Tremadocian
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The Tremadocian is the first internationally-recognized stage of the Ordovician Period of the Paleozoic Era. It lasted approximately 9.7 million years, from about 488.3 ± 1.7 to about 478.6 ± 1.7 Ma (million years ago).
The Tremadocian began at the start of a major sea level transgression event. This is when sea levels rise and flood lowlands. Evidence from areas around the Baltic Sea and in Scandinavia indicates that there were three or four such events during the Tremadocian, with sea levels subsiding to various degrees between each event. In any case, shallow seas occupied a much greater area of the world’s surface than they do today.
The boundary chosen for the beginning both of the Ordovician period and the Tremadocian stage is highly useful. Since it correlates well with the occurrence of widespread graptolite, conodont, and trilobite species, the base of the Tremadocian allows scientists not only to relate these species to each other, but to species that occur with them in other areas as well. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period.
| Ordovician period | ||
|---|---|---|
| Lower/Early Ordovician | Middle Ordovician | Upper/Late Ordovician |
| Tremadocian | Stage 2 | Stage 3 | Darriwilian | Stage 5 | Stage 6 Hirnantian |
