PATTERNS OF DIVERSITY IN CENOZOIC MARINE MOLLUSKS FROM THE PERUVIAN PROVINCE

The Pisco Basin of Peru has proven to be a rich desert repository of fossil marine mollusks of Eocene, Oligocene, and especially Miocene, Pliocene, and Pleistocene age. An abundance of microfossils and the occasional ash bed provide much-needed biochronological and geochronological control for molluscan events. Middle to late Eocene assemblages from the Paracas Peninsula ('Paracas fauna') are similar in most regards to the much richer assemblages of the Talara Basin in northern Peru. Latest Eocene and Oligocene assemblages in the Pisco Basin, dominated by Turritella and Ostrea in basal transgressive sandstones ("Otuma fauna'), are much less diverse and share very few taxa at the generic level with the Paracas fauna. Early Miocene assemblages ('Chilcatay fauna') have a much higher diversity and a significant complement of tropical taxa, but show a surprising affinity with assemblages of the Navidad Basin of central Chile. A sudden disappearance of the Chilcatay fauna around 15 Ma was followed initially by the appearance of immigrants (species of Turritella, Panopea, Anadara) from the Talara Basin ('Filudo fauna'). Soon afterwards, diversity grew ('Pisco fauna') with the radiation of endemic clades (muricids such as Chorus, Acanthina, Herminespina, and Concholepas; Turritella species of the T. cingulata lineage; Chlamys), the immigration of Panamic taxa (Dosinia, Amiantis, Cancellaria, Mitra, Terebra), and the introduction of southern taxa (fissurellids, Eurhomalea). A two-part mass extinction between 3 and 2 Ma eliminated 80 percent of the Pliocene species. The modern Peruvian fauna has persisted virtually unchanged for two million years.