Rostrum morphology and feeding strategy of the baleen whale indicate that right whales and pygmy right whales became skimmers independently

Baleen whales have lost their functional teeth and begun to use their baleen plates to feed on small prey. Modern baleen whales exhibit different types of feeding strategies, such as lunging, skimming and so on. The evolution of feeding strategy in the Chaeomysticeti is an important step in considering niche partitioning and diversification, feeding efficiency and gigantism, and evolution and extinction. This study analyses the rostrum morphology to test the hypothesis that specific rostral morphologies facilitate special feeding strategies, using modern species and their observed feeding strategies. By this means, the convergence of rostral morphology can be recognized in the closest groups in the morphospace. As a result, the two linages (Balaenidae and Caperea marginata) are recognized to have convergent rostral morphology. In addition, an early member of the Chaeomysticeti, Yamatocetus canaliculatus, and most fossil species are plotted in or close to the cluster of lunge feeders. The original feeding strategy of the Chaeomysticeti could be more similar to lunge feeding than to skim feeding. Fossil relatives of the two linages showing transitional conditions indicate that they shifted to skim feeding independently. The evolution of the feeding strategy of the Chaeomysticeti is possibly more complex than that was thought.