The time frame for opening of the Drake Passage, which resulted in the onset of Antarctic climatic cooling and then to the development of ice sheets on the Antarctic Peninsula, is hypothesized to be an early Oligocene event. Rock units from the topmost levels of the La Meseta Formation on Seymour Island, Antarctic Peninsula exhibit evidence of ice sheet formation. The date for ice sheet development is at the Eocene-Oligocene boundary. Thus the opening of the Drake Passage is hypothesized to be at Eocene-Oligocene boundary. However, fish teeth extracted from deep-sea cores were analyzed to provide data on a deepwater opening of the Drake Passage correlated to the Late Eocene (ca. 41 Ma). The data from vertebrate paleofaunas and associated paleofloras from the La Meseta Formation can be used to relate the opening of the Drake Passage to climatic indicators from these fossil remains.