Aetiocetidae gen et sp. indet., 2012

Tsai, Cheng-Hsiu, Kimura, Toshiyuki & Hasegawa, Yoshikazu, 2024, Coexistence of Oligocene toothed and baleen-assisted mysticetes in the northwestern Pacific, Fossil Record 27 (1), pp. 95-100 : 95

Aetiocetidae gen et sp. indet.

Fig. 2

Material.

YM-G-100208, including the posterior part of the skull. A 3D file of YM-G-100208 is freely available at: https://zenodo.org/record/8140997.

Locality and age.

YM-G-100208 was collected by Akito Makino on Umashima Island (about ten years ago, 33°57'58"N, 130°51'41"E; Fig. 1 ), Fukuoka Prefecture, Japan. YM-G-100208 was a floating nodule when discovered, resulting in an uncertain geological horizon. No microfossils, indicative of the geological age, have been recovered from the matrix with YM-G-100208. However, the matrix with YM-G-100208 is grayish and fine-grained sandstone, as typical of the Jinnobaru Formation of the Ashiya Group, which is the only exposed formation on Umashima ( Nakae et al. 1998). In addition, YM-G-100208 is extensively eroded but still preserves some skull sutures, suggesting that the specimen was likely not transported far from the original locality. Thus, we regard the geological horizon producing YM-G-100208 to be part of the Jinnobaru Formation of the Ashiya Group. The Ashiya Group includes the Yamaga, Norimatsu, Jinnobaru, Honjo, and Waita formations in ascending order stratigraphically ( Ozaki et al. 1993). The geological age of the Ashiya Group ranges from the latest Early to Late Oligocene based on fission-track dating, calcareous nannofossils, and planktonic foraminifera ( Saito 1984; Okada 1992; Ozaki et al. 1993), and the base of the Jinnobaru Formation was dated 28.91 ± 0.2 Ma by the sensitive high-resolution ion microprobe zircon U-Pb method ( Sakai et al. 2014). The upper boundary of the Jinnobaru Formation remains uncertain, and we consider YM-G-100208 to be slightly younger than 28 Ma, about the early Late Oligocene, similar to Yamatocetus canaliculatus . The Jinnobaru Formation has produced abundant vertebrate fossils, including the eomysticetid Yamatocetus canaliculatus ( Okazaki 1995; Okazaki 2012), the purported squalodontid " Metasqualodon " symmetricus ( Okazaki 1982), and plotopterids ( Olson and Hasegawa 1996).

Description.

YM-G-100208 preserves the post-frontal skull. The anteriormost serration likely indicates the frontal-parietal suture. Overall, the preserved part of the skull is eroded, and the natural sutures between bones are barely identifiable; the occipital complex is damaged. The right and left parietals meet at the dorsal midline, and the presence of the sagittal crest remains uncertain due to erosion. The anteriormost edge of the parietal is unclear, but the anteroposterior length of the parietal is much longer than its dorsoventral height. The posterior suture between the parietal and the supraoccipital is also eroded but shows a minor lateral extension of the supraoccipital, leaving a gentle overhang on the squamosal fossa posteriorly. The posterior-most margin of the parietal is also uncertain, but given the preserved morphology, it likely extends further back, only slightly anterior to the occipital condyle.

The supraoccipital is broadly triangular, and the anterior half is concave. Based on the surrounding morphology, the existence of a supraoccipital depression should be regarded as genuine. The suture between the supraoccipital and exoccipital likely remains partly unfused, but the post-mortem damage and compression hinder reliable judgment. The left occipital condyle is missing, but the overall preservation shows an oval shape of the magnum foramen (shorter dorsoventral height).Ventrally, the flat surface of the basioccipital is wide (about 63 mm), and the basioccipital crest is massive and bulbous. The basioccipital crest runs posterolaterally.

Anterior to the basioccipital, a partially well-developed keel of the vomer is observed, and the height reaches about 31 mm. The ventral margin of the keel is eroded, but it gently slopes to the surface of the basisphenoid/basioccipital posteriorly from the anterior margin of the pterygoid sinus. The vomer extends posteriorly at least to the level of the basioccipital crest. On the right side of the skull (the left side is eroded), the oval-shaped pterygoid sinus orients anteromedially, being much longer anteroposteriorly than wide. Posteriorly, the periotic is broken and eroded. The shape and degree of protrusion of the anterior process of the periotic remains uncertain due to erosion but shows a contact with the squamosal. The squamosal is also heavily eroded, but the base of the squamosal is robust based on the broken surface.

Body size and ontogenetic stage.

We estimated that the bizygomatic width of YM-G-100208 is about 28 cm. Based on this estimation, we used Pyenson and Sponberg’s equation (2011) for stem mysticetes:

log (TL) = 0.92*(log(BIZYG[in cm]) - 1.72) + 2.68

to assess the body size of YM-G-100208, resulting in 268 cm - typical for aetiocetids. The skull sutures that can help assess the ontogenetic stage ( Walsh and Berta 2011) are broadly eroded. But, given the fusion of some sutures, such as the suture between the basioccipital and basisphenoid, and robustness, we consider that YM-G-100208 represents a subadult at least.

Comment.

YM-G-100208 shows its aetiocetid affinity by displaying the following combination of characters: body size less than 3 m long, outline of the supraoccipital broadly triangular, an anteriorly-thrust supraoccipital, straight lateral margins of the supraoccipital, a moderate exposure of parietals on the skull roof, and a well-developed basioccipital crest. YM-G-100208 differs from llanocetids in its small body size, lack of the sagittal trough, bulbous basioccipital crests, and the ventral keel of the vomer extending posteriorly to the level of the basioccipital crest. YM-G-100208 further differs from mammalodontids in having a broadly triangular supraoccipital, a less elongate intertemporal region, a well-developed and bulbous basioccipital crests, and the ventral keel of the vomer extending posteriorly to the level of the basioccipital crest. YM-G-100208 differs from eomysticetids in lacking the sagittal crest on the skull roof and the ventral keel of the vomer extending posteriorly to the level of the basioccipital crest. YM-G-100208 further differs from other crown mysticetes in having the parietals exposed on the skull roof. Due to the incompleteness of YM-G-100208, we provisionally identify it as belonging to Aetiocetidae gen. et. sp. indet. This taxonomic identification allows for the first recognition of coexisting toothed and baleen-assisted mysticetes in the northwestern Pacific.

Figures

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.

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.