Abstract
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, Borealodon osedax gen. et sp. nov., from the Oligocene of Washington State, USA. Borealodon preserves multi-cusped teeth with apical wear; microCT scans of the inner ear indicate that the minimum frequency hearing limit of Borealodon was similar to mammalodontids. Borealodon 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 across the Rupelian–Chattian boundary. Furthermore, the presence of a putative chemosynthetic bivalve along with Osedax, a bone-boring annelid, found in association with the type specimen of Borealodon, offer more insights into the evolution of deep-sea whale-fall communities.
Highlights
Modern baleen whales are a clade of mammals that have evolved to include the largest vertebrates in history
These innovations occur prior to the origin of crown mysticetes, in the late Eocene and early Oligocene—well after the initial land– sea transition of stem cetaceans—suggesting (i) that they are not merely a function of returning to a marine environment and (ii) that they may be integral to the early success and diversification of the clade [1]
Stem baleen whales originate in the late Eocene [2,3] from toothed ancestors, and toothed, stem mysticetes persist into the Oligocene [4]
Summary
Modern baleen whales are a clade of mammals that have evolved to include the largest vertebrates in history Their evolutionary and ecological success is facilitated by key innovations that further aid their life in the water, such as low-frequency hearing and filter feeding with baleen plates [1]. At least four distinct clades of stem, toothed mysticetes exist, each with widely disparate tooth morphologies spanning a broad range of potential ecological roles [5,6] This dental diversity is in stark contrast to modern baleen whales, which are edentulous and feed with keratinous baleen plates. Understanding the diversity and trends associated with the evolution of stem mysticete teeth is integral to understanding the broader teeth-to-baleen transition
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