Ordovician nautiloids of central Australia, with a revision of Madiganella Teichert & Glenister.

Nautiloids mainly from the Lower Ordovician Horn Valley Siltstone of the Amadeus Basin are revised. Madiganella is shown to have unexpanded two-layered bullettes and well-developed endocones, the endosiphutobe of which have previously been mistaken for a free siphonal tube. These features do not allow assignment of Madiganella to the Ruedemannoceratidae as has been suggested: instead the genus belongs to a new family of Discosorida. Madiganella magna Teichert & Glenister 1952 and M. sp. (flower & Teichert, 1957) are shown to be synonyms of Madiganella tatei (Etheridge). Endoceras warburtoni Etheridge is assigned to Anthoceras Teichert & Glenister, and Anthoceras decorum Teichert & Glenister from Western Australia is considered a junior synonym. The specimens recently described by Crick & Teichert (1983) as A.decorum from Mount Arrowsith, New South Wales, belong to a different species, herein named A. arrowsmithense n. sp. The type specimens of Bactroceras gossei (Etheridge) are redescribed and refigured. A species of Aphetoceras is present, but it is not Aphetoceras delectans Teichert & Glenister. Also present is Lobendoceras emanueknse Teichert & Glenister, as well as a new genus of discosorid, known also from Thailand and possibly from Queensland. 'Trochoceras recticostatum' Tate is too poorly preserved to be assigned even to a family. Armenoceras is recorded from the Stairway Sandstone. -Authors