The bajocian ammonites of Western Australia

Abstract

The Bajocian (Middle Jurassic) ammonites of Western Australia are described on the basis of an extensive collection made in 1952-3 by Phillip E. Playford, who contributes a stratigraphical introduction and a geological map. In this introduction he subdivides the Jurassic sediments (total thickness at outcrop up to 550 ft.), names and defines most of the groups and formations for the first time, and elucidates complications due to lateritic alteration. All the ammonites come from the Newmarracarra Limestone (up to 38 ft. thick). The ranges of the species are determined so far as practicable. The ammonite fauna comprises at least twenty-three species (at least eleven new), now assigned to seven genera. The new collection enables Crick’s type specimens, named in 1894 on the basis of defective and inadequate material, to be reinterpreted, and necessitates complete generic revision. The age of the fauna is Middle Bajocian. Most of it belongs to the Sowerbyi Zone, but in places there is believed to be also a thin representative of the Humphriesianum Zone. A comparison (now possible for the first time) is made with the Bajocian ammonite faunas of circum-Pacific countries and central Asia: New Guinea, the Moluccas, Tibet, eastern Siberia, Alaska, western Canada, western United States, Mexico and the Andes. Photographs are given of comparable ammonites from Tibet, Canada and Argentina, not previously published photographically. Apart from the Moluccas, the peculiar Australian stephanoceratid ammonites, Pseudotoites and their allies, are not known from any of the extensive Bajocian outcrops in the Old World. Hitherto they have been thought to be confined to Western Australia. It is now shown that Pseudotoites occurs in the Moluccas, British Columbia, Alaska and Argentina, and that some rarer allied forms of Western Australia belong to the genus Zemistephanus , hitherto known only in Canada, Alaska and the United States. This distribution is held to indicate free migration across the Pacific Ocean. The regional basis of evolutionary radiation in several groups of Middle Bajocian ammonites is pointed out, and the significance of the facts for evolutionary and systematic theory is briefly stated. The Middle Bajocian stratigraphy of north-west Europe is historically summarized in order to provide a framework and scale of comparison for the Australian and circum-Pacific deposits.