Lower Cretaceous ammonites from the Neuquén Basin, Argentina: A Hauterivian Olcostephanus fauna

Abstract

The olcostephanid fauna of the Olcostephanus (O.) laticosta [formerly O. (O.) leanzai] Subzone (Agrio Formation) represents the last of four discrete invasions of olcostephanid ammonites into the Neuquén Basin of west-central Argentina. Olcostephanus (O.) laticosta (Gerth) dominates the fauna and is known only from the Neuquén Basin. But the co-occurrence of the distinctive subgenus Feannoticeras, newly recorded from Argentina, provides a link with the 'standard' West Mediterranean sequence and indicates that the O. (O.) laticosta Subzone is probably of mid Early Hauterivian age. This is supported by the discovery of two specimens of the widely distributed late Early Hauterivian species O. (O.) variegatus (Paquier) in the overlying Hoplitocrioceras giovinei Subzone. O. (Jeannoticeras) agrioensis sp. nov. is described.