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It correlates well with the occurrence of widespread graptolite, conodont, and trilobite species.The base (start) of the Tremadocian allows scientists to relate these species not only to each other, but also to species that occur with them in other areas.The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column.The faunal stages (subdivisions of epochs) from youngest to oldest are: Late Ordovician The Tremadoc corresponds to the (modern) Tremadocian.

Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction event.It lasted for about 42 million years and ended with the Ordovician–Silurian extinction event, about 443.8 Mya (ICS, 2004) which wiped out 60% of marine genera.The dates given are recent radiometric dates and vary slightly from those found in other sources.Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole.Early in the Ordovician, the continents of Laurentia (in present-day North America), Siberia, and Baltica (present-day northern Europe) were still independent continents (since the break-up of the supercontinent Pannotia earlier), but Baltica began to move towards Laurentia later in the period, causing the Iapetus Ocean between them to shrink.Carbonate hardgrounds were thus very common, along with calcitic ooids, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons.

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