The first appearance datum (FAD) of which fossil species demarcate the Cambrian-Ordovician boundary ?
FAD Iapetognathus fluctivagus
Chronostratigraphic boundaries are defined by a GSSP (golden spike), usually placed at the first appearance datum of a chosen fossil species.
The base of the Ordovician (Cambrian–Ordovician boundary) GSSP at Green Point, western Newfoundland, is placed at the FAD of the conodont Iapetognathus fluctivagus.
The trace fossil Trichophycus (Treptichnus) pedum marks the Ediacaran–Cambrian boundary; the trilobite Oryctocephalus indicus marks the base of the Wuliuan Stage within the Cambrian; the graptolite Tetragraptus approximatus marks the base of the Floian Stage within the Lower Ordovician.
So the Cambrian–Ordovician boundary is marked by the FAD of the conodont Iapetognathus fluctivagus.
Hence, FAD Iapetognathus fluctivagus.
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