Lower Bajocian ammonites (Middle Jurassic; Soninniidae) from the Newmarracarra Limestone, Western Australia

Abstract

Three species of sonniniid ammonites, previously described under Fontannesia Buckman, are placed in a new genus, Newmarracarroceras; Dorsetensia clarkei Crick is designated type species. These West Australian species differ from other known Fontannesia by having smooth body chambers, flanks which slope gradually toward the umbilical seam without development of an umbilical edge or wall, and simple septal sutures. Biometric studies show that ‘Fontannesia’ whitehousei Arkell is a smaller and more compressed variant of N. clarkei (Crick), with a complete gradation between the two ‘species’ for most morphological characters. While well-dated European Fontannesia occur in the uppermost Aalenian concavum Zone and lowermost Bajocian discites Zone, new dating of the circum-Pacific Pseudotoites faunas, with which Newmarracarroceras occurs in Western Australia, indicates correlation with the higher ovalis or laeviuscula Zones (lower Bajocian). An associated specimen of Sonninia playfordi Arkell is described.