<p>Baleen whales (mysticetes) underwent a profound transition from raptorial to filter feeding early in their history, allowing the exploitation of new food sources and environments. Their key structural innovation, baleen, is rarely fossilized, complicating reconstruction of the transition. Muscles used in food capture, locomotion, and respiration are anchored on developmentally discrete subunits of the sternum in living mysticetes, offering a novel approach for predicting muscle function and feeding style in extinct taxa. Elliptic Fourier Analysis was used to test the relationship between sternal shape and feeding style in extant balaenopteroid mysticetes. Results indicate that sternal shape changes during the ontogenetic transition from juvenile nursing to adult filter feeding and differs among adults of species with different styles of filter feeding, providing support for the use of sternal structure to predict the feeding habits of extinct taxa. The dense, block-shaped presterna and multiple sternebrae of early mysticetes suggest that they used static buoyancy control in nearshore environments and lacked the thoracic compliance necessary for deep diving. Variably enlarged sternohyoid muscle fields indicate that they all employed suction during prey capture. In sharp contrast, eomysticetes have osteoporotic, planar sterna without sternebrae and minimal sternohyoid fields, signaling deeper water habitats, diving facilitated by thoracic compliance, and little or no suction. Taxa crownward of eomysticetes display novel elaborations of the sternohyoid and sternocephalic muscle fields that characterize continuous and intermittent ram-feeding in living balaenopteroids.</p>