A crystal-rich volcaniclastic sandstone in the lower Peltura
scarabaeoides Zone at Ogof-ddû
near Criccieth, North Wales, yields a U–Pb zircon age of 491∓1
This late Late Cambrian date indicates
a remarkably young age for the Cambrian–Ordovician boundary whose
age must be less than 491 Ma.
Hence the revised duration of the post-Placentian (trilobite-bearing)
Cambrian indicates that local trilobite
zonations allow a biostratigraphic resolution comparable to that provided
by Ordovician graptolites and Mesozoic ammonites.