 Some of the earliest fossils were deposited at the end of the Ordovician on the continental shelf, including orthocerids and cystoids.In the Silurian, crustal extension began, opening the Paleotethys Ocean on the northern margin of Gondwana.Basins filled with sand, clay and calcareous sediments while basalt and andesite poured out of cracks, ultimately metamorphosing into amphibolite and greenschist now found in the Austroalpine and South Alpine superunits.