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Palaeozoic |
Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA
Studies by several workers show that major extinctions of trilobites, echinoderms, brachiopods and gastropods (but not conodonts) occurred at the horizon of a middle Ordovician K-bentonite (Deicke metabentonite) in the north central United States. Extinctions of Ostracoda are also represented at the Deicke horizon. The metabentonite, a Rocklandian stage ash fall the age of which is approximately 454 Ma, middle Caradocian, originated in the eastern United States.
About 160 species of Ostracoda have been recorded in the middle and upper Ordovician of the north central USA. Ostracode speciation was low prior to the Deicke extinction event, consisting of a few eoleperditiids, leperditellids, drepanellids, aparchitids and bairdiids of the Glenwood and Platteville formations (Blackriveran-early Rocklandian). Following the extinction event, during the next approximately 1.5 Ma, 85 ostracode species appeared in the uppermost Platteville Limestone and in the overlying Decorah Shale (late Rocklandian-early Shermanian). The known Ostracoda species durations averaged about 2 Ma, although some are only half that length of time.
Additional episodes of ash falls, following the Deicke and associated episodes, occurred in the next approximately 7.5 Ma in the region. The preceding Decorah ostracode fauna disappeared from the area and about 50 recorded new or immigrant taxa appeared following the last volcanic event in the late Edenian and Maysvillian stages (Dubuque and Maquoketa formations). Species duration during this time interval apparently averaged less than in the Decorah, about 1 Ma.
The volcanic episodes appear to have provided nutrient-rich habitats that resulted in evolutionary spurts in the Ostracoda, following at least two of the episodes. Duration of the species was perhaps related to the total nutrient budget provided by the ash falls as well as to salinity regimes, palaeoclimates and other factors.
