Miocene Aulacoseira species from the Virgin Valley Formation, Nevada, U.S.A.

Aulacoseira Thwaites is one of the oldest known diatom genera to inhabit freshwater ecosystems, having been recorded from multiple localities from the late Cretaceous. The genus continued to evolve throughout the Cenozoic and today continues to thrive in aquatic environments globally, often forming an important contributor to phytoplankton communities. Morphological characters that define Aulacoseira had largely evolved by the Eocene, and many additional species have been uncovered since that time, especially in the Miocene. The Virgin Valley Formation, located in the northwestern portion of Nevada, USA, represents an extensive deposit that dates to middle Miocene (~14.4 Ma) and spans a period of time that covers the end of the Middle Miocene Climatic Optimum (MMCO) and onset of the Middle Miocene Climatic Transition (MMCT). The formation contains strata consisting of organic lignite, mudstones and diatomite, ranging in thickness from a few cm to tens of meters. During an investigation of the biological remains, Aulacoseira was found to dominate many of the strata, indicating the importance of this diatom in these ancient aquatic ecosystems. The objective of this communication is to provide a detailed accounting of the most common Aulacoseira species found at the Virgin Valley locality, including the descriptions of six new species. Collectively, the new taxa represent a wide range of morphological characters, and their remains are used to infer aspects of the paleoecology of the ancient habitats.