Taxonomic revision of the genus Stiptognathus (Conodonta) from the Lower Ordovician of Australia and its biostratigraphical and palaeobiogeographical significance

Stiptognathus borealis from the Lower Ordovician Emanuel Formation and correlative subsurface Nambeet Formation of the Canning Basin in Western Australia is examined taxonomically. It is the type and only known species of the genus Stiptognathus, with a stratigraphical range restricted to the lower Floian (Lower Ordovician). The distribution of S. borealis suggests that this species might have inhabited a spectrum of environments, but it was palaeobiogeographically restricted to the tropical zone, possibly because of sea water temperature sensitivity. This study provides a new description of this species and redefines its constituent element types comprising a seximembrate apparatus. This is characterized by a pastinate Pa element and modified angulate Pb element. Stiptognathus borealis is morphologically most closely related to the genus Fahraeusodus. These taxa form a clade that might be phylogenetically basal to Plectodinidae and Cyatoniodontidae, which flourished in Middle and Late Ordovician times. Yong Yi Zhen [yong-yi.zhen@regional.nsw.gov.au], Geological Survey of New South Wales, W.B. Clarke Geoscience Centre, 947-953 Londonderry Road, Londonderry, NSW 2753, Australia.