The middle Eocene araphid diatom Rhaphoneis atlantica sp. Nov.: morphology and evolutionary significance

A middle Eocene deposit near the base of the continental slope 205 km east of Rehoboth Beach, Delaware, contains an assemblage of abundant diatoms, including a species herein described as Rhaphoneis atlantica. This species is substantially older than the better known species of Rhaphoneis of Miocene and younger age, and it shows morphological features that appear to be primitive for the genus. Rhaphoneis atlantica provides clues to the evolutionary history of the genus Rhaphoneis as well as other genera in the araphid subfamily Fragilarioideae.