Strontium isotopes reveal a globally unique assemblage of Early Miocene baleen whales.

The earliest Miocene (Aquitanian, 23-20 Ma) remains a critically under-sampled 'dark age' in cetacean evolution. This is especially true of baleen whales (mysticetes), Aquitanian specimens of which remain almost entirely unknown. Across the globe, the nature of the cetacean fossil record radically shifts at the Oligocene-Miocene boundary, with mysticetes and some archaic odontocete lineages suddenly disappearing despite the availability of cetacean-bearing rock units. New Zealand is the only place worldwide where this change is not readily apparent, with baleen whales apparently persisting into the earliest Miocene. Whether this is a genuine pattern has so far remained obscured by a lack of biostratigraphic resolution associated with the Oligo-Miocene boundary. Here, we report 23 new strontium (Sr/Sr) dates from shells associated with 16 mysticete and seven odontocete specimens, respectively. Of these, eight fall within the Early Miocene and seven - including five mysticetes - specifically within the Aquitanian. Our findings confirm the unique nature and global importance of the cetacean fossil record from New Zealand, and provide a foundation for investigations into the causes and effects of the Early Miocene cetacean 'dark age'.