Cranial anatomy of baleen whales was examined in order to identify evolutionary morphological novelties. Complex surfaces of the mandibles and craniums of 4 minke whales (Balaenoptera acutorostrata) were plotted in 3 dimensions by close-range photogrammetry. Photogrammetric data sets representing naturally opposed borders of the mandible and maxilla were used to determine 3-dimensional (3D) conformal coordinate transformations. These allowed the creation of precise 3D computer models of the skulls in which the strongly bowed mandibles were closely opposed along their entire length to the curved lateral borders of the rostrum. This simulated mouth closure. Subsequent internal measurements demonstrated a biomechanical specialization heretofore unknown in the class Mammalia—a maxillomandibular cam articulation. This novel articulation would operate as an adjunct to the temporomandibular joint in the final stage of mouth closure. Our functional interpretation is that it is at least a mechanism by which the energy cost of carrying a gigantic, expandable mouth at speed through an aqueous medium could be kept to a minimum. This articulation also may be part of a critically important trigger mechanism needed for precisely timed deployment of the feeding apparatus while it is under high hydrodynamic loads. We suggest that this evolutionary innovation was the root enabling cause of the adaptive radiation of rorquals (Balaenopteridae).