Abstract:
The last major turnover in deep-sea benthic foraminifera (the Stilostomella extinction) is documented in detail in six DSDP and ODP sites around New Zealand, South-west Pacific. This was the final phase in the progressive decline of elongate, cylindrical taxa (mostly stilostomellids, pleurostomellids and uniserial nodosariids), which reached their greatest relative abundance in the late Eocene, and exhibited major declines during periods of global cooling around the Eocene-Oligocene boundary, in late middle Miocene, and through the late Pliocene to middle Pleistocene. The Stilostomella extinction includes the extinction of all elongate species with cribrate (Chrysalogonium, Cribronodosaria), slit lunate, hooded with two teeth. (Pleurostomellidae), or secondarily toothed, necked (Stilostomellidae) apertures. Elongate and uniserial benthic foraminifera are separated into: an Extinction Group (53 taxa, including several elongate agglutinated and uvigerine forms, that became extinct in the South-west Pacific during the late Pliocene-middle Pleistocene); a Die-back Group (8 species that dramatically declined in abundance but survived to the Recent); and a Survivor Group (88 mostly rare species of uniserial nodosariids with little recognised decline). In the South-west Pacific, the absolute abundance of the Extinction and Die-back groups began to decline in the late Pliocene. The decline became more dramatic during the late early and middle Pleistocene (1.2-0.7 Ma). The rate of decline was pulsed, with major declines usually associated with the onset of cold intervals, and partial recoveries in intervening warm intervals. The pulses varied in timing between sites. The highest occurrences (HOs or local disappearances) of individual Extinction Group species are variable and mostly diachronous between sites. There was a progressively increasing overall rate of local disappearances per time through the late Pliocene (onset of northern hemisphere glaciation) to the middle Pleistocene climatic revolution, with the peak period of local disappearances (mostly 0.9-0.7 Ma) up to 0.5 Ma earlier at deeper and cooler water locations. The youngest occurrence of any member of the Extinction Group (Stilostomella extinction datum) is remarkably consistent in all sites (0.65-0.57 Ma). The timing of these abundance declines, highest occurrences (or withdrawals) and extinctions was essentially the same as in the Atlantic Ocean. The precise mechanistic cause of the Stilostomella extinction (cooling, increased oxygenation of bottom waters, food supply changes) is yet to be resolved. This study reveals a much larger extinction of taxa than previously recorded (middle Pleistocene extinction rate of 23% of the bathyal-upper abyssal fauna/myr). Becoming extinct, or virtually so, during this period were at least two families (Stilostomellidae, Pleurostomellidae), one subfamily (Plectofrondiculariinae), at least 17 genera (Awhea, Chrysalogonium, Cribronodosaria, Ellipsoglandulina, Ellipsopleurostomella, Ellipsopolymorphina, Haeuslerella, Mucronina, Myllostomella, Nodosarella, Orthomorphina, Parafrondicularia, Pleurostomella, ?Rectuvigerina, Siphonodosaria, Stilostomella, Strictocostella) and 53 species. Their taxonomy is reviewed, revealing many synonymies, often going back to Schwager's (1866) pioneering study of Pliocene deep-sea foraminifera of Car Nicobar, Indian Ocean. A revised generic subdivision of the Stilostomellidae is proposed based primarily on apertural features, and Myllostomella n. gen. described.