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Gill Rakers of Basking Sharks (Lamniformes: Cetorhinidae) from the Tertiary of Sakhalin Island, Russia

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Gill Rakers of Basking Sharks (Lamniformes: Cetorhinidae) from the Tertiary of Sakhalin Island, Russia ZOOSYSTEMATICA ROSSICA, 23(2): 269–275 25 DECEMBER 2014 Gill rakers of basking sharks (Lamniformes: Cetorhinidae) from the Tertiary of Sakhalin Island, Russia Жаберные тычинки гигантских акул (Lamniformes: Cetorhinidae) из третичных отложений острова Сахалин, Россия M.V. NAZARKIN М.В. НАЗАРКИН M.V. Nazarkin, Zoological Institute, Russian Academy of Sciences, 1 Universitetskaya Emb., St Petersburg 199034, Russia. E-mail: [email protected] Isolated gill rakers of basking sharks (Lamniformes: Cetorhinidae) were collected from the upper Oligocene Holmsk Formation and Middle-Upper Miocene Kurasi Formation of the Sakhalin Island. This is the second finding of fossil basking shark remains in Russia. Fossil gill rakers are similar to those in recent representatives of the genus Cetorhinus, but differ from the latter by wider and shorter medial processes and higher bases. These features are more typi- cal for basking sharks in the fossil genus Keasius. Additional material is needed for the exact taxonomic identification of the Tertiary basking sharks from Sakhalin. Сообщается о находке отдельных жаберных тычинок гигантских акул (Lamniformes: Cetorhinidae) в верхнее-олигоценовых отложениях холмской свиты и средне-верхне-ми- оценовых отложениях курасийской свиты острова Сахалин. Это второе обнаружение ис- копаемых остатков гигантских акул на территории России. Рассматриваемые тычинки напоминают таковые современного рода Cetorhinus, но отличаются более короткими и широкими медиальными отростками и более высокими основаниями. Эти признаки бо- лее характерны для гигантских акул вымершего рода Keasius. Необходимы более много- численные дополнительные материалы для точной идентификации третичных гигант- ских акул Сахалина. Key words: Oligocene, Miocene, Sakhalin, fossil basking shark, Cetorhinidae Ключевые слова: олигоцен, миоцен, Сахалин, ископаемая гигантская акула, Cetorhinidae INTRODUCTION (Compagno, 2002). Their filtering appara- tus consists of high and numerous gill rakers Cetorhinus maximus (Gunnerus, 1765) is (modified dermal denticles with extremely the only representative of the basking shark elongated crowns) which are densely placed family Cetorhinidae (Lamniformes) in the in two rows along each gill arch (Hovestadt modern fauna. After whale shark Rhincodon & Hovestadt-Euler, 2011). Number of gill typus Smith, 1828, this is the second largest rakers per gill arch is about 1260 (Bigelow shark attaining up to 12 m in total length & Schroeder, 1948). The jaw teeth are mon- (Compagno, 1984, 2002; Hovestadt & Hov- ocuspid, small and numerous (Compagno, estadt-Euler, 2011). This species is widely 1984; Cappetta, 1987). The gill raker sets are distributed in cold and warm temperate wa- annually shed during the low zooplankton ters worldwide (Compagno, 2002). One of abundance season and, after that, basking the most distinctive morphological features sharks presumably feed on benthic organ- of this species is very wide gill openings isms using their tiny jaw teeth (Compagno, which almost encircle the head. Basking 2002). Apparently, because of this biologi- sharks are pelagic filter-feeders which prey cal trait, fossilised remains of cetorhinid gill on small planktonic organisms and fish eggs rakers are commonly found on all continents 270 M.V. NAZARKIN. BASKING SHARKS FROM TERTIARY OF SAKHALIN in deposits from the middle Eocene through of the Holmsk Formation and one more – Pleistocene, whereas findings of the teeth, in the deposits of Middle-Late Miocene of vertebrae or skeletons are rather rare. In a the Kurasi Formation on the Sakhalin Is- detailed review of cetorhinid fossil records, land, Russia. These findings are the second Welton (2013) assigned two species of early known occurrence of fossil cetorhinids in and rather primitive cetorhinids in his new Russia. The structure of gill rakers is simi- extinct genus Keasius. One of them, K. tay- lar to that in known Cetorhinus. At the same lori Welton, 2013, is known only from the time, there are some differences from the late Eocene deposits in Oregon, USA (Wel- recent Cetorhinus that bring the Sakhalin ton, 2013). The other, K. parvus (Leriche, findings together with the extinct Keasius. 1908), is known from numerous localities in Thus, it seems important to describe these Oligocene and Miocene deposits in Europe fossils though without either generic or spe- and North America (Welton, 2013). cies identification until more representative A tooth from the Eocene deposits in material on this taxa is available. the Western Siberia is probably the earliest Remains of chondrichthyans in both representative of Cetorhinus (Malyshkina, mentioned formations are very rare. Earlier, 2006). Cetorhinus sp. has been also reported a single tooth of a thresher shark (Alopi- from the late Oligocene in Japan (Uyeno et idae) was described from the same local- al., 1984) and Mexico (Gonzalez-Barba & ity of the Holmsk Formation (Nazarkin Thies, 2000). Early to middle Miocene find- & Malyshkina, 2012) and a single tooth ings of Cetorhinus came from Europe, North of mako shark (Lamnidae) is known from and South America, and East Asia (Japan), the same locality of the Kurasi Formation and were usually identified as Cetorhinus sp. (Nazar kin, 2013). or, less often, as C. maximus (Welton, 2013). Almost all late Miocene through Pleisto- MATERIALS, METHODS cene cethorinids are referred to as C. maxi- AND LOCALITIES mus. Remains of this species were found also in Europe, both Americas and East Asia (Ja- Specimens are deposited in the paleon- pan) (Welton, 2013). The second (extinct) tological section of ichthyological collec- species of this genus – C. huddlestoni Wel- tion of the Zoological Institute of the Rus- ton, 2014 – was recently described from the sian Academy of Sciences, St Petersburg Middle Miocene of California, USA (Wel- (ZIN). Gill raker terminology follows Wel- ton, 2014). ton (2013). Measurements are made with All data on fossil basking sharks in East calipers to the nearest 0.1 mm. Drawings Asia (the north-western Pacific) are from are made according to digital photos. Japan. More than 360 separate gill rakers The Late Oligocene findings from the of Cetorhinus sp. were discovered from the Holmsk Formation are represented by the lower Oligocene beds on Ainoshima Island imprints of two separate gill rakers in the (Tomita & Oji, 2010). Remains of Cetorhi- matrix with both rakers’ bodies themselves nus sp. are reported also from the upper Oli- preserved in its most part. Specimen ZIN gocene and middle Miocene localities (Uy- 306p-1 with filament is broken and dis- eno et al., 1984; Itoigawa et al., 1985; Kara- placed in the proximal part. In specimen sawa, 1989; Yabumoto & Uyeno, 1994). Ce- ZIN 306p-2, the medial process is missing. torhinus maximus remains from Japan were The Middle-Late Miocene fossil raker from found in deposits of the middle Miocene the Kurasi Formation (ZIN 307p) is rep- (Uyeno et al., 1983; Karasawa, 1989) and resented as an imprint in matrix; with the Pleistocene (Uyeno & Matsushima, 1974). bone itself completely lost. Recently, two gill rakers of basking shark The fossils were found on the coastal cliffs were found in the deposits of Late Oligocene of the Tatar Strait. Fossiliferous beds of the © 2014 Zoological Institute, Russian Academy of Scienсes, Zoosystematica Rossica 23(2): 269–275 M.V. NAZARKIN. BASKING SHARKS FROM TERTIARY OF SAKHALIN 271 Holmsk Formation are located about one ki- trict, Sakhalin Province, Russia; Holmsk For- lometre south from the settlement of Nevod- mation, Upper Oligocene; coll. M.V. Nazarkin, skoye and three kilometres north from the 2012 and 2013. ZIN 307p, one gill raker. Coastal town of Tomari on the south-western coast cliff of Tatar Strait about five kilometers south of settlement of Penzenskoye, Tomari District, of the island (47°47.921´N 142°04.602´E). Sakhalin Province, Russia; Kurasi Formation, The outcrop of Kurasi Formation is placed Middle-Upper Miocene; coll. M.V. Nazarkin, approximately 8 km to the north from the 2014. ´ ´ former (47°52.224 N 142°05.813 E), about Description. Total length of gill rakers five kilometres south from the settlement of from the Holmsk Formation are approxi- Penzenskoye at the same district. mately 56 mm in specimen ZIN 306p-1 and The cliffs of both localities exposed lami- 55.7 mm in specimen ZIN 306p-2; length nated clayish aleurolites, superficially simi- of the filament exceeds the basal height by lar to each other. The fossiliferous beds of 12.5 and 14.7 times, respectively (Fig. 1a, the both sites were formed in comparatively b). The surface of the filament is densely deep marine environments (Savitskyi, 1982; covered with weak longitudinal wrinkles. Zhidkova, 1982, 1986). Periods of sedimen- The central part of the filament is depressed tation coincided with two largest phases of and bordered by two flat ridges which ex- the Tertiary sea transgression on Sakhalin. tend from the base joint to halfway to the During these periods, two strongest phases apex. Curvature of the filament base is of the accumulation of siliceous deep-water weak. The bight is comparatively deep, deposits were noted which were connected with intermediate width and of subangu- with periods of high biomass production of lar shape (Fig. 2a). The bight in specimen diatoms (Gladenkov et al., 2002). Numer- ZIN 306p-1 seems slightly wider than that ous remains of deep-water fish taxa such as of specimen ZIN 306p-2. The bight depth of myctophids, bathylagids, macrourids, poly- the former specimen is slightly (by 2%) ex- merichthiids, and stomiids were discovered ceeding the basal height; the bight depth of from both localities. The remains of neritic the second specimen makes, probably, about or near-bottom fishes of the families Clu- 98% of basal height. Thus, the basal height peidae, Gasterosteidae, Sebastidae are less is medium. The medial process is of medi- frequent. Structure of both, Late Oligocene um size, comparatively wide, of a triangle and Middle-Late Miocene fish communities shape, with a slightly concave mesial edge are very close at the levels of families and and a rounded apex. The basal protuberance genera of fishes. is weakly developed. The basal edge of the base is almost straight.
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