Evolutionary trends in coiling of tropical Paleogene planktic foraminifera

Populations of planktic foraminifera display "proportionate" coiling (appro ximately 50% sinistral and dextral individuals given the data at hand) or m ay have "biased" coiling, in which populations are dominated by either sini stral or dextral individuals. The major radiations of planktic foraminifera in the Late Cretaceous, the Paleocene to early Eocene, the middle Eocene, and the Neogene were each initiated by clades with proportionate coiling bu t subsequently accumulated sinistral and dextral species over time. Upper M aastrichtian foraminifera were predominantly dextral, but only the small nu mber of species with proportionate coiling actually survived the Cretaceous /Paleogene mass extinction. The first Paleocene species with biased coiling appeared about four million years after the extinction and gradually came to represent as much as 50-60% of the tropical species diversity by the lat est Paleocene. Tropical taxa with biased coiling suffered a second extincti on in the late early Eocene and renewed a trend toward an increased abundan ce of species with biased coiling in the middle Eocene. Our results for the Paleogene reflect a recurring theme in foraminifer evol ution. In each radiation. once the founding species of a clade developed a biased-coiling mode, the descendants tended to maintain biased coiling unti l the extinction of the clade. The iterative evolution of biased coiling ap pears to represent an example in which a fundamental feature of development becomes fixed in a clade and inhibits reversion to an ancestral state. App arently, coiling patterns are heritable in contrast with previous interpret ations that coiling is environmentally controlled. On evolutionary timescal es, species with proportionate coiling are less susceptible to extinction t han species dominated by sinistral or dextral forms. Differential survivors hip ensures that each radiation is initiated from founders with proportiona te coiling following mass extinction. Hence, coiling preferences represent a case where the establishment of an evolutionary trend is caused by drift away from a "limiting boundary," much like the evolution of large body size from ubiquitous small ancestors.