Multiple origins of gigantism in stem baleen whales
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Living baleen whales (Mysticeti) include the world’s largest animals to have ever lived—blue whales (Balaenoptera musculus) can reach more than 30 m. However, the gigantism in baleen whales remains little explored. Here, we compiled all published stem mysticetes from the Eocene and Oligocene and then mapped the estimated body size onto different phylogenies that suggest distinct evolutionary histories of baleen whales. By assembling all known stem baleen whales, we present three novel findings in early mysticete evolution. Results show that, regardless of different phylogenetic scenarios, large body size (more than 5-m long) evolved multiple times independently in their early evolutionary history. For example, the earliest known aetiocetid (Fucaia buelli, 33–31 Ma) was small in size, about 2 m, and a later aetiocetid (Morawanocetus-like animal, 26–23 Ma) can reach 8-m long—almost four times the size of Fucaia buelli—suggesting an independent gigantism in the aetiocetid lineage. In addition, our reconstruction of ancestral state demonstrates that the baleen whales originated from small body size (less than 5 m) rather than large body size as previously acknowledged. Moreover, reconstructing the evolution of body size in stem baleen whales suggests that the initial pulse of mysticete gigantism started at least back to the Paleogene and in turn should help to understand the origin, pattern, and process of the extreme gigantism in the crown baleen whales. This study illustrates that Cope’s rule is insufficient to explain the evolution of body size in a group that comprises the largest animals in the history of life, although currently the lack of exact ancestor-descendant relationships remains to fully reveal the evolutionary history of body size.
KeywordsBody size Cetacea Evolution Mysticeti Oligocene
We thank the editor, Sven Thatje, and two anonymous reviewers for commenting and suggesting; Robert Boessenecker for reading and commenting on the earlier version; Ewan Fordyce (University of Otago), Hiroshi Sawamura, Tatsuro Ando, and Tatsuya Shinmura (Ashoro Museum of Paleontology), Erich Fitzgerald (Museum Victoria), Nicholas Pyenson and David Bohaska (National Museum of Natural History, Smithsonian Institution), Lawrence Barnes and Samuel McLeod (Natural History Museum of Los Angeles County), Patricia Holroyd (University of California Museum of Paleontology) for access to collections and stimulating discussion during CHT and/or NK’s visits; Takashi Oda (†L. denticrenatus), Tatsuya Shinmura (Morawanocetus-like animal), Nobu Tamura (†H. umarere, †J. hunderi, and †Y. canaliculatus), and Yoshimi Watanabe (B. musculus and E. glacialis) for providing illustrations. CHT thanks James Mead (Washington DC), Robert Boessenecker (California), and Erich Fitzgerald, Karen Roberts (Melbourne) for accommodating and discussing during various research trips. CHT was supported by a Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowship for Foreign Researchers (P15329) and a JSPS Grant-in-Aid for Foreign Fellows (15F15329, granted to CHT and NK).
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Conflict of interest
The authors declare that they have no conflict of interest.
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