Modern Mysticete Whales are noteworthy for the presence of a keratinous baleen, which they use as a filter feeding device, having lost all conventional dentition. The earliest members of the group, which retained teeth, first appeared in the Late Eocene, with the group undergoing a significant diversification event in the Oligocene. To date, only two species of Mysticete Whales are known from the Eocene, both from the Southern Hemisphere (one from Peru and one from Antarctica). This has led to the hypothesis that the Mysticetes are of Southern Hemisphere origin, and that the Oligocene radiation within the group is likely to have been driven by the End Eocene opening of a seaway between Antarctica and South America, and the subsequent formation of the Antarctic Circumpolar Current, which in turn led to a Southern Ocean with superabundant and highly-concentrated nutrients, an environment which could have favoured the evolution of the baleen filter-feeding system.
In a paper published in the journal Current Biology on 28 March 2024, Cheng-Hsiu Tsai of the Department of Life Science and Institute of Ecology and Evolutionary Biology at the National Taiwan University, and the Department of Geology at the Japanese National Museum of Nature and Science, James Goedert of the Burke Museum of Natural History and Culture at the University of Washington, and Robert Boessenecker, formerly of the Charleston Center for Paleontology*, describe a third species of Eocene Mysticete Whale, from the Lincoln Creek Formation of Washington State, the first known Mysticete Whale from the Eocene of the Northern Hemisphere.
The new species is placed in the genus Fucaia, which contains two previously described species from the Oligocene of the northwest coast of North America, and given the specific name humilis, meaning 'humble', in reference to the apparent minor role of Mysticete Whales in Eocene ecosystems. The new species is described from a single specimen (UWBM 109363), comprising posterior portion of a skull with right periotic and bulla attached, partial mandible, and the hyoid apparatus.
The precise location that the specimen was collected from is not given, but is described as being a site on the Middle Fork of the Satsop River, Mason County, dated to 34.5 million years before the present, which is 600 000 years before the Eocene/Oligocene boundary.
Based upon the size of the specimen, the living Whale is estimated to have been between 286 and 206 cm in length, which is fairly typical for small Aetiocetid Whales (the group of toothed Mysticete Whales to which the genus Fucaia belongs).
The discovery of Fucaia humilis in deposits dated to the Latest Eocene of North America has serious implications for our understanding of the evolution of Mysticete Whales. Both previously described Eocene Mysticete Whales came from the Southern Hemisphere, which led palaeontologists to the conclusion that the group came from that part of the world, and that the subsequent Oligocene radiation of Mysticete Whales was driven by events there, notably the formation of the Antarctic Circumpolar Current.
The presence of a member of the genus Fucaia, which has been found nowhere else, on the Pacific Northwest of North America in the Latest Eocene suggests a presence of Aetiocetid Whales in this region for around 10 million years, whereas the occurrence of toothed Mysticete Whales in other parts of the world appears to have been somewhat sporadic, with Whales only present some of the time, gaps in the record, and a high turnover of genera, implying unstable environments intermittently colonised by Whales. Notably, the appearance of Fucaia humilis appears to coincide with the development of a Kelp forest ecosystem in this area, something which persists to this day. Kelp forests provide a stable, nutrient rich environment, which was apparently favourable to small Aetiocetid Whales.
*Robert Boessenecker was arrested on 14 March 2024 along with his wife, the former museum collection manager at the Mace Brown Natural History Museum, accused of stealing fossils with a value of US$8634 from the collection of the Mace Brown Museum when both were employed there.
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