Ulnaria asymmetrica sp. nov. (Bacillariophyta), a new fossil species from the Trout Creek Miocene locality, Oregon, USA

A new species of Ulnaria (K & uuml;tzing) Comp & egrave;re is described from the Miocene Trout Creek fossil locality, situated in Harney County, Oregon, USA. The rock material from which the new species is described consisted of a subsample taken from the matrix harboring a plant fossil collected in 1932 or 1933 and archived at the Field Museum of Natural History, Chicago, Illinois, USA. The new species differs from all other taxa in the genus by possessing club-shaped valves that are asymmetric about the transverse axis, and frustules that are cuneate-shaped in girdle view. In addition to its unique shape, valves of the new species have a well-developed ocellulimbus (pore field) at each apex, a single rimoportula at one or both poles and closed girdle bands. The virgae are thick relative to the vimines, areolae possess exterior closing plates, and on larger specimens, the uniseriate striae can become biseriate near the valve margins. Material at the Trout Creek locality was deposited approximately 15.6-14 Ma during the Middle Miocene Climatic Optimum (MMCO). Given the diatom species co-occurring with the new Ulnaria species, it likely inhabited the plankton.