Journal of Paleontology, 92(4), 2018, p. 634–647 Copyright © 2018, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.137
New and Mesozoic-relict mollusks from Paleocene wood-fall communities in Urahoro Town, eastern Hokkaido, northern Japan
Kazutaka Amano,1 Robert G. Jenkins,2 and Hiroshi Kurita3
1Department of Geoscience, Joetsu University of Education, Joetsu 943-8512, Japan ⟨
amano@juen.ac.jp⟩ 2College of Science and Engineering, Kanazawa University, Kanazawa 920-1192, Japan ⟨
robertgj@staff.kanazawa-u.ac.jp⟩ 3Department of Geology, Faculty of Science, Niigata University, Niigata 950-2181, Japan ⟨
kurita@sc.niigata-u.ac.jp⟩
Abstract.—Five species of bivalves and two species of gastropods are described from late Selandian to earliest Thanetian wood-fall communities from the Katsuhira Formation in Urahoro Town, eastern Hokkaido, northern Japan. Three bivalves and two gastropods are new to science: Thyasira (Thyasira) oliveri Amano and Jenkins, new species, Astarte (Astarte) paleocenica Amano and Jenkins, new species, Poromya katsuhiraensis Amano and Jenkins, new species, Neverita majimai Amano and Jenkins, new species, and Biplica paleocenica Amano and Jenkins, new species. Poromya katsuhiraensis n. sp. and Neverita majimai n. sp. are the earliest records of their genus. Astarte paleocenica n. sp. is the last species before the genus disappeared from the northern Pacific region during the Eocene, only to reappear with the opening of the Bering Strait during the latest Miocene. Moreover, two bivalve species and one gastropod genus are Cretaceous relict forms: Propeamussium yubarense (Yabe and Nagao, 1928), Myrtea ezoensis (Nagao, 1938), and Biplica Popenoe, 1957. These species and other relict protobranch bivalves had wide geographical ranges in the deep sea during the Cretaceous, which helped them to survive the end-Cretaceous mass extinction. The chemosynthesis-based species Bathyacmaea? sp., Myrtea ezoensis, and Thyasira oliveri n. sp. were recovered, but small bathymodioline mussels have not been found. This confirms that the small deep-sea mussels did not appear in the wood-fall communities at least by the earliest Thanetian.
UUID:
http://zoobank.org/125bd4ab-b172-43d9-80b9-57b75a805150 Introduction
Wood-fall communities are considered an evolutionary first step for shallow-water invertebrates in adapting to vent and seep sites (e.g., Distel et al., 2000; Lorion et al., 2010). Therefore, it is essential to reconstruct the evolution of these communities over geological time, especially for so-called ‘Cenozoic type’ chemosynthetic communities, including the vesicomyids and bathymodiolines, which first appeared in the middle Eocene (Amano and Kiel, 2007; Kiel and Amano, 2013). Although many Cretaceous chemosynthetic communities have been recorded (e.g., Campbell, 2006), only two Paleocene chemo- synthetic communities have been reported, from the Panoche Hills in California (Schwartz et al., 2003) and Spitsbergen Island (Hryniewicz et al., 2016). Neither of these communities contained vesicomyids or bathymodiolines. In Urahoro Town, eastern Hokkaido (Fig. 1), many species
of deep-sea mollusks have been recovered from the Paleocene Katsuhira Formation around its type locality (Oda et al., 1959) by the authors and their colleagues (Amano and Jenkins, 2014, 2017; Amano and Oleinik, 2014; Amano et al., 2015a, b, 2016b). These are defined as wood-fall communities because they include some chemosymbiotic species such as limpets, lucinids, and thyasririds, as well as an aporrhaid gastropod, a detritus feeder on plant fragments, deposit feeders such as
protobranch bivalves, suspension feeders such as Bentharca Verrill & Bush, 1898, and predators such as buccinoideans and cancellariids (Amano et al., 2016a), adding to wood-boring bivalves such as Xylophaga? sp. Among the protobranchs, two species and two genera survived the end-Cretaceous mass extinction, but disappeared by the end of the Paleocene (Amano and Jenkins, 2017). Moreover, it has been elucidated that Bentharca steffeni Amano, Jenkins, and Nishida, 2015a, Admete katsuhiraensis Amano, Oleinik, and Jenkins, 2016b, and Tindaria paleocenica Amano and Jenkins, 2017 are the oldest records of their genera worldwide. Thus, in the Paleocene, it is possible that the deep sea in the northwestern Pacific could have acted as a refuge from the end-Cretaceous extinction and as a place of origin of Cenozoic-type mollusks. To address this hypothesis, it is necessary to carry out detailed taxonomic research on the fauna of the Katsuhira Formation. In this paper, we describe some new species and Mesozoic-relict species from the formation in detail.
Geologic setting
Molluscan fossils were collected by the authors from 45 local- ities of the Katsuhira Formation (Fig. 1). Stratigraphically, use of the Katsuhira Formation by Kaiho (1984) has caused some confusion. In this paper, we use Katsuhira Formation
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