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 Neuquen Basin of west-central Argentina. Olcostephanus ( O .) laticosta (Gerth) dominates the fauna and is known only from the Neuquen Basin. But the co-occurrence of the distinctive subgenus Jeannoticeras , 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.