 The morphology of acoustically isolated ear bones in basilosaurid archaeocetes indicates that this order had directional hearing underwater at low to mid frequencies by the late middle Eocene.However, with the extinction of archaeocete at the onset of the Oligocene, two new lineages in the early Oligocene period (31.5-28 million years ago) comprised a second radiation.These early mysticetes (baleen whales) and odontocetes can be dated back to the middle Oligocene in New Zealand.