The Ordovician graptolite sequence of Australasia

Abstract

The classical Ordovician graptolite succession of Victoria has long been taken as standard for the region and widely used for subdivision and correlation of Ordovician sequences around the world. We review the Victorian succession and incorporate other Australian and New Zealand graptolitic sequences into an Australasian set of zones and stages. Thirty zones are recognized and defined, of which one (Ca4, Isograptus victoriae maximodivergens) is new and two (La1, Da4) each comprise two subzones. Several zones are redefined, but the previous zone notation has been retained. Nine stages are recognized (as previously) and defined, six (Lancefieldian, Bendigonian, Chewtonian, Castlemainian, Yapeenian, Darriwilian) in the Early Ordovician and three (Gisbornian, Eastonian, Bolindian) in the Late Ordovician; reference sections for each are nominated. The Bolindian stage contains 5 zones, here designated Bol-5. The Australasian stages are correlated internationally. A census of Australasian graptolite taxa lists the currently recognized 313 species and subspecies, with references. A range chart shows the zonal distribution of all species, and reveals an average duration of 2·46 zones per species, approximately equivalent to 5 Ma. The pattern of diversity change shows maxima during the Bendigonian-Chewtonian (peaking in Be1 with 55 taxa), late Castlemainian-mid Darriwilian, and late Gisbornian-early Eastonian. Species originations peak in Be1 and Da3 and are high throughout much of the Late Ordovician. Extinctions peak in the Da3 and are high towards the end of the Ordovician.