New Fossil Record of the Genus Bathynomus

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New Fossil Record of the Genus Bathynomus (Crustacea: Isopoda: Cirolanidae) from the Middle and Upper Miocene of Central Japan, with Description of a New Supergiant Species Author(s): Hisayoshi Kato , Yukito Kurihara and Toru Tokita Source: Paleontological Research, 20(2):145-156. Published By: The Palaeontological Society of Japan DOI: http://dx.doi.org/10.2517/2015PR029 URL: http://www.bioone.org/doi/full/10.2517/2015PR029

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Paleontological Research, vol. 20, no. 2, pp. 145–156, April 1, 2016 © by the Palaeontological Society of Japan doi:10.2517/2015PR029

New fossil record of the genus Bathynomus (Crustacea: Isopoda: Cirolanidae) from the middle and upper Miocene of central Japan, with description of a new supergiant species HISAYOSHI KATO1, YUKITO KURIHARA2 AND TORU TOKITA3 1Natural

History Museum and Institute, Chiba, 955-2, Aoba-cho, Chiba 260-8682, Japan (e-mail: [email protected]) Department of Geology, Faculty of Education, Mie University, 1517 Kurimamachiya-cho, Tsu, Mie, 514-8507, Japan 32-14-2, Iwasaki, Ichihara, Chiba 290-0047, Japan 2

Received May 31, 2015; Revised manuscript accepted September 18, 2015

Abstract. Three species of the cirolanid isopod genus Bathynomus are reported from the Miocene deposits of central Japan. Bathynomus kominatoensis sp. nov. and an unidentified Bathynomus sp. 1 are described from the upper Miocene Amatsu Formation in the Kominato-Osawa area, Chiba Prefecture. Another large species, Bathynomus sp. 2, was obtained from the middle Miocene Negishi Formation in the Iwadono Hills area, Saitama Prefecture. The new species most resembles Bathynomus giganteus, inhabiting the deep-sea floor of the Western Atlantic and Caribbean Sea. In addition to the large-sized body, Bathynomus kominatoensis sp. nov. and Bathynomus sp. 2 are included in the supergiant species group of the genus based upon morphology of the pleotelson and uropods. Both occurrences extend the geographic and stratigraphic ranges for the supergiant species group of Bathynomus into the Miocene of central Japan, although no species of the supergiant group is distributed in modern Japanese waters. Key words: Amatsu Formation, Bathynomus, Crustacea, Isopoda, Negishi Formation, supergiant species

Introduction The large cirolanid isopod genus Bathynomus A. Milne-Edwards, 1879, lives on the deep-sea floor of tropical to warm-temperate oceans and contains 18 extant species (Lowry and Dempsey, 2006). Bathynomus giganteus A. Milne-Edwards, 1879, the type species inhabiting the western Atlantic, is well known to have the largest body among all living isopods, with a body length that reaches to a maximum of 46 cm (Wetzer, 1986). Lowry and Dempsey (2006) reexamined all the living Bathynomus species and divided them into two species groups, the “giants” that range from 80 mm to 140 mm and the “supergiants” that range from 170 mm to 500 mm in mature body length. According to their definition, the sole Japanese extant species Bathynomus doederleini Ortmann, 1894, inhabiting the lower sublittoral to bathyal waters of the Pacific Ocean side south of central Japan (Sekiguchi et al., 1982; Lowry and Dempsey, 2006), belongs to the giant species group. The fossil record of Bathynomus is mostly documented from the Cenozoic of Japan until now (Karasawa et al.,

1992a; Obata, 2006; and references therein). Except for Bathynomus sp. from the Pliocene of Okinawa (Karasawa and Nobuhara, 2008), which is the only known fossil record of the supergiants, all of those fossils are assigned to the giant species group. In this paper, we describe a new supergiant and two unnamed species of Bathynomus from the middle and upper Miocene of central Japan and discuss their taxonomic implications. All material described herein is deposited in the Natural History Museum and Institute, Chiba (CBM-PI).

Materials and geologic settings The fossil specimens described herein were collected from the Kominato-Osawa area, Chiba Prefecture and the Iwadono Hills area, Saitama Prefecture, central Japan (Figure 1). Kominato-Osawa area Bathynomus fossils were found in mudstone blocks at locs. Km-1 to Km-10 and Os-1, 2 (Figure 2A). The mud-

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stone blocks are derived from the nearby cliffs of the upper Miocene to the lowest Pliocene Amatsu Formation of the Awa Group (13–5 Ma: Kameo et al., 2002). Bathynomus fossils were obtained from the middle part of the Amatsu Formation and the stratigraphic interval ranges from the pyroclastic key beds Am 44 to above Am

Figure 1. Index map of the study areas and the Miocene strata in southern Kanto region.

46 of Nakajima et al. (1981) and Takano et al. (1998) (Figure 2B). Tokuhashi et al. (2000) reported a radiometric age of 8.5±0.5 Ma from the Am 40 key bed. According to Kameo et al. (2002), the horizon just below the Am 44 key bed is correlated to the upper border of NN9 of the nannofossil biozonation of Martini (1971). Judging from these data, the fossil-bearing horizons are considered to be the middle upper Miocene. Molluscan data suggest that the middle part of the Amatsu Formation in the Kominato-Osawa area was deposited in a bathyal environment (Kurihara and Tokita, 2010). Based upon benthic foraminifera, Kitazato (1991) indicated that the mudstone layer of the Amatsu Formation was deposited in deep water at depths of 1000 to 2000 m. Iwadono Hills area In the Iwadono Hills area, a large specimen referred to Bathynomus sp. was obtained from the Negishi Formation of the Tokigawa Group at loc. Iwd-22 of Kurihara (2010). This locality is assigned to the interval between I-1 and I-3 tuff beds of the upper part of the Negishi Formation (Kurihara et al., 2003). According to the diatom biostratigraphy (Kurihara et al., 2003), this interval correlates to the lower part of the Denticulopsis praedimorpha Zone (NPD5B) between biohorizons D51 and D52 of Yanagisawa and Akiba (1998). The numerical age of D51 and D52 is at 12.7 Ma and 12.4 Ma, respectively (Yanagisawa and Kudo, 2011). Therefore, the Bathynomusbearing horizon in the Iwadono Hills area is upper mid-

Figure 2. Fossil localities and stratigraphic horizons in the Kominato-Osawa area. A, Bathynomus fossil localities. Base map is “Awakominato” 1:25,000 topographic map published by the Geographic Survey Institute of Japan. B, Columnar sections indicating Bathynomus-fossilbearing horizons in the Amatsu Formation. Pyroclastic key beds “Am 44, 45, 46” are based on Nakajima et al. (1991) and Takano et al. (1998).

Miocene Bathynomus from Japan dle Miocene. Bathynomus sp. was found in a muddy mediumgrained sandstone facies in association with a deep-water molluscan assemblage including Limopsis nodai Kurihara, Phanerolepida pseudotransenna Ozaki and Halicardia sp. Molluscan data suggest that the Negishi Formation was deposited at a marginal shelf environment (Majima, 1989; Kurihara, 2015).

Systematic paleontology by H. Kato and T. Tokita Order Isopoda Latreille, 1817 Suborder Cymothoida Wägele, 1989 Family Cirolanidae Dana, 1852 Genus Bathynomus A. Milne-Edwards, 1879 Type species.—Bathynomus giganteus A. MilneEdwards, 1879, by original designation. Remarks.—As already discussed by many authors (Hessler, 1969; Feldmann and Goolaerts, 2005; Polz et al., 2006; Feldmann and Rust, 2006; Pasini and Garassino, 2012; Jones et al., 2014), the taxonomy of the extinct genus Palaega Woodward, 1870 and distinguishing from the genus Bathynomus have long been controversial. Mainly due to their biphasic molting (Schöbl, 1880; George, 1972), the fossils considered to be exuviae invariably lack the anterior portion of the body (Wieder and Feldmann, 1989). Many species, including the type species Palaega carteri Woodward, 1870, have been described based only upon the posterior half of the body. In addition, detailed features of the mouthparts and pereopods are scarcely preserved in the fossil material (Wieder and Feldmann, 1992), although they are important characters for the taxonomy of living isopod crustaceans (Brandt and Poore, 2003). Consequently, Palaega was thought to be a “form genus” that might be a mixture of several genera or even families (Hessler, 1969) and a group assemblage (Bowman, 1971), and the generic assignments of the previously described species were considered to be questionable (Feldmann and Goolaerts, 2005; Feldmann and Rust, 2006). In fact, some species of Palaega described based only on the posterior half of the body are difficult to distinguish from the genus Bathynomus. In consideration of these situations, the ICZN made the decision that the generic name Bathynomus A. Milne-Edwards, 1879, was given precedence over the genus Palaega Woodward, 1870 (Opinion 1668: Anonymous, 1992). Recently, Hyžný et al. (2013) proposed a taxonomic key to the fossils of Cirolanidae Dana, 1852 and Urdidae Kunth, 1870, and distinguished Palaega sensu lato,

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Palaega sensu Woodward, 1870 and Bathynomus. According to their definitions, the pleotelson of Bathynomus does not have the serrated lateral margin that is recognized in Palaega, and bears 7 to 13 subequal distinct pleotelsonic spines. The present material is consistent with their definition and is clearly referred to Bathynomus. Fossil occurrences of Bathynomus have been hitherto documented in the Japanese Neogene and Quaternary. Bathynomus undecimspinosus (Karasawa, Nobuhara and Matsuoka, 1992) is known from the lower to middle Miocene of Japan (Imaizumi, 1953; Obata and Omori, 1993; Karasawa et al., 1995; Karasawa, 1997; Takakuwa, 2004; Obata, 2006). Bathynomus aff. doederleini was reported from the lower Pleistocene Hijikata Formation, Kakegawa Group (Karasawa et al., 1992a). Other unnamed Bathynomus fossils from Japan include the uppermost Miocene Senhata Formation, Awa Group (Karasawa et al., 1992b), the upper Pliocene Nakatsu Group (Fujiyama, 1982), and the middle to upper Pliocene Yonahama Formation, Shimajiri Group (Karasawa and Nobuhara, 2008). The stratigraphic ranges and distributions of these Japanese fossil Bathynomus species were given by Takakuwa (2004) and Obata (2006). Geologic range.—Latest early Miocene (Obata, 2006; Takakuwa, 2004) to Recent. Bathynomus kominatoensis Kato and Tokita, sp. nov. Figures 3.1, 2, 4, 5, 4.1, 2

Type materials.—Holotype (CBM-PI 0002772: pereonites 5–7, pleonites 1–5, pleotelson) from Km-5; paratype 1 (CBM-PI 0002766: pereonite 6–7, pleonites 1–5, pleotelson) from Km-7; paratype 2 (CBM-PI 0002760: pleonites 3–5, pleotelson, uropod) from Km-4; paratype 3 (CBM-PI 0002771: pereonites 5–7, pleonites 1–5, pleotelson, uropod) from Km-8; paratype 4 (CBM-PI 0002768): pleotelson, uropodal exopod) from Os-1. Other materials examined.—CBM-PI 0002761 (pleotelson, uropodal exopod, endopod), 0002765 (pleonites 2–5, pleotelson, uropodal exopod), 0002767 (pereonites 6–7?, pleonites 1–5), 0002770 (pleonites 3–5, pleotelson, uropodal exopod), 0002773 (pleonites 4–5?, pleotelson), 0002774 (pleonites, uropodal exopod), 0002775 (pereonites 6–7?, pleonites 1–5, pleotelson, uropodal exopod). Type locality.—Km-5. Cliff northeast of the Kobentenjima Islet, Kominato, Kamogawa City, Chiba Prefecture (35°6ƍ49ƎN, 140°11ƍ57ƎE). Occurrence.—Km-2, 4–9, Os-1. Middle part of the Amatsu Formation, Awa Group. Etymology.—The species name refers to Kominato district of Kamogawa City, Chiba Prefecture.

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Figure 3. Bathynomus spp. from the Amatsu Formation. 1–2, 4–5, Bathynomus kominatoensis sp. nov.; 1a, paratype 4, CBM-PI 0002768 from Os-1; 1b, sketch of 1a; 2, internal mold of holotype showing the left uropodal exopod (arrow), CBM-PI 0002772 from Km5; 4a, paratype 3. CBM-PI 0002771 from Km-8; 4b, sketch of 4a; 5a, holotype. CBM-PI 0002772; 5b, sketch of 5a; 3, Bathynomus sp. 1; 3a, CBM-PI 0002776 from Km-3; 3b, sketch of 3a. Scale bar is 1 cm in Figure 3.1–3.3; 5 cm in Figure 3.4, 3.5. Abbreviation: V–VII, pereonite; 1–5, pleonite; Pt, pleotelson; Ex, uropodal exopod; Ed, uropodal endopod. Abbreviations are same hereafter.

Miocene Bathynomus from Japan

149

Figure 4. Bathynomus kominatoensis sp. nov. 1, paratype 2; 1a, CBM-PI 0002760 from Km-4; 1b, sketch of 1a; 2, paratype 1, CBMPI 0002766 from Km-7; 2a, internal mold; 2b, external mold; 2c, sketch of 2b; 3, annelid fossils on the pleotelson of paratype1. Scale bar is 1 cm in Figure 4.1; 5 cm in Figure 4.2; 5 mm in Figure 4.3.

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Diagnosis.—Large-sized Bathynomus. Pleurae of pleonites narrow, gently arcuate. Pleotelson semicircular, wider than long with 13 distinct posterior marginal spines; medial longitudinal carina indiscernible or faintly observed. Uropodal exopod elongate oval; lateral margin convex, medial margin straight. distomedial corner of endopod a nearly right angle. Description.—Pereonites 5–7, pleonites 1–5, pleotelson, uropods and fragmentary pereopod preserved. Pereonite 5 longest, anterior and posterior margins straighter than other somites. Although pereonite 7 exhibits strongly arcuate anterior and posterior margins, pereonite 6 with somewhat straighter posterior margin. Coxal plates mostly displaced. Pleonite with gently forwardly convex tergite and moderately arcuate pleurae. Pleonite 1 overlapped by pereonite 7 and overlapping pleonite 2. Pleonites 2–4 nearly equal in length; broader than pleotelson; extending to narrow abdominal pleurae. Pleurae moderately long, gently curved posterolaterally with a median longitudinal carina; tip acute. Pleura of pleonite 3 extending up to pleonite 5. Pleotelson semicircular, wider than long without distinct median longitudinal carina. Surface evenly punctuate. Thirteen short, triangular spines directed posteriorly. Medial spine distinct with a pointed tip; lateral ones slightly smaller; third and fourth spines prominent; outermost spine small, but distinct. Propodus of pereopod 7 slender, dactylus gently curved. Uropodal exopod elongate oval; lateral margin convex, medial margin straight. Endopod longer than exopod; distolateral corner obtuse, not produced, with a minute spine; distomedial corner rounded, nearly a right angle. Remarks.—The present new species most resembles Bathynomus giganteus in the general shape of the pereonites, pleonites and pleotelson. However, B. kominatoensis sp. nov. can be distinguished from B. giganteus in that the pleotelson is much wider than long. According to Cocke (1987), the ratio of maximum width to length of the pleotelson ranges from 0.676–0.911 (mean: 0.809) in B. giganteus but that of B. kominatoensis sp. nov. is smaller, 0.644–0.817 (mean: < 0.721). The distomedial corner of the uropodal endopod of the new species is nearly at a right angle instead of strongly curved as in B. giganteus. Thirteen distinct pleotelsonic spines of the new species also support distinguishing it from B. giganteus. Although the number of pleotelsonic spines of B. giganteus is usually 11 or 13 (Lowry and Dempsey, 2006) including a small to faint outermost one, that of the present new species is 13 including a rather distinct outermost one. The present new species also resembles B. keablei

Lowry and Dempsey, 2006, known from the Indian Ocean, in having the wider pleotelson. However, B. keablei bears 11 short, straight pleotelsonic spines, fewer than B. kominatoensis sp. nov. The present new species is distinguished also from another supergiant species, B. kensleyi Lowry and Dempsey, 2006, known from the South China Sea, Sulu Sea and Coral Sea, which is characterized by 11 remarkably elongated and upwardly curved pleotelsonic spines. Magalhães and Young (2003) noted the lengths of the pleotelson of three living species (B. giganteus, B. miyarei, B. obtusus) are about 1/4 of the total body lengths. Based on this ratio, the total length of paratype 1 (CBM-PI 0002766) is estimated to be larger than 240 mm. Bathynomus sp. from the Pliocene Shimajiri Group, Okinawa, Japan (Karasawa and Nobuhara, 2008) retains pleonites 2 to 5 and the pleotelson, and the total length of the remaining portion attains 115 mm. This is larger than the corresponding parts of the largest specimen of B. kominatoensis sp. nov. Judging from the remaining portion, it had at least 11 pleotelsonic spines. Unfortunately, however, the poorly preserved sole specimen does not allow further comparisons. Several minute coiled annelids referable to Spirorbidae or Serpulidae are observed on the dorsal surface of the pleonites and pleotelson of paratype 1 (Figure 4.3). Such epizoans, including cirripedes, gastropods, annelids, branchiurans and hydrozoans, are found on the dorsal and ventral surfaces of the Recent Bathynomus species (Bruce, 1986; Cocke, 1987; Briones-Fourzán and Lozano-Alvarez, 1991) as well as fossil Palaega (Jones et al., 2014). Measurements (mm).—Given in Table 1. Bathynomus sp. 1 Figure 3.3

Material examined.—CBM-PI 0002776 (internal mold of pereonites 5–7, pleonites 1–5, pleotelson, uropods) from Km-3. Occurrence.—Km-3. Middle part of the Amatsu Formation, Awa Group. Description.—Moderate-sized for giant species group of Bathynomus. Fragments tentatively attributable to coxal plates of anterior pereonites are recognized. Pereonites 5 and 6 long, nearly equal in length. Pereonite 7 much shorter than 5 and 6. Pleonites noticeably narrower than pereonites. Pleonites 2–4 nearly equal in length. Pleotelson wider than long with a distinct median carina. Two pleotelsonic spines including medial one are discernible. Distolateral angle of uropodal endopod makes an acute triangle, directed posteriorly. Remarks.—The sole specimen was obtained from

Miocene Bathynomus from Japan Table 1.

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Measurements of Bathynomus spp. from the Amatsu and Negishi formations. L, length; W, width. Pereonite 5 Pereonite 6 Pereonite 7 Pleonite 1 Pleonite 2 Pleonite 3 Pleonite 4 Pleonite 5 L

W

L

W

L

W

L

W

L

W

L

W

L

W

L

W

Pleotelson Pleon total L.

L

W

Bathynomus kominatoensis sp. nov. CBM-PI 0002760

7.1

CBM-PI 0002765 CBM-PI 0002766

7.5 18.7

84.1 18.0 79.4 16.1

12.4

CBM-PI 0002770 CBM-PI 0002771

6.6 65.3+ 7.6

6.2

26.1 10.8 51+

CBM-PI 0002772

8.3 50+ 11.2

9.6 51.6

9.3+ 50.5 5.7 54.5+ 6.2 53.0+

9.4 52+

6.2 56.6+ 5.5 56.5+

3.3 25.5

7.2

35.5+ 53.9+ 32.4+

45.5+ 64+

43.7

61+

4.2 23.0

7.2 53.5+ 6.3 49+

78.4+

23.5+ 23.2 32.7

35.3

43.2+

5.5 55.9+ 6.4 56.1+ 29.6

39.3

54+

51.5

71.2

CBM-PI 0002773 Bathynomus sp. 1 CBM-PI 0002776

3.3 28.8

2.9

3.3

17.7

9.5 64.0

9.2 61.1

7.8 57.5+ 39+

53.4

Bathynomus sp. 2 CBM-PI 0002611

nearby the locality of Bathynomus kominatoensis sp. nov. Based upon the assumption that the length of the pleotelson is 1/4 of the total body length (Magalhães and Young, 2003), the length of this specimen in life is estimated as about 70 mm. In addition to the smaller size, it is distinguished from B. kominatoensis sp. nov. by having a pleotelson with a distinct median longitudinal carina and the uropodal endopod with an acute distolateral angle. Whereas the marginal features of the pleotelson cannot be observed sufficiently, this species resembles the extant B. doederleini in having a proportionally shorter and narrower pleotelson. In contrast, this species less resembles Bathynomus sp. from the uppermost Miocene Senhata Formation, Awa Group (Karasawa et al., 1992b) and B. undecimspinosus from the lower to middle Miocene of Japan because those species have a relatively broader pleotelson. Measurements (mm).—Given in Table 1. Bathynomus sp. 2 Figures 5.2, 5.3

Material examined.—CBM-PI 0002611 (internal and external mold of pereonites 6–7, pleonites and pleotelson) from Iwd-22. Occurrence.—Iwd-22. Upper part of the Negishi Formation. Description.—Large-sized Bathynomus retaining fragmentary pereonites 6–7, pleonites 1–5, and pleotelson. Pereonite 6 long, length nearly twice that of pereonite

9.2 65.3

57.0

7. Pleonites 2 and 3 nearly equal in length. Pleonite 4 longest. Pleurae of pleonites 1–4 preserved, directed posterolaterally. Pleura of pleonite 3 broad, largest. Strong median carina on pleura of pleonite 3 and 4. Pleotelson broad, obviously wider than long. A medial pleotelsonic spine, 4 spines on right side and 5 on left side present on posterior margin. Uropodal exopod showing elongate oval outline; lateral margin strongly convex, distal margin rounded. Distomedial corner of endopod rounded, forms a nearly right angle. Remarks.—Although several characters such as the number of the pleotelsonic spine or the shape of the uropods are not distinct, Bathynomus sp. 2 is distinguished from B. kominatoensis sp. nov. in having a relatively longer pleotelson and narrower pleonites. Bathynomus sp. 2 is readily distinguished from the early to middle Miocene B. undecimspinosus by its broad pleotelson without a median longitudinal carina. The length of the remaining posterior part of the body, from the anterior margin of the pereonite 7 to the tip of the pleotelson, exceeds 100 mm, and the total body length is estimated to have been approximately 210 mm. Originally there were more than 11 pleotelsonic spines, as extrapolated from the remaining 10 spines. The median longitudinal carina on the pleotelson is indiscernible. Measurements (mm).—Given in Table 1.

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Figure 5. Bathynomus sp. 2 from the Negishi Formation and reference specimen of Bathynomus undecimspinosus. 1, Bathynomus undecimspinosus (Karasawa, Nobuhara and Matsuoka, 1992), CBM-PI 0001340, right uropodal exopod, early middle Miocene Higashibessho Formation, Yatsuo Group; 2, 3, Bathynomus sp. 2, CBM-PI 0002611 from Iwd-22; 2, external mold of pleotelson showing right uropodal exopod (arrow); 3a, internal mold; 3b, sketch of 3a. Scale bar is 1 cm in Figure 5.1, 5.2; 5 cm in Figure 5.3.

Discussion The two species groups of the genus Bathynomus proposed by Lowry and Dempsey (2006), the giants and supergiants, were defined primarily by a combination of

the total body length and the morphology of the pleotelson and uropods. Concurrently, they described and figured the detailed diagnostic characters for each species, some of which may be useful to distinguish the two species group (Table 2). According to Lowry and Dempsey

Miocene Bathynomus from Japan

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Table 2. Comparisons of characters of the pleotelson and uropods, species diversity and habitat between living supergiant and giant species groups in the genus Bathynomus. Data based on Lowry and Dempsey, 2006; Magalhães and Young, 2003; Bruce and Bussarawit, 2004. Supergiant species group Total body length

Giant species group

170–500 mm

80–140 mm

Longitudinal carina

Mostly incospicuous

Conspicuous

Number of spines

11–13 (2 lateral, 9–11 distal)

7–11 (2 lateral, 5–9 distal)

Shape of spine

Straight or upwardly corved

Straight

Medial spine

Simple

Simple or bifid

Outline

Elongated oval outline, tip obtsuse or minutely pointed

Lanceolate, tip pointed

Setal fringe on the outer margin

Medium

Varying from short to continuous

Uropodal endpod

Distolateral corner usually obtsuse with pointed tip

Distolateral corner triangular with pointed apex

Number of species

9

9

Geographic range

W Atlantic (SE USA to Brazil), Carribean, Indian Ocean, Andaman, South China Sea, Phillipines, Sulu Sea, Coral Sea, E Australia

W Pacific (Japan, Taiwan, Philippines to SE Australia), N and NW Australia, Brazil

Pleotelson

Uropodal exopod

(2006), species of the giants have 7–11 pleotelsonic spines whereas the supergiants have 11–13 spines. Four species of the giants have a bifid medial pleotelsonic spine whereas none of the supergiants has such a spine. Two species of the supergiants have upwardly curved pleotelsonic spines whereas none of the giants have such spines. All of the giants have a conspicuous median longitudinal carina on the pleotelson, in contrast to the carina being inconspicuous or indiscernible in most of the supergiants. Furthermore, the general outlines of the uropods are useful to distinguish the giants and supergiants (Table 2, 3). Although the specimens described herein lack the cephalon and anterior pereon together with the appendages, the outlines of the uropodal exopod and endopod can be observed. Judging from the figures in Shih (1972, figs. 9–11), Bruce (1986, figs. 87–91), Soong (1992, fig. 1), Magalhães and Young (2003, figs. 5, 10, 14), Bruce and Bussarawit (2004, fig. 4), and Lowry and Dempsey (2006, figs. 2–22), members of the supergiants typically have a broader uropodal exopod exhibiting an elongate oval outline in contrast to the slender, lanceolate ones in the members of the giants. In addition, the distolateral corner of the uropodal endopod of the supergiants usually subtends an obtuse angle with the rounded or minutely pointed tips. However, that of the giants is evi-

dently triangular with a sharp apex. In the three species described in this study, two supergiants and one giant are recognized (Table 3). Bathynomus kominatoensis sp. nov. and Bathynomus sp. 2 belong to the supergiants on the basis of the large body length, the lack of a distinct median longitudinal carina on the pleotelson, and the elongated oval uropodal exopod. On the other hand, Bathynomus sp. 1 is referred to the giants on the basis of its small body length and the presence of a distinct median longitudinal carina. Hyžný et al. (2013) remarked that the early to middle Miocene Bathynomus undecimspinosus might be assigned to the giants. We agree with them because the body size is nearly the same as that of the extant B. doederleini. In addition, a conspicuous median longitudinal keel on the pleotelson, and the triangular pointed apex of the uropodal exopod, also support this assignment (Tables 2, 3). Among the hitherto fossil record of Bathynomus, an unnamed species from the Pliocene Shimajiri Group, Okinawa, Japan (Karasawa and Nobuhara, 2008) is recognized as a sole member of the supergiants because of its large length of the pleon and pleotelson. Therefore, the occurrences of Bathynomus kominatoensis sp. nov. and Bathynomus sp. 2 extend the geographic and stratigraphic ranges of the supergiant species of Bathynomus into the Miocene of central Japan,

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Table 3. Comparisons of the discussed fossil with living species. Maximum body lengths of fossil species were estimated from the length of pleotelson and indicated with asterisk (*). See text for details. Species group

Maximum body length

Pleotelsonic spine number

supergiant

> 240 mm*

13

B. sp. 1

giant

70.8 mm*

unknown

B. sp. 2

supergiant

210 mm*

11≤

Outline of uropodal exopod

Distolateral angle of uropodal endopod

Median longitudinal carina of pleotelson

elongate oval

obtuse, tip faintly pointed

absent or inconspicuous

This study

unclear

acute triangular

conspicuous

This study

elongate oval

unknown

absent

This study

References

This paper B. kominatoensis sp. nov.

Early to middle Miocene of Japan B. undecimspinosus

giant

123.2 mm*

11 or 9

lanceolate, tip pointed

unknown

conspicuous

Takakuwa, 2004, Obata, 2006

Living species B. giganteus

supergiant

421 mm

11 or 13

elongate oval

obtuse, tip faintly pointed

absent or inconspicuous

Soto & Mincarone, 2001, Magalhães & Young, 2003

B. kensleyi

supergiant

300 mm

10 or 11

elongate oval

obtuse, tip faintly pointed

inconspicuous

Lowry & Dempsey, 2006

B. keablei

supergiant

270 mm

11

elongate oval

obtuse, tip faintly pointed

conspicuous

Lowry & Dempsey, 2006

giant

133 mm

7 or 9

lanceolate, tip pointed

acute triangular

conspicuous

Lowry & Dempsey, 2006

B. doederleini

and suggest that the two lineages, giants and supergiants, had existed in Miocene time. In the recent Western Pacific, two supergiant species of Bathynomus are known from the deep sea (Lowry and Dempsey, 2006). Bathynomus kensleyi inhabits depths of 300–1000 m in the South China Sea, Philippines and eastern Australia, and B. richeri occurs at depths of 586– 950 m off New Caledonia (Lowry and Dempsey, 2006). The Atlantic species Bathynomus giganteus inhabits depths of 170–2140 m (Cocke, 1987), but it is commonly found between 365 m and 730 m (Holthuis and Mikulka, 1972) or 349–733 m (Briones-Fourzán and LozanoAlvarez, 1991). On the other hand, Bathynomus doederleini occurs at depths of 150–800 m but it is abundant between 250 and 550 m (Soong and Mok, 1994) or at continental shelf and slope depths of 100–680 m (Lowry and Dempsey, 2006). These depths of living species are consistent with other paleoecological data of the Amatsu Formation (Kitazato, 1991; Kurihara and Tokita, 2010) and the Negishi Formation (Majima, 1989; Kurihara, 2015).

Acknowledgements The authors wish to express our sincere gratitude to

the late Warren C. Blow and to William Moser (National Museum of Natural History, Smithsonian Institution), Mary Wicksten (Texas A & M University), and Yusuke Ando (Mizunami Fossil Museum) for their help in accessing the specimens and literature of Bathynomus. Thanks are also due to Tomoyuki Komai (Natural History Museum and Institute, Chiba) and Hiroaki Karasawa (Mizunami Fossil Museum) for providing many useful comments. We are grateful to Robert Jenkins (Kanazawa University), Naoki Takahashi (Natural History Museum and Institute, Chiba) and Yukio Yanagisawa (Geological Survey of Japan/AIST) for providing valuable information about the geology and geochronology of the Kominato-Osawa and Iwadono Hills areas. We gratefully acknowledge Rodney M. Feldmann (Kent State University, Ohio) and two reviewers, H. Karasawa and Matúš Hyžný (Natural History Museum Vienna) for improvements of the manuscript.

References Anonymous, 1992: Opinion 1668. Bathynomus A. Milne Edwards, 1879 (Crustacea, Isopoda): given precedence of Palaega Woodward, 1870. Bulletin of Zoological Nomenclature, vol. 49, p. 86– 87. Bowman, T. E., 1971: Palaega lamnae, new species (Crustacea:

Miocene Bathynomus from Japan Isopoda) from the Upper Cretaceous of Texas. Journal of Paleontology, vol. 45, p. 540–541. Brandt, A. and Poore, G. C. B., 2003: Higher classification of the flabelliferan and related Isopoda based on a reappraisal of relationships. Invertebrate Systematics, vol. 17, p. 893–923. Briones-Fourzán, P. and Lozano-Alvarez, E., 1991: Aspects of the biology of the giant isopod Bathynomus giganteus A. Milne Edwards, 1879 (Flabellifera: Cirolanidae), off the Yucatan Peninsula. Journal of Crustacean Biology, vol. 11, p. 375–385. Bruce, N. L., 1986: Cirolanidae (Crustacea: Isopoda) of Australia. Records of the Australian Museum, Supplement, no. 6, p. 1–219. Bruce, N. L. and Bussarawit, S., 2004: Bathynomus lowryi n. sp. (Crustacea: Isopoda: Cirolanidae), the first record of the “giant” marine isopod genus from Thailand waters. Phuket Marine Biological Centre Research Bulletin, vol. 65, p. 1–8. Cocke, B. T., 1987: Morphological Variation in the Giant Isopod Bathynomus giganteus (Suborder Flabellifera: Family Cirolanidae) with Note on the Genus, 129 p. Master’s thesis, Texas A & M University, Texas. Dana, J. D., 1852: On the classification of the Crustacea Choristopoda or Tetradecapoda. American Journal of Science and Arts, Second Series, vol. 14, p. 297–316. Feldmann, R. M. and Goolaerts, S., 2005: Palaega rugosa, a new species of fossil isopod (Crustacea) from Maastrichtian rocks of Tunisia. Journal of Paleontology, vol. 79, p. 1031–1035. Feldmann, R. M. and Rust, S., 2006: Palaega kakatahi n. sp.: the first record of a marine isopod from the Pliocene of New Zealand. New Zealand Journal of Geology and Geophysics, vol. 49, p. 411–415. Fujiyama, I., 1982: Cenozoic Insecta, Crustacea and Arachnida. In, Fujiyama, I., Hamada, T. and Yamagiwa, N. eds., Illustrated Handbook of Fossils in Japan, Student Edition, p. 366–368. Hokuryukan, Tokyo. (in Japanese; original title translated) George, R. Y., 1972: Biphasic moulting in isopod Crustacea and the finding of an unusual mode of moulting in the Antarctic genus Glyptonotus. Journal of Natural History, vol. 6, p. 651–656. Hessler, R. R., 1969: Peracarida. In, Moore, R. C. ed., Treatise on Invertebrate Paleontology, Part R, Arthropoda 4, p. R360–R393. Geological Society of America, New York and University of Kansas Press, Lawrence. Holthuis, L. B. and Mikulka, W. R., 1972: Notes on the deep-sea isopods of the genus Bathynomus A. Milne Edwards, 1879. Bulletin of Marine Science, vol. 22, p. 575–591. Hyžný, M., Bruce, N. L. and Schlögl, J., 2013: An appraisal of the fossil record for the Cirolanidae (Malacostraca: Peracarida: Isopoda: Cymothoida), with a description of a new cirolanid isopod crustacean from the early Miocene of the Vienna Basin (Western Carpathians). Palaeontology, vol. 56, part 3, p. 615–630. Imaizumi, R., 1953: Notes on Bathynomus sp. (Crustacea) from the Miocene in Japan. Short Papers, Institute of Geology and Paleontology, Tohoku University, Sendai, no. 5, p. 84–87. Jones, W. T., Feldmann, R. M. and Garassino, A., 2014: Three new isopod species and a new occurrence of the tanaidacean Niveotanais brunnensis Polz, 2005 from the Jurassic Plattenkalk beds of Monte Fallano, Italy. Journal of Crustacean Biology, vol. 34, p. 739–753. Kameo, K., Mita, I. and Fujioka, M., 2002: Calcareous nannofossil biostratigraphy of the Amatsu Formation (Middle Miocene to Lower Pliocene), Awa Group, distributed in the central part of the Boso Peninsula, central Japan. Journal of the Geological Society of Japan, vol. 108, p. 813–828. (in Japanese with English abstract) Karasawa, H., 1997: A monograph of Cenozoic stomatopod, decapod, isopod and amphipod Crustacea from west Japan. Monograph of

155

the Mizunami Fossil Museum, no. 8, p. 1–81. (in Japanese with English abstract) Karasawa, H. and Nobuhara, T., 2008: Decapoda and Isopoda (Crustacea) from the Pliocene Shimajiri Group in the Miyako-jima island, Ryukyus, Japan. Bulletin of the Mizunami Fossil Museum, no. 34, p. 23–30. Karasawa, H., Nobuhara, T. and Matsuoka, K., 1992a: Fossil and living species of the giant isopod genus Palaega Woodward, 1870 of Japan. Science Report of the Toyohashi Museum of Natural History, no. 2, p. 1–12. Karasawa, H., Suzuki, A. and Kato, H., 1995: Bathynomus undecimspinosus (Karasawa, Nobuhara & Matsuoka, 1992) (Crustacea, Isopoda) from the Miocene Hidarimatagawa Formation of southwestern Hokkaido, Japan. Bulletin of the Mizunami Fossil Museum, no. 22, p. 121–125. Karasawa, H., Tomida, S. and Inoue, K., 1992b: Palaega sp. (Isopoda: Crustacea) from the Neogene Senhata Formation, Chiba Prefecture, Japan. Journal of the Natural History Museum and Institute, Chiba, vol. 2, p. 25–30. Kitazato, H., 1991: Paleoenvironments of the Tanzawa and adjacent area based on foraminiferal evidences. In, Kanagawa Prefectural Museum of Natural History ed., Tanzawa that Came from the South Sea: Wonder of Plate Tectonics (Minami no Umi kara Kita Tanzawa: PurƝtotekutonikusu no Fushigi), p. 813–828. Yurindo, Yokohama. (in Japanese; original title translated) Kunth, A., 1870: Ueber weing bekannte Crustaceen von Solenhofen. Zeitschrift der Deutschen Geologischen Gesellschaft, vol. 22, p. 771–802. Kurihara, Y., 2010: Middle and Late Miocene marine Bivalvia from the northern Kanto region, central Japan. National Museum of Nature and Science Monographs, no. 41, p. 1–87. Kurihara, Y., 2015: Miocene molluscan fossils collected from the Godo and Negishi formations by the Kuzubukuro Fossil Survey. Reports on the Geology and Paleontology of the Kuzubukuro District, Higashimatsuyama City, Saitama Prefecture, Central Japan, p. 98–102. (in Japanese) Kurihara, Y., Horiuchi, S. and Yanagisawa, Y., 2003: Lithostratigraphy and diatom and calcareous nannofossil biostratigraphy of the Miocene sequence in the Iwadono Hills area, Saitama Prefecture, central Japan. Journal of the Geological Society of Japan, vol. 109, p. 215–233. (in Japanese with English abstract) Kurihara, Y. and Tokita, T., 2010: Occurrence of Halicardia sp. cf. houbricki Poutiers & Bernard, 1995 (Bivalvia: Verticordiidae) from the Miocene of Japan. Venus (Japanese Journal of Malacology), vol. 68, p. 179–182. Latreille, P. A., 1817: Nouveau Dictionnaire d’Histoire Naturelle, Appliquée aux Arts, à l’Agriculture, à l’Economie Rurale et Domestique, à la Médecine, etc. Tome 10, 404 p. Déterville, Paris. Lowry, J. K. and Dempsey, K., 2006: The giant deep-sea scavenger genus Bathynomus (Crustacea, Isopoda, Cirolanidae) in the IndoWest Pacific. In, Richer de Forges, B. and Justone, J.-L. eds., Tropical Deep-Sea Benthos, Vol. 24. Mémoires du Muséum National d’Histoire Naturelle, vol. 193, p. 163–192. Magalhães, N. and Young, P., 2003: Bathynomus A. Milne Edwards, 1879 (Isopoda, Cirolanidae) from the Brazilian coast, with description of a new species. Arquivos do Museu Nacional do Rio de Janeiro, vol. 61, no. 4, p. 221–239. Majima, R., 1989: Neogene stratigraphy along the Arakawa River near Yorii, and of the Ogawa Basin, Hiki Hills, and Iwadono Hills, central Saitama Prefecture, central Japan. Geoscience Reports of Shizuoka University, vol. 15, p. 1–9. Martini, E., 1971: Standard Tertiary and Quaternary calcareous nannoplankton zonation. In, Farinacci, A. ed., Proceedings of the

156

Hisayoshi Kato et al.

Second Planktonic Conference, Roma, 1970, p. 739–785. Tecnoscienza, Rome. Milne-Edwards, A., 1879: Sur un isopode gigantesque des grandes profondeurs de la mer. Comptes Rendus de l’Académie des Sciences, Paris, vol. 88, p. 21–23. Nakajima, T., Makimoto, H., Hirayama, J. and Tokuhashi, S., 1981: Geology of the Kamogawa District. Quadrangle Series, Scale 1: 50,000, 107 p. Geological Survey of Japan, Tsukuba. (in Japanese with English abstract) Obata, K., 2006: The characters of Bathynomus undecimspinosus (crustacean Isopoda) from the Miocene Nenokami Formation in the Chichibu Basin, Central Japan. Bulletin of the Saitama Museum of Natural History, no. 23, p. 1–9. (in Japanese with English abstract) Obata, K. and Omori, M., 1993: Fossil Bathynomus from the Lower Miocene Nenokami Sandstone Formation in the Chichibu Basin, Saitama Prefecture, Japan. Bulletin of the Saitama Museum of Natural History, no. 11, p. 57–64. (in Japanese with English abstract) Ortmann, A., 1894: A new species of the isopod-genus Bathynomus. Proceedings of the Academy of Natural Sciences of Philadelphia 1894, p. 191–193. Pasini, G. and Garassino, A., 2012: Palaega pisana n. sp. (Crustacea, Isopoda, Cirolanidae) from the Pliocene of Orciano Pisano, Pisa (Toscana, Central Italy). Atti della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano, vol. 153, p. 3–11. Polz, H., Schweigert, G. and Maisch, M. W., 2006: Two new species of Palaega (Isopoda: Cymothoida: Cirolanidae) from the Upper Jurassic of the Swabian Alb, South Germany. Stuttgarter Beiträge zur Naturkunde, Serie B (Geologie und Paläontologie), no. 362, p. 1–17. Schöbl, J., 1880: Über die Fortpflanzung isopoder Crustaceen. Archiv für Mikroskopische Anatomie, Band 17, p. 125–140. Sekiguchi, H., Yamaguchi, Y. and Kobayashi, H., 1982: Geographical distribution of scavenging giant isopods bathynomids in the Northwestern Pacific. Bulletin of the Japanese Society of Scientific Fisheries, vol. 48, p. 499–504. Shih, C. T., 1972: Note on the giant isopod genus Bathynomus Milne Edwards, 1879 with description of a new species. Publications of the Seto Marine Biology Laboratory, vol. 21, p. 81–82. Soong, K., 1992: Occurrence of the giant isopod Bathynomus giganteus A. Milne Edwards, 1879 (Isopoda Flabellifera, Cirolanidae) in the West Pacific. Crustaceana, vol. 63, p. 291–295. Soong, K. and Mok, H. K., 1994: Size and maturity stage observations

of the deep-sea isopod Bathynomus doederleini Ortmann 1894 (Flabellifera: Cirolanidae), in eastern Taiwan. Journal of Crustacean Biology, vol. 14, p. 72–79. Soto, J. M. R. and Mincarone, M. M., 2001: Distribution and morphology of the giant isopods Bathynomus giganteus and Bathynomus miyarei (Flabellifera, Cirolanidae) off southern Brazil. Mare Magnum, vol. 1, p. 141–145. Takakuwa, Y., 2004: Fossil Bathynomus (Crustacea: Isopoda) and “Isocrinus?” (Crinoidea: Isocrinida) from the Miocene deposit in the southeastern area of Gunma Prefecture, Central Japan. Bulletin of the Gunma Museum of Natural History, no. 8, p. 27–34. (in Japanese with English abstract) Takano, H., Kurokawa, A., Kondo, K., Hashimoto, N., Oki, J., Takahashi, N. and Nakajima, T., 1998: Key beds in the middle part of the Amatsu Formation, Miura Group. In, Nakajima, T., Takano, J. and Takahashi, N., eds., Catalogue of Key Tuff Beds in the Middle Part of the Amatsu Formation, the Miura Group (the Edition of 1997), p. 12–52. Earth Science Catalogue Series, Natural History Museum and Institute, Chiba. (in Japanese) Tokuhashi, S., Danhara, T. and Iwano, H., 2000: Fission track ages of eight tuffs in the upper part of the Awa Group, Boso Peninsula, central Japan. Journal of the Geological Society of Japan, vol. 106, p. 560–573. Wägele, J. W., 1989: Evolution und phylogenetisches System der Isopoda. Stand der Forschung und neue Erkenntnisse. Zoologica (Stuttgart), vol. 140, p. 1–262. Wetzer, R., 1986: Bathynomus. A living monster. Terra, vol. 25, p. 26– 29. Wieder, R. W. and Feldmann, R. M., 1989: Palaega goedertorum, a fossil isopod (Crustacea) from late Eocene to early Miocene rocks of Washington State. Journal of Paleontology, vol. 63, p. 73–80. Wieder, R. W. and Feldmann, R. M., 1992: Mesozoic and Cenozoic fossil isopods of North America. Journal of Paleontology, vol. 66, p. 958–972. Woodward, H., 1870: Contributions to British fossil Crustacea. Geological Magazine, vol. 7, p. 493–497. Yanagisawa, Y. and Akiba, F., 1998: Refined Neogene diatom biostratigraphy for the northwest Pacific around Japan, with an introduction of code numbers for selected diatom biohorizons. Journal of the Geological Society of Japan, vol. 104, p. 395–414. Yanagisawa, Y. and Kudo, T., 2011: Data report of the Neogene and Quaternary diatom biostratigraphy in the Kamo District, Niigata Prefecture, Japan. Open-File Report of the Geological Survey of Japan, AIST, no. 537, p. 1–59.