ARENIG SERIES

also called Arenigian Series, the second of six divisions (in ascending order) constituting the Ordovician System, representing all those rocks deposited worldwide during the Arenigian Age (488 to 478 million years ago). The series' name is derived from the Arenig mountains in Gwynedd county in North Wales, which serve as the type area in Great Britain for rocks of this age. Welsh rock strata belonging to the series attain a thickness of about 1,200 m (4,000 feet), but unfortunately they are not rich in fossils. One possible alternative global stratotype section is found in western Newfoundland, but a global stratotype section and point (GSSP) has not yet been formally proposed by the International Commission on Stratigraphy. The base of the fossil zone of the graptolite species Tetragraptus approximatus is widely accepted as closely approximating the base of the Arenig Series in Britain. This level also corresponds to the base of the fossil zone of the conodont Paroistodus proteus. The Arenig Series is overlain by the Llanvirn Series.