Seeking a better life in the plankton.

F oraminifers are single-celled, testate protists with a rich geologic record spanning at least 500 million years of Earth’s most recent history. Their abundance, diversity, and diagnostic distribution patterns make them very useful to Earth scientists for purposes of relative age determination, correlation from one locality to another, and paleoenvironmental reconstructions. Benthic foraminifers occupy nearly every conceivable marine habitat ranging from coastal salt marshes and estuaries to marginal silled basins and deep-sea trenches. They range from pole to pole and are particularly diverse and abundant in shallow waters of the tropics. Planktic foraminifers have a shorter geologic history that begins 180 Ma in the late Early Jurassic. Traditional classification has assigned all of the planktic forms to a single suborder Globigerinina (1). Such a classification scheme implies a monophyletic origin for the planktic foraminifers, although many specialists have long suspected a polyphyletic origin from benthic ancestors. In a recent issue of PNAS, Darling et al. (2) provide compelling evidence to further support the polyphyletic hypothesis, while also demonstrating a very peculiar dual mode of life for one particular species of foraminifer. Here, for the first time, Darling et al. conclusively demonstrate that the benthic Bolivina variabilis and planktic Streptochilus globigerus are one and the same species. This taxon can actively grow in seafloor sediments of a continental shelf or in surface waters of the open ocean. The existence of such a tychopelagic mode of life in foraminifers provides a glimpse at a possible mechanism by which planktic lineages may have evolved multiple times from benthic ancestors since Jurassic time. The Earth has experienced a dynamic history of global change during the Mesozoic and Cenozoic eras (past 200 million years). As the Supercontinent Pangaea broke apart, new ocean basins were created, oceanic gateways opened and others closed, new mountain belts formed, and the redistribution of land and sea altered global climate and heat dispersal around the planet. Plate tectonic activity, chemical weathering, ocean circulation, and metabolic processes of the biosphere have modulated atmospheric greenhouse gas concentrations, while orbital forcing has modulated incoming solar radiation. Importantly, global climate change has been both gradual and abrupt. During this time, foraminiferal evolution occurred in the context of global warming and cooling, growth and decay of continental ice sheets, fluctuating global sea level including the creation of vast epicontinental seas, and major events such as the Oceanic Anoxic Events (OAEs) of the Mesozoic, an eradefining bolide impact and mass extinction at the Cretaceous–Paleogene boundary (65.5 Ma), and the Paleocene–Eocene Thermal Maximum (PETM; 55 Ma). The well-known iterative pattern of planktic foraminiferal evolution (3) points to multiple turnovers in planktic foraminiferal communities with subsequent opportunities to invade and repopulate the pelagic realm (Fig. 1). Phytoplankton evolution accelerated during the Mesozoic as dinoflagellates, and coccolithophorids rose to dominate marine phytoplankton communities (4–6). Sea level rose during this time, peaking in the Late Cretaceous with the creation of vast epicontinental seas (7). At times, low oxygen waters of the oxygen minimum zone (OMZ) shoaled into the neritic zone and spread into epicontinental seas during global OAEs (8). This was also a time of