution than most other continental or marine sequences and 2) the lake environment seems conducive to speciation, as evidenced by the many examples of lacustrine species flocks. In this paper I present distributional data for 26 species of Semionotus in four temporally distinct lake deposits in the Newark Basin (Late Triassic-Early Jurassic). These data suggest that the effect of environmental cyclicity--the repeated cycles of formation and evaporation of lakes in the same basin--is not to accumulate species diversity over the long term. Rather, the effect is to decrease diversity when lakes evaporate and, alternately, to provide ecological opportunity during lake expansion, thus allowing the proliferation of species.