The search for the origin of the planktic Foraminifera

Abstract

Planktic Foraminifera are an extremely abundant, important and successful group of marine protists. They are particularly useful in reconstructing past environments and for biostratigraphic dating. Despite their importance, the origin of the group is uncertain. Previous work has suggested that they evolved from a benthic ancestor during the Triassic or, perhaps, the Mid-Jurassic (?Bajocian), but a reason for their origination has remained unclear. Here, we present evidence from the Toarcian (early Jurassic) of NW Europe that the origin of the planktic Foraminifera may have been one of the results of the early Toarcian oceanic anoxic event. This event appears to have been associated with a massive dissociation of gas hydrates and other, perhaps related, water chemistry changes.

Surviving mass extinction by bridging the benthic/planktic divide

“…Some early planktic foraminifera have been described as living in inner shelf as well as more open ocean regions (24,35,36), but the possibility that a tychopelagic lifestyle exists (as we demonstrate for B. variabilis/S. globigerus) has not been taken into account in phylogenetic studies based on the planktic foraminiferal fossil record, including the discussion of the first evolution of planktic foraminifera in the Jurassic as well as radiations in the Early Cretaceous (13,24,(35)(36)(37)(38)(39) Our view of biserial foraminifera as potentially tychopelagic species requires another look at the planktic environment and pattern of survival after the Cretaceous mass extinction, as well as the Mesozoic and Cenozoic evolution of microperforate foraminifera (8,35). All species surviving the end-Cretaceous mass extinction have been described as nerito-plankton, living predominantly in coastal, relatively shallow waters, although no evidence has been presented that these species were actually living as plankton, i.e., floating in the water column, rather than as bottom-dwellers in the neritic environment (SI Text).…”

“…Traditionally, all planktic foraminifera have been seen as monophyletic [Suborder Globigerinina (12)], descended from a single Early-Middle Jurassic ancestor (13), similar to the monophyletic origins of other planktic groups (14). It has, however, not been possible to derive a satisfactory cladogram on fossil data, and the current foraminiferal molecular phylogeny has limited resolution for the prediction of deep-time ancestral relationships.…”

Seeking a better life in the plankton

“…The first planktic foraminiferal genus, Conoglobigerina, possesses a microperforate, aragonitic wall and was likely derived from an aragonitic benthic of the Family Duostominidae (11,12). Planktic foraminifers of the Jurassic occupied shallow seas and hugged the margins of Tethys (12)(13)(14).…”

“…Planktic foraminifers of the Jurassic occupied shallow seas and hugged the margins of Tethys (12)(13)(14). They expanded their range in the Early Cretaceous (15), but then experienced an explosion of diversity and morphologic form during the mid-Cretaceous that continued through the Late Cretaceous (e.g., refs.…”

“…OAEs of the Jurassic and Cretaceous may also have been important contributing factors in some benthic foraminifera evolving a planktic mode of life. Hart et al (12) suggested that the evolution of the earliest planktic foraminifers during the late Early Jurassic may have been related to the development of an OAE in the Toarcian.…”