Australian Middle Cambrian molluscs and their bearing on early molluscan evolution

Thirty ammonite taxa are recognised in the Wangarlu Mudstone of the Bathurst Island Group from the Cox Peninsula and Shoal Bay, Northern Territory, Australia. Included are a new heteromorph genus Notostreptites, type species N. exilis, and four additional new species and subspecies: Pseudhelicoceras gracilis, Labeceras (L.) tumidum, Labeceras (Appurdiceras) decorum, and Idiohamites dorsetensis laticostatus. The assemblage is best collectively correlated with the Mortoniceras (M.) inflatum Zone of the standard European Albian zonation but some of its members may represent the lower part of the Stoliczkaia dispar Zone. It is broadly correlative with faunas from the Eromanga Basin, which relate to an extensive eastern Australian Late Albian epicontinental sea, but is strikingly different in aspect. The Wangarlu Mudstone assemblage has relatively high diversity and an abundance of cosmopolitan heteromorph taxa well known from Europe and elsewhere, whereas Eromanga Basin assemblages are of relatively low diversity and dominated by Austral heteromorph genera known only from Australasia, southern Africa and Malagasy. The Austral character of Eromanga Basin assemblages is attributed to evolution in a restricted epicontinental sea environment and modest dispersal whereas the continental margin position of the Wangarlu Mudstone ensured an influx of pandemic elements drawn from the mid-Cretaceous world ocean.