Systematics, biostratigraphy and evolution of the Late Ludlow and Pridoli (Late Silurian) graptolites of the Yass district, New South Wales, Australia

Abstract

Graptolites from the Yass district of New South Wales include important material from: low in the Black Bog Shale; from the Yarwood Siltstone Member; 2 levels high in the Black Bog Shale; 2 levels low in the Rosebank Shale; low in the Cowridge Siltstone; and in the lower part of the Elmside Formation. The faunas from the lower 4 levels are late Ludlow (early Late Silurian), and the higher 4 levels are P^ ridoli (late Late Silurian). Twenty-seven graptolite taxa, a considerable increase on previous records from Yass, have been identified in the late Ludlow and P^ ridoli of the district. These taxa enable the Ludlow-P ^boundary to be identified some 20 m above the base of the Rosebank Shale (Booroo Ponds Group); our stratigraphically highest collection from the Elmside Formation is latest P ^ supporting the previous placement of the base of the Devonian approximately midway through the Elmside Formation (Barambogie Group). The following graptolite Biozones have been identified: praecornutus, cornutus, parultimus, bouceki and transgrediens. Twenty Yass taxa are described, including the new species Bohemograptus paracornutus, Pristiograptus shearsbyi, Neocucullograptus? yassensis and N.? mitchelli and the new subspecies Monograptus perneri elmsidensis and M. formosus jenkinsi. The following are recorded from Australia for the first time: Bohemograptus praecornutus Urbanek, 1970; Crinitograptus operculatus Munch, 1938; and Pristiograptus kolednikensis P^ 1940. Dictyonema sp. cf. D. sherrardae Rickards et al., 1995 and D. elegans Bulman, 1928 are considered late evolutionary derivatives of long-ranging dendroid species. Linograptus posthumus introversus Rickards & Wright, 1997 is interpreted as a short-lived, late Ludlow offshoot of the long-ranging L. p. posthumus Richter, 1875. Bohemograptus praecornutus is regarded as the ancestor of B. paracornutus, the B. cornutus evolutionary plexus being recognised for the first time in Australia. Late forms of Pristiograptus dubius (Suess, 1851) probably gave rise to P. shearsbyi n.sp. and Pristiograptus kolednikensis probably arose in the basal P^ ridoli from the late Ludlow P. fragmentalis (Boucek, 1936). Some material described and discussed by Brown & Sherrard (1952) is reinterpreted. Bohemograptus paratenuis n.sp. is proposed for material assigned by Urbanek (1970) to B. bohemicus aff. tenuis (Boucek, 1936); this species is known only from Poland.