Phylogenetic Analysis Matrix from <i>Borealodon osedax</i>, a new stem mysticete (Mammalia, Cetacea) from the Oligocene of Washington State and implications for fossil whale-fall communities B. K. Shipps Carlos Mauricio Peredo Nicholas D. Pyenson 10.6084/m9.figshare.8852114.v1 https://rs.figshare.com/articles/dataset/Phylogenetic_Analysis_Matrix_from_i_Borealodon_osedax_i_a_new_stem_mysticete_Mammalia_Cetacea_from_the_Oligocene_of_Washington_State_and_implications_for_fossil_whale-fall_communities/8852114 Baleen whales (mysticetes) lack teeth as adults and instead filter feed using keratinous baleen plates. They do not echolocate with ultrasonic frequencies like toothed whales but are instead known for infrasonic acoustics. Both baleen and infrasonic hearing are separately considered key innovations linked to their gigantism, evolutionary success and ecological diversity. The earliest mysticetes had teeth, and the phylogenetic position of many so-called toothed mysticetes remains debated, including those belonging to the nominal taxonomic groups Llanocetidae, Mammalodontidae and Aetiocetidae. Here, we report a new stem mysticete, <i>Borealodon osedax</i> gen. et sp. nov, from the Oligocene of Washington State, USA. <i>Borealodon</i> preserves multi-cusped teeth with apical wear; microCT scans of the inner ear indicate that the minimum frequency hearing limit of <i>Borealodon</i> was similar to mammalodontids. <i>Borealodon</i> is not recovered within a monophyletic Mammalodontidae nor a monophyletic Aetiocetidae; instead, it represents an unnamed lineage of stem Mysticeti, adding to the diversity of stem mysticetes, especially in across the Rupelian–Chattian boundary. Furthermore, the presence of a putative chemosynthetic bivalve along with <i>Osedax</i>, a bone-boring annelid, found in association with the type specimen of <i>Borealodon</i>, offer more insights into the evolution of these deep-sea whale-fall communities. 2019-07-10 15:43:57 baleen Cetacea Mysticeti Oligocene Pysht Formation