Ordovician
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The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 43.75 million years from t
External mold of the Ordovician bivalve Anomalodonta gigantea showing that the original aragonite shell dissolved on the sea floor, leaving a cemented mold for biological encrustation (Waynesville Formation of Franklin County, Indiana).
A diorama depicting Ordovician flora and fauna
Fossiliferous limestone slab from the Liberty Formation (Upper Ordovician) of Caesar Creek State Park near Waynesville, Ohio.
The trilobite Isotelus from Wisconsin
Silurian
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The Silurian is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at 443.1 Ma to the beginning of the Devonian Period, 419.62 Ma. The Silurian is the thir
Ordovician-Silurian boundary on Hovedøya, Norway, showing brownish late Ordovician mudstone and later dark deep-water Silurian shale. The layers have been overturned by the Caledonian orogeny.
Fossils of the late Silurian sea bed, England
Diorama of a Silurian seafloor
Cooksonia, the earliest vascular plant, middle Silurian