It is useful to consider where "Eucalyptus" fossils have not been found. Extensive research has gone into the fossil floras of the Paleocene to Oligocene of South-Eastern Australia, and has failed to uncover a single "Eucalyptus" specimen. Although the evidence is sparse, the best hypothesis is that in the mid-Tertiary, the contintental margins of Australia only supported more mesic noneucalypt vegetation, and that eucalypts probably contributed to the drier vegetation of the arid continental interior. With the progressive drying out of the continent since the Miocene, eucalypts were displaced to the continental margins, and much of the mesic and rainforest vegetation that was once there was eliminated entirely.