Jour. Geol. Soc. Japan, Vol. 114, No. 10, p. 536－539, October 2008

Squaliform teeth of the Centroselachus from the Miocene of Japan

Hideshi Suzuki ＊†

Received April 21, 2008. Accepted September 24, 2008. ＊ Graduate school of Natural Science and Technology, Division of Environmental Science and Engineering, Kanazawa Uni- versity, Kakuma, Kanazawa 920-1192, Japan † Tateshina Senior High School, 3652, Ashida, Tateshina Town, Kitasaku Gun, Nagano 384-2305 Japan.

Abstract: Newly found fossil shark teeth of a sleeper shark are described. This is probably an undescribed species of the genus Centroselachus belonging to the Family . These fossil teeth were discovered from the Middle Miocene Iseyama Formation（Northern Fossa Magna Region）, Ueda City, Nagano Prefecture, central Japan. These teeth indicate the shape of a part of a symphysial tooth row, which belong to a left side of Fig. 1. Map showing the fossil locality of Centroselachus sp. ※: parasymphysial teeth and a left first anterior tooth. 138 °18 ′00.11 ″E, 36 °25 ′12.05 ″N, using the topographical map Judging from the main characters of a parasymphysial of“Sanada”, scale 1: 25,000, published by Geographical Survey tooth, such as distal and mesial blades presented both Institute. joining in a notch, it is considered that the tooth differs from teeth of the genus . This fossil is identified as the genus Centroselachus sp.. This paper constitutes the first fossil record of the genus Cen- troselachus from the Miocene of Japan.

Key words: Centroselachus, Somniosidae, Middle Miocene, Iseyama Formation, Northern Fossa Magna Region, Nagano Prefecture, symphysial tooth row.

Introduction

The purpose of this study is to report newly found fossil shark teeth of the sleeper shark of the genus Centroselachus from the Miocene of Japan. These specimens（GNSTKU-PV0010a･b） are Fig. 2. Photograph of the outcrop at the fossil locality. well preserved which belong to a symphysial tooth row, a left side of parasymphysial teeh and a left side first anterior tooth, Geological setting which are useful to be classified into the genus of the Family Somniosidae. Sleeper are classified into eighteen deep- Tertiary sedimentary rocks which consist of Early to Middle water benthic and oceanic species in seven genera: Cen- Miocene marine deposits, are distributed in the middle part of troscymnus, Centroselachus, Proscymnodon, Scymnodala- Ueda City, Nagano Prefecture, central Japan（Fig. 1）. Geologic tias, Scymnodon, and Zameus（Compagno et al., sections of this area are lithologically subdivided into following 2005）. Formerly, the genus Centroscymnus are classified into formations in ascending order: the Oomineyama, Tarouyama, three species, C.coelolepis, C.owstoni and C.crepidater, on the Yokoo and Iseyama Formations（Yamagishi, 1964）. The northern basis of external characteristic differences. However, Cen- part of Ueda City, Yokoo and Motohara area strata, are divided troscymnus crepidater differs markedly in its tooth morpholo- into Yokoo and Iseyama Formations which are correlated to the gy from other species, C.crepidater was moved to the genus upper part of Uchimura and Bessho Formations（Yamagishi, Centroselachus（Garman, 1913）. The present author named 1964, Kosaka et al., 1992）. The upper part of the Uchimura For- them as Centroselachus crepidater（Garman, 1913）. This is the mation and the lower part of the Bessho Formation biostati- first fossil record of the genus Centroselachus from Japan. graphically correspond to Zone N8 to N9 of Tertiary planktonic foraminiferal fossil zone of Middle Miocene in age（Blow, 1969,

CThe Geological Society of Japan 2008 536 Jour. Geol. Soc. Japan 114（ 10 ）Centroselachus sp.（Elasmobranchii,）from the Miocene of Japan 537

River in Motohara area, Sanada machi, Ueda City, Nagano Pre- fecture.（Figs. 1, 2, 3）. This fossil locality corresponds with the lower part of the Iseyama Formation, which is situated between Yokoo and Iseyama Formations. This fossil horizon strata consist of parallel laminated mudstone with well-sorted thin sandstone layer, which include many other fossils, ‘Gennou-ishi’, cal- careous nodule, and pyrite grain. Main associated fossils which indicate the paleoenvironment, are as follows: Acharax toku- nagai（Yokoyama（）Suzuki, 2005b, 2007b）, Mizuhobalis izu- moensis（Yokoyama（）Suzuki, 2005b, 2007b）, Ophiotrichidae （Echinodermata:Ophiuroidea）, Alepisaurus sp.（Suzuki, 2008）, and many fishes（Suzuki, 2005a, b, 2007a, b, 2008）.

Systematic description

Class Huxley, 1880 Subclass Elasmobranchii Bonaparte, 1838 Order Squaliformes Goodrich, 1909 Family Somniosidae Jordan, 1888 Genus Centroselachus Garman, 1913 Centroselachus sp. （Fig. 5.a., 5.b） Specimens. -GNSTKU（Graduate School of Natural Science and Technology, Kanazawa University）-PV0010a･b. Fig. 3. Columnar section at the fossil locality, Kangawa River, Ueda City, Nagano Prefecture, central Japan. Locality. -Kangawa-River in Motohara area. Horizon. -The lower part of the Iseyama Formaition（after Yam- agishi, 1964）. Remarks. -Dental terminology is summarized by Kuga and Goto （1980）, Cappetta（1987）, Herman et al.（1989）, Yabe and Goto （1999）, and Adnet and Cappetta（2001）, whose terminology is widely accepted, is shown in Fig. 4. The classification is referred to Herman et al.（1989）, Compagno et al.（2005）and Cappetta （2006）. According to Herman et al.（1989: 119-121）, the genus Cen- troselachus belongs to the Family Somniosidae Jordan, 1888, and is described as follows:“This genus has markedly dignathic heterodonty. The lower teeth are strongly compressed and have a triangular principal cusp. The cusp slightly inclined distally. Fig. 4. Dental terminology（after Kuga and Goto, 1980, Cappetta, 1987, Herman et al., 1989, Yabe and Goto, 1999, modified from The mesial cutting edge is sigmoidal. The distal cutting edge Adnet and Cappetta, 2001）. tends to be almost vertical and joins the distal heel in a notch. Both cutting edges are smooth. The crown and root are labiolin- Kosaka et al., 1992）. And planktonic foraminiferal fossil, Glo- gually flattened but the labial face of the crown is weakly convex. bigerinoides sicanus and Praeorbulina circularis, occurred A short quadrangular apron is present. The apron is not over- in the upper part of the Yokoo Formation, which correspond to hanging the root but forms one part with it. The labial face of the Zone N8 to N9（Kubota and Kosaka, 1990）. This means that the root shows numerous foramina, mainly surrounding the apron. boundary of the Yokoo and Iseyama Formations Middle corre- The basal notch is situated in the central root end. The tooth is sponds to Miocene. As this setting is supported, it is considered twice as high as its base width. that the fossil horizon was deposited in the period about 15Ma. The parasymphysial teeth are present. The tooth has a trian- The lower part of the Iseyama Formation is mainly composed of gular principal cusp. The cusp erects and has distinct distal and mudstone-rich alternation and laminated mudstone（Suzuki, mesial blades both joining in a notch. Other lower teeth are lack- 2005a）. The fossil teeth of the sleeper shark reported in this ing a mesial blade. The basal notch is situated in the central root paper was discovered from outcrop at the riverside of Kangawa- end. The crown is as high as the crown base width.” 538 Hideshi Suzuki 2008―10

Fig. 6（left）. Symphysial tooth row of extant sleeper shark, Cen- troscymnus coelolepis（labial view of lower parasymphysial posi- tion）. Registration number 1662, Department of Marine Biology, School of Marine Science and Technology, Tokai University. Fig. 7（right）. Symphysial tooth row of extant sleeper shark, Cen- troscymnus owstoni（labial view of lower parasymphysial posi- tion）. Registration number 3224, Department of Marine Biology, School of Marine Science and Technology, Tokai University.

The anterior tooth has following characters: 1）teeth labiolin- gually flattened, 2）triangular cusp slightly inclined distally, 3） Fig. 5. a）Fossil symphysial tooth row of sleeper shark, Cen- troselachus sp.（GNSTKU-PV0010a） from the lower part of the cutting edges smooth, 4）mesial cutting edge sigmoidal, 5）distal Iseyama Formation（labial view of lower parasymphysial position）. cutting edge tends to be almost vertical, 6）mesial blade lacked, GNSTKU, Graduate School of Natural Science and Technology, 7 distal heel joining cusp in a notch, 8 labial face of crown Kanazawa University, Kanazawa City, Ishikawa Prefecture. b） ） ） GNSTKU-PV0010b; counter part of GNSTKU-PV0010a. slightly convex, 9）large upper mesial and distal foramina situat- ed surrounding below the apron, 10）basal notch situated central Description. -This fossil is a symphysial tooth row which consists root end, 11）tooth length two times bigger than tooth width. of a left side of parasymphysial teeth and left first anterior tooth. The parasymphysial tooth has following characters: 1）cusp The left anterior tooth is strongly compressed and has a trian- erected, 2）distinct distal and mesial blades presented both join- gular principal cusp. The cusp slightly inclined distally. The ing cusp in a notch, 3）large mesial and distal foramina situated mesial cutting edge is sigmoidal and the distal cutting edge tends surrounding below the apron, 4）basal notch situated central to be almost vertical and joins the distal heel in a notch. Both cut- root end, 5）tooth length two times bigger than tooth width. ting edges are smooth. The crown and root are labiolingually flat- Conclusion tened but the labial face of the crown is slightly convex. A narrow vertically elongated apron is present reaching the central root The genus Centroscymnus includes two species, Cen- end. The labial face of the root shows numerous foramina which troscymnus coelolepis and C.owstoni（Compagno et al., 2005）. are mainly surrounding below the apron. The large upper mesial The genus Centroselachus is monotypic, Centroselachus crep- and distal foramina are present. The basal notch is situated in the idater, which was separated from the genus Centroscymnus central root end. The mesial blade is absent. The tooth is twice as because the parasymphysial tooth has distinct distal and mesial high as its base width. blades, and both blade joining cusp in a notch（Garman, 1913）. The left side of parasymphysial teeth is present. The tooth has Comparison of the characters of these specimens with Recent a triangular principal cusp. The cusp erects and has distinct dis- species, Centroselachus crepidater, Centroscymnus tal and mesial blades both joining in a notch. The labial face of the coelolepis（Fig. 6）, and C.owstoni（Fig. 7）are shown in root shows numerous foramina situated surrounding below the Table.1. These fossil characters are almost similar to the Recent apron. The tooth is twice as high as its base width. species, C. crepidater. Judging from these characters, it is iden- Measurements. -Left parasymphysial tooth height is 2.9 mm, tified as the genus Centroselachus belonging to the Family Som- tooth width is 1.5 mm. Left first anterior tooth height is 3.0 mm, niosidae, Centroselachus sp. And also it is concluded that this tooth width is 1.5 mm. fossil is very close to Centroselachus crepidater Bocage and Capello, 1864 in morphology. Comparison Acknowledgements I would like to express my sincere grat- Comparing characters of these fossils with those of the Recent itude to Masatoshi Goto（Junior College of Tsurumi University）, species, Centroselachus crepidater Bocage and Capello, 1864 Kazue Tazaki（Kanazawa University）, Sho Tanaka（Tokai Uni- （Herman et al., 1989）, tooth height, width, outline and following versity）, Tomoyoshi Kosaka（Shinshu University）, Yuji Taka- characters are obviously belonging to the genus Centroselachus. kuwa（Gunma Museum of Natural History）, Irmhild Obinata. Jour. Geol. Soc. Japan 114（ 10 ）Centroselachus sp.（Elasmobranchii,Squaliformes）from the Miocene of Japan 539

Table. 1. Comparison with the tooth characters of Recent genera, Centroselachus and Centroscymnus（after Herman et al., 1989）.

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