The lineages of rorquals and right whales split almost 20 mya. It is unknown where this occurred, but it is generally believed that they, like their descendants, followed plankton migrations. These primitive baleen whales had lost their dentition in favor of baleen, and are believed to have lived on a specialized benthic, plankton, or copepod diet like modern baleen whales. Baleen whales experienced their first radiation in the mid-Miocene. It is thought this radiation was caused by global climate change and major tectonic activity when Antarctica and Australia separated from each other, creating the Antarctic Circumpolar Current. Balaenopterids grew bigger during this time, with species like "Balaenoptera sibbaldina" perhaps rivaling the blue whale in terms of size, though other studies disagree that any baleen whale grew that large in the Miocene.