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

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 Prídolí (late Late Silurian). Twenty-seven graptolite taxa, a considerable increase on previous records from Yass, have been identified in the late Ludlow and P rídolí of the district. These taxa enable the Ludlow-Prídolí 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 Prídolí, 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 Münch, 1938; and Pristiograptus kolednikensis Pribyl, 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 Prídolí from the late Ludlow P. fragmentalis (Bouek, 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 (Bouek, 1936); this species is known only from Poland. The Yass Silurian sequence of New South Wales has been recognised as one of the most important developments of the system in Australia (Jell & Talent, 1989). Among the fossil groups abundantly represented in the Yass strata, conodonts and graptolites are the most important for determining the age relationships of the strata; the conodont faunas were described by Link & Druce (1972) but the graptolites have been, by comparison, poorly known. In this paper we identify the graptolite faunas from a number of stratigraphic levels in the Yass sequence, and determine the ages and evolutionary relationships of the faunas. The early explorer-geologist Strzelecki (1845) was the first to note the occurrence in the Yass district of strata we now recognise as Silurian. Graptolites from the Yass Silurian were first noted by the local schoolteacher, John Mitchell (1886, 1888: from "Bowning Beds, Bowning" and "Bell [sic] Vale"). The first brief graptolite description (as "allied to M. dubius") and illustration were by T.S. Hall (1903: Bowning Series, Belle Vale). Although "Belle Vale" is still a well-known property on the Black Range Road in the Yass district (Fig. 1), these reports are of historic interest only as the locality and material are not recognisable from published data. A.J. Shearsby, a local photographer and keen amateur geologist, noted in 1912 the graptolites Monograptus (?) and Dendrograptus from strata identified by him as the "Barrandella shales". The stratigraphic terminology for the Silurian strata of the area has evolved considerably from the pioneering descriptions by Shearsby (1912) and Brown (1941), to the current stratigraphic terminology proposed by Link (1970), followed by Link & Druce (1972) in their seminal studies of conodonts from the Yass Silurian, and finally modified by Cramsie et al. (1978). Sherrard & Keble (1937) described Ordovician and Silurian graptolites from the Yass area. Surprisingly (in view of the abundant graptolites at other levels in the Yass Silurian sequence) the Silurian graptolites they described were Prídolí forms from near "Silverdale" NNW of Yass (Fig. 1), under the names of Monograptus flemingii (Salter), M. Figure 1. Locality map for the Yass district. Geological details can be obtained from Link & Druce (1972). The 3 and 4 digit numbers adjacent to short lines along the figure margins indicate the metric grid for the Yass 8628-S 50 000 map sheet.