古生代放散虫の分類と消長史

7回の絶滅事件

抄録

The Paleozoic Radiolaria, a major zooplankton group with a siliceous skeleton, comprises 147 genera, 29 families, and 5 suborders. The compilation of 270 literatures on secular diversity change of Paleozoic radiolarians clarifies 7 distinct radiolarian extinction events : Event 1 at the Tremadoc/Arenig boundary (Early Ordovician), Event 2 at the Llandovery/ Wenlock boundary (Early Silurian), Event 3 at the Late Ludlow, Event 4 at the Early/Middle Devonian boundary, Event 5 at the Frasnian/Famennian (F/F) boundary, Event 6 at the Devonian/Carboniferous (D/C) boundary, and Event 7 at the Early/Late Carboniferous boundary.
The Paleozoic radiolarians are divided into 3 major groups : a spherical-shell group composed of Spumellaria and Entactinaria ; a cylindrical to conical-shell group composed of Albaillellaria and Nassellaria; and, a flattened-shell group composed of Stauraxonaria. The spherical-shell group declined remarkably at Events 1, 2, 3, 4, and 5, while the cylindrical to conical-shell group declined at Events 6 and 7. The spherical-shell group dominated the Late Paleozoic epicontinental realm, while the cylindrical to conical-shell group dominated the pelagic realm. Events 1-5 represent diversity loss of radiolarians in the epicontinental habitat, while Events 6 and 7 represent diversity loss of pelagic inhabitants. Events 6 and 7 coincide with the timings of continental glaciation, suggesting that the decline of the pelagic radiolarians may have been related to global cooling.
It is noteworthy that a significant extinction of radiolarians occurred at the F/F boundary (Event 5), although Radiolaria has previously been regarded to have flourished even across the boundary.
Events 1 and 4 are the most significant radiolarian extinction events among 7 events, and occurred synchronously with faunal changes of other planktonic biota. Although terrestrial and benthic records suggest minor crises at these events; a global turnover is emphasized for marine planktonic biota.