Abstract:
Populations of planktic foraminifera display "proportionate" coiling (approximately 50% sinistral and dextral individuals given the data at hand) or may have "biased" coiling, in which populations are dominated by either sinistral 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 but subsequently accumulated sinistral and dextral species over time. Upper Maastrichtian foraminifera were predominantly dextral, but only the small number 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 latest Paleocene. Tropical taxa with biased coiling suffered a second extinction in the late early Eocene and renewed a trend toward an increased abundance of species with biased coiling in the middle Eocene. Our results for the Paleogene reflect a recurring theme in foraminifer evolution. In each radiation, once the founding species of a clade developed a biased-coiling mode, the descendants tended to maintain biased coiling until the extinction of the clade. The iterative evolution of biased coiling appears to represent an example in which a fundamental feature of development becomes fixed in a clade and inhibits reversion to an ancestral state. Apparently, coiling patterns are heritable in contrast with previous interpretations that coiling is environmentally controlled. On evolutionary timescales, species with proportionate coiling are less susceptible to extinction than species dominated by sinistral or dextral forms. Differential survivorship ensures that each radiation is initiated from founders with proportionate 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.