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| Fucaia humilis Tsai, Goedert & Boessenecker, 2024 |
Highlights:
• A new fossil mysticete is described from Washington State, USA
• This fossil represents the oldest known mysticete from the Northern Hemisphere
• The small body size contrasts with larger coeval whales in the Southern Hemisphere
• The coastal kelp ecosystem likely fostered the diversification of early whales
Summary
Extant baleen whales (Mysticeti) uniquely use keratinous baleen for filter-feeding and lack dentition, but the fossil record clearly shows that “toothed” baleen whales first appeared in the Late Eocene.1 Globally, only two Eocene mysticetes have been found, and both are from the Southern Hemisphere: Mystacodon selenensis from Peru, 36.4 mega-annum (Ma) ago and Llanocetus denticrenatus from Antarctica, 34.2 Ma ago. Based on a partial skull from the lower part of the Lincoln Creek Formation in Washington State, USA, we describe the Northern Hemisphere’s geochronologically earliest mysticete, Fucaia humilis sp. nov. Geology, biostratigraphy, and magnetostratigraphy places Fucaia humilis sp. nov. in the latest Eocene (ca. 34.5 Ma ago, near the Eocene/Oligocene transition at 33.9 Ma ago), approximately coeval with the oldest record of fossil kelps, also in the northeastern Pacific.5 This observation leads to our hypothesis that the origin and development of a relatively stable, nutrient-rich kelp ecosystem5,6 in the latest Eocene may have fostered the radiation of small-sized toothed mysticetes (Family Aetiocetidae) in the North Pacific basin, a stark contrast to the larger Llanocetidae (whether Mystacodon belongs to llanocetids or another independent clade remains unresolved) with the latest Eocene onset of the Antarctic Circumpolar Current in the Southern Hemisphere.7,8,9 Our discovery suggests that disparate mechanisms and ecological scenarios may have nurtured contrasting early mysticete evolutionary histories in the Northern and Southern hemispheres.
Systematics
Cetacea
Mysticeti
Aetiocetidae
Fucaia humilis sp. nov.
Diagnosis: Fucaia humilis sp. nov. shares with other species of Fucaia a newly identified synapomorphy: a shallow and rounded secondary squamosal fossa on the posterolateral margin of the squamosal (Figure 1). Eomysticetids possess a similar character, but the homology within Mysticeti remains uncertain. Furthermore, Fucaia humilis sp. nov. differs from all other aetiocetids and other Fucaia species by having the following unique character combination: a shorter intertemporal constriction exposed on the skull vertex; the presence of a distinct nuchal tubercle at the junction of parieto-squamosal suture and supraoccipital; a broadly rounded, dorsally low, and lobate coronoid process of the mandible; the presence of a postcoronoid elevation; and rounded anterior border of the tympanic bulla.
Etymology: In Latin, humilis refers to humble in English, alluding to the previously unknown humble early mysticete evolution in the Northern Hemisphere.
Cheng-Hsiu Tsai, James L. Goedert and Robert W. Boessenecker. 2024. The Oldest mysticete in the Northern Hemisphere. DOI: 10.1016/j.cub.2024.03.011
