Tooth Loss Precedes the Origin of Baleen in Whales

Whales use baleen, a novel integumentary structure, to filter feed; filter feeding itself evolved at least five times in tetrapod history but demonstrably only once in mammals [1]. Living baleen whales (mysticetes) are born without teeth, but paleontological and embryological evidence demonstrate that they evolved from toothed ancestors that lacked baleen entirely [2]. The mechanisms driving the origin of filter feeding in tetrapods remain obscure. Here we report Maiabalaena nesbittae gen. et sp. nov., a new fossil whale from early Oligocene rocks of Washington State, USA, lacking evidence of both teeth and baleen. The holotype possesses a nearly complete skull with ear bones, both mandibles, and associated postcrania. Phylogenetic analysis shows Maiabalaena as crownward of all toothed mysticetes, demonstrating that tooth loss preceded the evolution of baleen. The functional transition from teeth to baleen in mysticetes has remained enigmatic because baleen decays rapidly and leaves osteological correlates with unclear homology; the oldest direct evidence for fossil baleen is ∼25 million years younger [3] than the oldest stem mysticetes (∼36 Ma). Previous hypotheses for the origin of baleen [4, 5] are inconsistent with the morphology and phylogenetic position of Maiabalaena. The absence of both teeth and baleen in Maiabalaena is consistent with recent evidence that the evolutionary loss of teeth and origin of baleen are decoupled evolutionary transformations, each with a separate morphological and genetic basis [2, 6]. Understanding these macroevolutionary patterns in baleen whales is akin to other macroevolutionary transformations in tetrapods such as scales to feathers in birds.