Oligocene mysticetes display an unparalleled diversity and morphological disparity in the evolutionary history of Mysticeti. However, their paleoecological aspects, such as the patterns of coexistence of different morphotypes, remain poorly explored. Here we describe an aetiocetid (toothed mysticete) from the Jinnobaru Formation (lower upper Oligocene, about 28 million years ago) of Umashima Island, Kitakyushu, Japan. Our description of a toothed mysticete from the Oligocene of Umashima exemplifies the coexistence of toothed and baleen-assisted mysticetes in the northwestern Pacific. Hopefully, new finds of Oligocene mysticetes will lead to a well-sampled dataset for analyzing this and other related paleoecological traits to understand the demise of “archaic” Oligocene mysticetes and the subsequent rise of the modern-looking baleen-bearing whales in Miocene times.

Aetiocetidae, Chattian, Jinnobaru Formation, Mysticeti, toothed baleen whale, Umashima

The coexistence of closely related species often shows niche partitioning under various mechanisms, such as resource, spatial, and temporal partitioning, habitat differentiation, or size disparity (Schoener 1974). Baleen whales (Cetacea: Mysticeti), the largest animals ever generally avoid intense competition through food partitioning. For example, blue whales consume euphausiids primarily, whereas fin whales predominantly forage on small fishes but also include euphausiids when abundant (Wursig et al. 2018). Yet, coexistence or competition of closely related mysticetes in the deep past has rarely been documented or discussed. The few remarkable examples include a large aetiocetid from the Oligocene of Hokkaido, which implies an early case of niche partitioning by size disparity in toothed mysticetes along the northwestern Pacific coast (Tsai and Ando 2016), and a likely competitive exclusion between cetotheriid and eschrichtiid whales (Collareta et al. 2021). Here, we describe a newly collected aetiocetid specimen from the Oligocene of Umashima Island (Fukuoka, Japan; Fig. 1). This new specimen represents the first formally recognized toothed “baleen” whale from the Jinnobaru Formation. Given that the Jinnobaru Formation has also produced the well-preserved eomysticetid Yamatocetus, our discovery provides an opportunity to discuss the coexistence of different morphotypes of early mysticetes.

Figure 1. 

Geological horizon and locality of YM-G-100208. (A) Geological column to indicate the collecting horizon. (B) Geographic map to show the collecting locality.

Our description of YM-G-100208 represents a formal recognition of the presence of aetiocetids in the Jinnobaru Formation, likely coexisting with the eomysticetid Yamatocetus canaliculatus. Interestingly, “Metasqualodon” symmetricus was originally named and recognized as an odontocete (Okazaki 1982), but later cladistic analyses suggested its placement in the mysticete lineage (Geisler et al. 2017; Boessenecker et al. 2023). The updated description and formal taxonomic revision of “Metasqualodon” symmetricus, which require a new generic identification, remain under progress (pers. comm. with Y. Okazaki). In addition, the lack of overlapping materials between YM-G-100208 and the holotype of “Metasqualodon” symmetricus hinders our interpretations of taxonomic similarities and the detailed composition of the mysticete communities of the Jinnobaru Formation. That said, the description of YM-G-100208 indicates a complex Oligocene ecosystem in the Jinnobaru Formation, with the co-occurrence of baleen-assisted eomysticetids and toothed mysticetes.

In the northwestern Pacific, two units, the Ashiya Group (including Jinnobaru Formation) in Kyushu and the Morawan Formation in Hokkaido, have yielded abundant Oligocene fossil cetaceans. However, the faunal composition of the two fossil whale-rich formations features an apparent discrepancy. Various species of toothed mysticetes were recovered from the Morawan Formation of Hokkaido, but no eomysticetids (Barnes et al. 1995; Tsai and Ando 2016), whereas both toothed mysticetes (this study) and eomysticetids (Okazaki 2012} have been documented (Fig. 2) from the Jinnobaru Formation of the Kyushu area. This faunal discrepancy, coupled with the fact that the Jinnobaru Formation (about 28 Ma) is older than the Morawan Formation (ranging from 26 to 24 Ma), may reflect an evolutionary scenario of the ecosystem structure along the northwestern Pacific. The competition between toothed mysticetes and baleen-assisted eomysticetids should have been less intense (a common resource-dependent niche partitioning). Then, toothed mysticetes may have dispersed north and differentiated to occupy various niches in the Oligocene waters of Hokkaido, resulting the size disparity of toothed mysticetes (Tsai and Ando 2016).

Figure 2. 

The skull of YM-G-100208 and a well-preserved skull of Aetiocetus weltoni (UCMP 122900) for comparison. (A) Dorsal view of Aetiocetus weltoni (UCMP 122900). (B) Ventral view of Aetiocetus weltoni (UCMP 122900). (C) Dorsal view of YM-G-100208. (D) Ventral view of YM-G-100208.

Alternatively, the faunal discrepancy between the Jinnobaru and Morawan formations may indicate the need for more research effort in the Ashoro area, Hokkaido, which has produced various aetiocetid species but no baleen-assisted mysticetes (Barnes et al. 1995;Tsai and Ando 2016). On the other side of the North Pacific (Fig. 2), abundant Oligocene mysticetes, including both toothed and baleen-assisted species, inhabited both the northern and southern waters of the eastern North Pacific (ranging from Washington of the USA to Mexico) (Barnes et al. 1995; Marx et al. 2015; Peredo and Uhen 2016; Hernández-Cisneros 2022; Hernández-Cisneros and Nava-Sanchez 2022). Of note, a toothed mysticete, Chonecetus sookensis, has also been recovered from Canada (Russell 1968). Still, given the geographic locality and geological horizon, Chonecetus sookensis from the Oligocene of Canada can be considered as part of the faunal composition from the Oligocene sediments in Washington, USA. This distribution pattern suggests more abundant Oligocene mysticetes from the western North Pacific. For example, the Oligocene mysticete composition between New Zealand and Australia also shows a prominent discrepancy; New Zealand includes various baleen-assisted and toothed species (Fordyce and Marx 2016; Boessenecker and Fordyce 2017; Tsai and Fordyce 2018), whereas toothed mysticetes represent the only Oligocene mysticetes in Australia (Fitzgerald 2006; Fitzgerald 2010). Interestingly, the supplementary information of a recent publication (Marx et al. 2019) listed 12 specimens (earbones: bullae) and identified as eomysticetids or chaeomysticetes from the Oligocene of Australia. Future finds should substantiate this identification and provide a better understanding of the Oligocene mysticete communities of the Southern Ocean.

Oligocene mysticetes show an unparalleled diversity and morphological disparity in the evolutionary history of Mysticeti (Barnes et al. 1995; Fitzgerald 2010; Boessenecker and Fordyce 2015; Tsai and Fordyce 2015; Geisler et al. 2017; Peredo et al. 2018; Tsai 2023), and some of the most challenging questions in the mysticete evolution are whether toothed aetiocetids possessed proto-baleen and how the mysticete toothed-baleen transition occurred (Ekdale and Deméré 2022; Peredo et al. 2022). Such a transition in the structure of the feeding apparatus of baleen whales indeed played a critical role and attracts intense research efforts. Our description of a toothed mysticete from the Oligocene of Umashima (Kyushu, Japan) and a compilation of Oligocene mysticetes across the globe (Fig. 3) should draw more attention to finding more Oligocene mysticetes and lead to a well-sampled dataset for analyzing the coexistence pattern or other related paleoecological traits to understand the demise of “archaic” Oligocene mysticetes and give rise to the modern-looking baleen whales.

Figure 3. 

The global distribution of Oligocene toothed and baleen-assisted mysticetes.

We thank A. Makino for collecting and donating YM-G-100208; A. Kameya and E. Akazaki of the Yamaguchi Prefectural Museum for access to YM-G-100208; P. Holroyd of the University of California Museum of Paleontology for access to UCMP 122900; Alberto Collareta and Oliver Hampe for constructive and helpful comments.. This research is partly supported by the Taiwan Ministry of Science and Technology (MOST 111-2621-B-002-006, 112-2621-B-002-005; now known as National Science and Technology Council, Taiwan), public donations (NTU FD107028), and Bestland Co., Ltd. based in Tsukuba to CHT.