Before They Were Giants: The Fossil Record of Toothed Baleen Whales

Whales diverged from a lineage of terrestrial mammals in the early Eocene, approximately 54-53 million years ago, in a dramatic story of adaptation and descent. Modern whales and dolphins arose from toothed ancestors, the Archaeoceti, splitting into two groups about 35 million years ago. One of these groups, the Mysticeti, then evolved into the largest animals ever to live on earth. All living members of the Mysticeti use baleen to filter food from the oceans, but their earliest ancestors had teeth like primitive whales. These toothed mysticetes were a diverse group that varied in size, dentition and feeding ecology. They lived throughout the North and South Pacific and Atlantic Ocean basins, and at times were successfully living alongside archaeocetes, toothless filter-feeding mysticetes, and echolocating odontocetes. Ecologically, toothed mysticetes played a similar role as modern pinnipeds and odontocete whales, and included predatory, suction feeding, and filter feeding types. They persisted until the early Miocene, and are represented by a diversity of forms even late in their existence, with four different species known from a single early Miocene site in southern California. Toothed mysticetes gave rise to large, toothless baleen whales as near shore and oceanic upwelling ecosystems became increasingly productive from the Oligocene through the later Miocene.