Late Silurian (Ludlow) and Early Devonian (Pragian) conodonts from the Cobar Supergroup, western New South Wales, Australia

摘要

Late Silurian and Early Devonian conodonts are documented from outcropping limestones at nine Cobar Supergroup localities: the Booth Limestone, the Mountain Dam Limestone, the Beloura Tank Limestone Member of the Baledmund Formation, the “Lerida Limestone Member” of the Amphitheatre Group, and limestones in Stoney Creek in the Gundabooka National Park, in the Troffs Formation in the Trundle Group and in the Derriwong Group at Myola. These (with the exception of Myola) and conodonts previously documented from the White Tank and Rookery limestone members of the Meryula Formation are Pragian in age, ranging from sulcatus Zone to the imprecisely defined pireneae Zone, with the uppermost part of one of them (the Booth Limestone) possibly extending into the earliest Emsian. The Derriwong Group carbonates are late Ludfordian, crispa Zone, in age and are the only remnants discovered to date of a Silurian marine incursion. Ages for the Devonian limestones are: Booth Limestone, late in the sulcatus Zone to “pireneae Zone” with the uppermost horizon possibly extending into the dehiscens Zone; Mountain Dam Limestone, sulcatus Zone with the uppermost preserved horizon inferred to have been late in the sulcatus Zone; the Beloura Tank Limestone Member, sulcatus Zone; the ?olistoliths constituting the “Lerida Limestone Member”, sulcatus Zone; and limestones in Stoney Creek in the Gunderbooka National Park, “pireneae Zone”. Conodonts previously documented from “The Rookery” - the White Tank and Rookery limestone members - are inferred to be sulcatus Zone. In terms of recently refined absolute ages, Devonian carbonate sedimentation in the Cobar Supergroup, though localised, thus spans a six-or-seven-million-year time slice. The Devonian limestones investigated were apparently deposited during a single, basin-wide sedimentary event that was conducive to accumulation of carbonate sediments at various loci. Several conodont species documented here are taxa found in Central Asia and Arctic North American localities. Previous examples of such apparent provincialism, based on conodont distribution, have been noted to occur in faunas from the pesavis and kindlei zones, the imprecisely defined pireneae interval, and now for the sulcatus Zone. A plethora of new species and subspecies identified from the Cobar Supergroup limestones appear in the middle to late portion of the sulcatus Zone, an interval from which conodonts have not been documented as copiously as from other time slots globally. New conodont taxa consist of one new genus, eight new species - one kept in open nomenclature - and six new subspecies: Heliagnathus gen. nov. (type species Heliagnathus parvilabiatus sp. nov.), Panderodus rhytiodus sp. nov., Panderodus sp. nov. A, Icriodus ampliatus sp. nov., Pelekysgnathus inaequalis sp. nov., Oulodus astriatus sp. nov., Ozarkodina multistriola sp. nov., Ozarkodina scoliciformis sp. nov., Amydrotaxis corniculans truncus subsp. nov., Eognathodus sulcatus lanei subsp. nov., Eognathodus sulcatus sicatus subsp. nov., Ozarkodina selfi lenticula subsp. nov., Ozarkodina selfi lanceola subsp. nov. and Ozarkodina selfi cordata subsp. nov. Biostratigraphic data from this study indicate a separation in the ranges of Ozarkodina selfi and Eognathodus sulcatus . A polyphyletic origin of Eognathodus is suggested. Biometric data are presented to demonstrate that Icriodus ampliatus sp. nov. occupies a morphological gap between I. steinachensis and I. claudiae and its first appearance is a marker of the upper sulcatus zone.