Late Silurian strophomenate brachiopods from Yass, New South Wales

Abstract

The Yass Sysncline succession, of possibly late Wenlock and certainly Late Silurian to earliest Devonian age, has yielded eleven species and subspecies described here, belonging to the Superfamilies Strophomenoidea, Plectambonitoidea, Chonetoidea and Chilidiopsoidea. The scope of the chilidiopsine genus Morinorhynchus is discussed. Two species, Leptaena compitalis and P. (Plectodonta) psygmata, are new. Three species, Desistrophia papilio. Epelidoaegiria minuta chilidifera and Morinorhynchus oepiki, were originally described from the late Wenlock to early Ludlow of Canberra. Leptaena compitalis is also recorded from the Ludlow of Molong in central N.S.W. The 17 species originally assigned to Stropheodonta by Mitchell are fully revised: two are unusable, the others are now reduced to four: M. (Mesoleptostrophia) quadrata, Mesopholidostrophia bendeninensis, Epelidoaegiria minuta minuta, and P. (Plectodonta) bipartita. The greatest species diversity is in the lower Ludfordian Black Bog Shale: none of the species originally described from Canberra extends above that level. The fauna of the upper Ludfordian Rainbow Hill Member (basal Rosebank Shale) is also diverse. Only two species are common to those two units, reflecting environmental differences. One of them (Strophochonetes kemezysi) extends into the Plidoli, where it is joined by P. (Plectodonta) bipartita. No brachiopod species is known from the basal Devonian at the top of the Yass sequence.