Surviving mass extinction by bridging the benthic/planktic divide

Via PNAS Early Edition:
Kate F. Darling, et. al. provide evidence for a tychopelagic existence for the planktic foraminifera Streptochilus globigerus citing its relation to the benthic Bolivina variabilis. The evolution from a benthic to planktic existence was generally viewed as a one-time event per clade. This finding shows the possibility that the tychopelagic foraminifera have the capability to quickly recolonize the planktic domain after extinction events.

Evolution of planktic organisms from benthic ancestors is commonly thought to represent unidirectional expansion into new ecological domains, possibly only once per clade. For foraminifera, this evolutionary expansion occurred in the Early–Middle Jurassic, and all living and extinct planktic foraminifera have been placed within 1 clade, the Suborder Globigerinina. The subsequent radiation of planktic foraminifera in the Jurassic and Cretaceous resulted in highly diverse assemblages, which suffered mass extinction at the end of the Cretaceous, leaving an impoverished assemblage dominated by microperforate triserial and biserial forms. The few survivor species radiated to form diverse assemblages once again in the Cenozoic. There have, however, long been doubts regarding the monophyletic origin of planktic foraminifera.

We present surprising but conclusive genetic evidence that the Recent biserial planktic Streptochilus globigerus belongs to the same biological species as the benthic Bolivina variabilis, and geochemical evidence that this ecologically flexible species actively grows within the open-ocean surface waters, thus occupying both planktic and benthic domains. Such a lifestyle (tychopelagic) had not been recognized as adapted by foraminifera. Tychopelagic are endowed with great ecological advantage, enabling rapid recolonization of the extinction-susceptible pelagic domain from the benthos. We argue that the existence of such forms must be considered in resolving foraminiferal phylogeny.

Published online before print July 2, 2009, doi: 10.1073/pnas.0902827106

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