644
Journal of Paleontology 92(4):634–647
and Beisel, 2005 from the Maastrichtian Gankin Formation in western Siberia is similar to B. paleocenica n. sp. in having 24 spiral grooves. However, the lower spire, the outer lip with a varix, and the single columellar fold of B. siberica enable us to separate it from B. paleocenica n. sp. Biplica miniplicata Popenoe, 1957 from the uppermost Cretaceous deposits of California can be easily separated from B. paleocenica n. sp. by its more numerous spiral grooves (~30) and single columellar fold.
Discussion
When Heinberg (1999) compared the Maastrichtian chalk fauna with the fauna from the Danian Cerithium Limestone at Stevens Clint in Denmark, he listed 24 common species among a total of 123 species in his table 2. Among these, only three species (12.5%) are protobranch bivalves. However, it is plausible that some Paleocene species might be reworked from the Maas- trichtian chalk because, like the chalk, the Paleocene Cerithium Limestone was deposited under shallow-water conditions. In contrast, the Katsuhira fauna consists of wood-fall communities and might have lived in deep water during the late Selandian to earliest Thanetian, as mentioned above. Thus, it is very unlikely that any taxa in the Katsuhira Formation were derived from underlying Cretaceous deposits. As the result of this study, the pectinoidean Propea-
mussium yubarense and the lucinid Myrtea ezoensis are newly recognized as surviving the end-Cretaceous mass extinction at the species level (Table 8), in addition to the two protobranchs Acila (Truncacila) hokkaidoensis (Nagao, 1932) (Table 8) and Pristigloma? sachalinensis (Salnikova, 1987) pointed out by Amano and Jenkins (2017). At the genus level, Astarte Sowerby, 1816 and Biplica Popenoe, 1957 survived the event but became extinct before the Eocene, as did Ezonuculana Nagao, 1938 and Menneroctenia Kalishevich, 1973 (Amano and Jenkins, 2017). Moreover, aporrhaid gastropods suffered a severe extinction at the end of the Cretaceous, as pointed out by Roy (1994, 1996). In the northwestern Pacific, the following two aporrhaid species survived the end-Cretaceous extinction:
Kangilioptera inouei Amano and Jenkins, 2014 from the Kat- suhira Formation, and Drepanocheilus grammi Kalishevich in Kalishevich et al., 1981 from the Danian to early Paleocene Sinegorsk horizon in southeastern Sakhalin. Among these taxa, it is interesting to consider the evolution
of astartids from a biogeographical point of view. Marincovich et al. (2002) pointed out that the occurrence of Astarte parvula Kalishevich in Kalishevich et al., 1981 from the Danian to lower Paleocene of Sakhalin is the last occurrence of astartids before they disappeared from the Pacific. They then reinvaded the North Pacific region after the opening of the Bering Strait during the latest Miocene (Ogasawara, 1986; Gladenkov et al., 1991; Amano, 1994; Marincovich and Gladenkov, 1999; Marincovich et al., 2002). The occurrence of A. paleocenica n. sp. newly described herein reveals that astartids continued to live until at least the late Selandian to earliest Thanetian in eastern Hokkaido. Among the surviving species, the Cretaceous occurrence of
Acila (Truncacila) hokkaidoensis, Propeamussium yubarense, and Myrtea ezoensis have wide occurrences from Kyushu to Hokkaido or from Hokkaido to the Koryak Upland. Such wide geographical distributions possibly helped these species to survive through the extinction event (Jablonski and Raup, 1995; Jablonski and Hunt, 2006; Jablonski, 2008; Robertson et al., 2013; Landman et al., 2014). Some authors have pointed out the low extinction rate of
protobranchs or deposit feeders (Sheehan and Hansen, 1986; Jablonski and Raup, 1995; Jablonski, 1996; Levinton, 1996; Robertson et al., 2013). This trend can be seen in the late Selandian to earliest Thaneetian Katsuhira fauna. Among the nine relict Mesozoic taxa described above, four are proto-
branchs.Although Jablonski and Raup (1995) denied the role of water depth as a cause of a low extinction rate, they only examined the depth range of taxa on the continental shelf, not on slopes or in basins. Based on the Recent bathymetric range of protobranchs, the Katsuhira Formation was deposited in an upper bathyal depth. The end-Cretaceous mass extinction by an asteroid impact is thought to have depended on climatic change (e.g., Kaiho et al., 2016). However, deep-sea benthic
Table 8. List of mollusks from the Katsuhira Formation, indicating Mesozoic-relict taxa (Mes. R.) and oldest records as genera and species (Oldest Rec.). *=chemosynthesis-based species, + =species level, ++ =genus level.
Species
Leionucula yotsukurensis (Hirayama, 1955) Acila (Truncacila) hokkaidoensis (Nagao, 1932) Ezonuculana aff. obsoleta Tashiro, 1976
Bentharca steffeni Amano, Jenkins, and Nishida, 2015a Propeamussium yubarense (Yabe and Nagao, 1928) Myrtea ezoensis (Nagao, 1938)*
Meganuculana alleni Amano and Jenkins, 2017 Malletia poronaica (Yokoyama, 1890) Menneroctenia plena Kalishevich, 1973 Neilonella alleni Amano and Jenkins, 2017 Tindaria paleocenica Amano and Jenkins, 2017 Pristigloma? sachalinensis(Salnikova, 1987)
Thyasira (Thyasira) oliveri Amano and Jenkins, n. sp.* Astarte (Astarte) paleocenica Amano and Jenkins, n. sp. Poromya katsuhiraensis Amano and Jenkins, n. sp. Bathyacmaea? sp.*
Mes. R. + ++ +
+ +
+ ++
Neverita majimai Amano and Jenkins, n. sp. Kangilioptera inouei Amano and Jenkins, 2014 Urahorosphaera kanekoi Amano and Oleinik, 2014 Admete katsuhiraensis Amano, Oleinik, and Jenkins, 2016b Biplica paleocenica Amano and Jenkins, n. sp.
++ ++
++ ++
++
++ ++
Oldest Rec. Reference
Amano and Jenkins (2017) Amano and Jenkins (2017) Amano and Jenkins (2017) Amano and Jenkins (2017) Amano and Jenkins (2017) Amano and Jenkins (2017) Amano and Jenkins (2017) Amano and Jenkins (2017) Amano and Jenkins (2017) Amano et al. (2015a) This study This study This study This study This study This study This study
Amano and Jenkins (2014) Amano and Oleinik (2014) Amano et al. (2016b) This study
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