Cetacea, Delphinoidea

Japan Cetology (23):13-20(2013)

The first Pliocene albireonid( Cetacea, Delphinoidea) periotic from the western North Pacific and paleobiogeographic significance of fossil delphinoid ear bones of Na-arai Formation of Choshi, Chiba, central Japan Mizuki Murakami1) and Yoshiki Koda2)

Abstract A right periotic of Albireonidae (Cetacea, Delphinoidea), from the Pliocene Na-arai Formation of Chiba, central Japan, is described. The albireonid specimen is the second reported from the western North Pacific and the first recorded from the Pliocene. Thus, this discovery largely extends our knowledge of paleobiogeography of Albireonidae. The Na-arai specimen shares following features with the type species of Albireo: having the large aperture for the cochlear aqueduct; the trapezoidal cochlear portion and the large angle between the caudal tympanic process and the lateral edge of the cochlear portion in ventral view; the mediolaterally narrow posterior bullar facet; shallow grooves on the posterior bullar facet. However, the present specimen probably belongs to different species from the type species because of differences of morphology in the internal acoustic meatus and anterior process. The fossil ear bone assemblage of Delphinoidea from the Na-arai Formation provides important data for comparing Pliocene delphinoid faunas of the western North Pacific with those from other areas of the Pacific. The paleoclimate of the Na- arai Formation was probably more similar to those of delphinoid localities in the eastern regions of the North and South Pacific than to those of other fossil localities in the western North Pacific.

Introduction Mysticeti from the Na-arai Formation in detail. However, The basal conglomerate bed of the Na- ear bones of Delphinoidea from the formation, which are arai Formation of Choshi, Chiba, central Japan mostly undescribed and housed in private collections, is known for yielding large number of cetacean have not been thoroughly investigated. In this study, fossils（Ozaki 1954 1958, Hasegawa 1968 1982, we describe a well-preserved and isolated periotic of Shikama 1975, Oishi and Hasegawa 1994, Kohno Albireonidae（Cetacea, Delphinoidea） and discuss their 2002）. All of them are isolated remains and are taxonomic and paleobiogeographic significance. derived as conglomerates. The Na-arai Formation Institutional Abbreviations—INM ,Ibaraki unconformably overlies the Jurassic Atagoyama Nature Museum, Ibaraki, Japan; UCMP ,University of Group, the Cretaceous Choshi Group, and the lower California Museum of Paleontology, Berkley, California, USA. Miocene Metogahana Formation（Ozaki 1954, Measurements and Anatomical Terms— Takahashi et al. 2003; Fig. 1）. The age of a tephra Measurements of periotic are listed in Table 1. These bed（In 3） near to above the basal conglomerate measurements were made according to the methods of the Na-arai Formation is 2.65 Ma（Tamura et al. used by Bianucci（1996 ）. The anatomical terminology 2006）. The occurrences of calcareous nannofossil of periotics follows that of Mead and Fordyce（2009 ）. （Reticulofenestra pseudoumbilicus, Last Appearance Datum 3.8 Ma and Sphenolithus abies, 3.6 Ma; Sato SYSTEMATIC PALEONTOLOGY in Oishi and Hasegawa 1994） and planktonic foraminiferans（ Globoturborotalita nepenthes, LAD 4.36 CETACEA Brisson, 1762 Ma and Sphaeroidinellopsis seminulina, 3.57 Ma; Matoba ODONTOCETI Flower, 1867 1967） from the basal conglomerate indicates that DELPHINOIDEA Gray, 1821 the layer is early Pliocene（Zanclean ）. The molluscan ALBILEONIDAE Barnes, 1984 assemblages（Ozaki 1958） and occurrences of shark ALBILEONIDAE gen. et sp. indet. teeth（ Carcharodon carcharias; Itoigawa et al. 1975）（Figs. 2A-F, 3A-C） also indicate that the age of the basal conglomerates of Na-arai Formation is of Pliocene. Oishi and Hasegawa Material—INM-4-014129, an isolate right periotic. （1994） investigated fossil ear bone assemblages of

1）Department of Earth Sciences, Faculty of Education and Integrated Arts and Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku, Tokyo 169-8050, Japan

2）Ibaraki Nature Museum, 700 Ohsaki, Bando City, Ibaraki 306-0622, Japan

13 M.Murakami &Y.Koda

level as the fossa for the stapedial muscle. The area enclosing the cochlear window, the caudal tympanic process, and the aperture for the cochlear aqueduct is concave（Fig. 2A）. The angle between the lateral edge of the cochlear portion and the caudal tympanic process in ventral view is approximately 135°. In dorsal view, the angle between the anterior edge of the cochlear portion and the anterior process is greater than 90（Fig.° 2B）. The anterior part of the anterior process is relatively thick mediolaterally. The medial and posterior edges of the cochlear portion are rather rectangular. Consequently, the cochlear portion is trapezoidal or rectangular in dorsal and ventral views（Fig. 2B, E）. The internal acoustic meatus is long mediolaterally Fig.1 The fossil locality of an albireonid periotic (INM-4-014129). and inverted teardrop-shaped（13.1 mm in major axis; Fig. 2B）. The lateral margin of the aperture for Table1 Measurements(mm) of the periotic (INM-4-014129; + indicates incomplete to some extent) the vestibular aqueduct lies medial to the level of the

Measurement points INM-4-014129 lateral margin of the internal acoustic meatus. The dorsal tuberosity is moderately developed（Fig. 2B- Greatest length of the periotic 34.1+ D, F）. In cross-section, at mid-length, the anterior Greatest width of the periotic 23.3 process is ovoid（Fig. 2C）. Width of the periotic at level of upper 23.3 tympanic aperture The parabullary ridge is weak（Fig. 2A, D, E）. Greatest thickness of the periotic 18.2 The ventral edge of the anterior process is concave. Thickness of the periotic at level of 14.8 upper tympanic aperture The tip of the posterior bullar facet is weathered（Fig. Length of the cochlear portion 24.5 2D-F）. In ventral view, the posterior bullar facet is Thickness of the cochlear portion 14.5 mediolaterally narrow. The anterior part of posterior Length of the anterior process of 20.2 bullar facet is concave; several indistinct grooves are periotic present on the facet, but these ridges are weak as Greatest length of the posterior 12.5+ process of periotic along its axis compared with those of delphinids and monodontids Length of the ventral tuberosity + 16.7 （Fig. 2E）. The angle between the posterior process and parabullary ridge of the periotic the cochlear portion of the periotic is approximately 155°. The fossa for the stapedial muscle is wide. Description Periotic—The anterior process anterior Comparisons to the cochlear portion is short（Fig. 2A, B, E）. The INM-4-014129 from the Na-arai Formation length is less than one fourth the length of the shares the following characteristics of the periotic cochlear portion. The apex of the anterior process of with the type specimen of Albireo whistleri Barnes, the periotic abruptly curves anteroventrally and the 1984（UCMP 314589; Fig. 3D-F）: relatively large anterior edge of the process is rectangular（Fig. 2A, size; having a trapezoidal and anteroposteriorly long D）. The anteroventral angle of the process is slightly cochlear portion（Fig. 3B, C, E, F）; a large aperture weathered（Fig. 2A, C）. The aperture for the cochlear for the cochlear aqueduct（Fig. 3A, B, D, E）; the aqueduct is large（major axis, 3.6mm） and located large angle between the caudal tympanic process on the posteromedial part of the cochlear portion. and the lateral edge of the cochlear portion in The cochlear window opens distinctly at the ventral medial view（Fig. 3A, D）; the mediolaterally narrow

14 The first Pliocene albireonid periotic from Japan

Fig.2 The right periotics of INM-4-014129 (A-F). A, medial; B, dorsal; C, anterior; D, lateral; E, ventral; F, posterior views.

Fig.3 Comparisons of the right periotic of INM-4-014129 (A-C) and the left periotic of Albireo whistleri (UCMP 314589, D-F). A, D, medial; B, E, dorsal; C, F, ventral views.

posterior bullar facet（Fig. 3C, F）; shallow grooves facet of the periotic is mediolaterally narrow. The on the posterior bullar facet（Fig. 3C, F）. Barnes Na-arai specimen differs from the type specimen in （1984, 2008） noted that the posterior bullar facet having the narrow anterolateral part of the internal of the periotic in the type specimen of A.whistleri is acoustic meatus（Fig. 3B）; the anteroposteriorly short smooth, but Barnes（1984:35） also said that it is anterior process（Fig. 3B）. In the Na-arai specimen, only faintly striated. Barnes（1984, 2008） described the length of the anterior process anterior to the the posterior bullar facet is large. However, the cochlear portion to that of the cochlear portion is posterior bullar facet of the periotic of A. whistleri is less than 25%; in contrast, the ratio in A. whistleri is mediolaterally narrower than those of delphinids. 40%. Therefore, the INM-4-014129 may belong to a Consequently, here we described the posterior bullar different species or genus than that of A.whistleri.

15 M.Murakami &Y.Koda

Discussion the Late Miocene（e.g., Bianucci and Landini 2002）. Taxonomic significance Consequently, the Pacific Ocean is suspected as Albireonidae, established by Barnes the location of origin or early evolution of all （1984）, is an extinct family of Delphinoidea. Muizon non-kentriodontid families among Delphinoidea. （1988a） considered Albireonidae as a sister group Importantly, no con-generic delphinoid taxa of the of Phocoenidae. On the other hand, Barnes（1990） Late Miocene to Pliocene age have been identified and Geisler et al.（2012） considered Albireonidae between the western North Pacific and the eastern as more basal than Kentriodontidae, which is an North Pacific or eastern South Pacific Oceans, extinct paraphyletic delphinoid group. In contrast, although recently five genera six species of delphinoid Geisler et al.（2011） and Murakami et al.（2012a of these age have been described from Japan b） considered Albireonidae as the second basal （Ichishima and Kimura 2000 2005 2009, Murakami position of Delphinoidea, next to paraphyletic et al. 2012a b）. On the other hand, only two genera Kentriodontidae. Fossil occurrences of Albireonidae （kentriodontid Atocetus and phocoenid Piscolithax） are extremely rare, although the holotype of Albireo are common to the eastern North Pacific and eastern whistleri is one of the most complete delphinoid South Pacific regions（Barnes 1985a）. Thus, Late fossils known to date（Barnes 2008）. Until now, Miocene to Pliocene delphinoid faunas appear to be only two species and six specimens are known and endemic to each area of the Pacific Rim, as previously their distributions are restricted in the North Pacific noted by Barnes（1977 ）.This phenomenon is strange, region of the Late Miocene to Pliocene（Barnes considering that no geographic barriers（such as an 2008）. Among six specimens except the holotype of isthmus between these regions） were present during A.whistleri, three are isolated incomplete tympanic this period. According to Jefferson et al.（2008 ）, bulla; second species consists of postcranial elements many species of extant delphinoids show a worldwide alone; a well-preserved Japanese specimen has distribution. 15 of 16 genera（and 18 of 20 species） not been described（Barnes and Furusawa 2001, of delphinoids present in the region of Japan also Furusawa and Barnes 2002, Barnes 2008）. The occur near the west coast of the North America, present specimen represents the second fossil record and 14 of 16 genera（and 14 of 20 species） are of Albireonidae from the western North Pacific and also along the west coast of the South America. Two the first fossil record from that region of the Pliocene. possible reasons explain the apparent endemism of The present specimen indicates that the geographic delphinoid faunas during Late Miocene to Pliocene; range of Albireonidae in the Pliocene was wider than （1） zoogeographic differences between these regions previously thought. Delphinoid fossils from Japan and（2） insufficiencies of the fossil record. have often reported based on non-diagnostic isolated In the western North Pacific region, a teeth or postcranial bones（e.g. Tomida and Sakura delphinid from the Pliocene Tatsunokuchi Formation, 1988）. The periotic includes more useful information Sendai, Tohoku, northern Japan was described to consider paleodiversity than such a material. at a latitude of 38°N（Hatai et al. 1963）. The co- occurring molluscan fauna has been assigned to Paleobiogeographic significance the cool-temperate Tatsunokuchi Fauna（ Otsuka All non-kentriodontid families among 1941）. Most well-preserved delphinoid fossils from Delphinoidea except Odobenocetopsidae（e.g. the upper Miocene to Pliocene occur at latitudes Albireonidae, Delphinidae, Monodontidae, and of 43°N–45°N of Hokkaido and the co-occurring Phocoenidae; sensu Murakami et al. 2012a b） molluscan faunas in this area have been assigned appeared in the Late Miocene of the Pacific Ocean. to the cool to temperate Lower Togeshita Fauna No reliably identified species or materials of all non- （Amano 1983 1986; ca. 14-10 Ma）, the temperate kentriodontid families among Delphinoidea have Upper Togeshita Fauna（Amano 1983 1986 ca. been discovered from out of the Pacific Ocean in 10-6 Ma） or the cool-temperate to subarctic

16 The first Pliocene albireonid periotic from Japan

Takikawa-Honbetsu Fauna（Fujie and Uozumi 1957, although the existence of many undescribed species Uozumi 1962; ca. 6-2Ma）. Furthermore, the diatom has been suggested, for example the Purisima assemblage（e.g., Coscinodiscus marginatus） co- Formation of California. More taxonomic studies of occurring with the Hokkaido specimens shows the fossil delphinoids are required so as to compare the influence of the cold-water environments（Shimada faunas of the western North Pacific with those from et al. 1998）. Thus, fossil delphinoid specimens of other regions. the Late Miocene to Pliocene of the western North In contrast to the Tohoku and Hokkaido Pacific are concentrated in northern Japan. These localities, molluscan fossil of the Na-arai Formation delphinoid assemblages probably represent subarctic indicate a warm-temperate environment influenced to cool-temperate, and some of the species may be by a warm-water current（Ozawa and Tomida 1992）. eurhythermal. Consequently, the delphinoid fossils of the Na-arai In the eastern South Pacific, most delphinoid Formation are expected to include eurhythermal and fossil localities are in Peru, at latitudes of 14°S-16°S sub-tropical species. Therefore, the delphinoid faunas （Muizon, 1988b）, and in Chile, at a latitude 27°S described from the eastern North and South Pacific （Walsh and Naish 2002）. The fish, reptile, molluscan are probably more similar to those of the Na-arai and crustacean faunas of the Peruvian localities Formation than to those of Tohoku and Hokkaido. indicate that this region was a tropical-water In summary, fossil delphinoid ear bones environment during the Late Miocene to Pliocene from the Na-arai Formation provide important （Muizon and Devries 1985）. The Chilean delphinoid evidence for the delphinoid faunas of the late fossils may include eurhythermal and cool-temperate Neogene of the Pacific Ocean, and for the evolution species; however, taxonomic studies of Chilean of Delphinoidea. Knowledge of the generic or delphinoid fossils are still in a preliminary phase. morphotypic composition of the fossil delphinoid In the eastern North Pacific, most fossils of ear bones from the Na-arai Formation would be the Late Miocene delphinoids and other delphinidans an important contribution to our understanding of yielded from the Isla Sedros, Baja California, Mexico, delphinoid paleobiogeography, and should be the at a latitude of 28°N（Barnes 1984）. Diatom target of future studies. assemblages from the upper Miocene of this area indicate cool-temperate environments under the Acknowledgments influence of the cool California Current （Moreno-Ruiz The authors are grateful Ogai, K. for and Carreno 1994）, whereas the vertebrate fossil providing the opportunity to study the present fauna of Isla Sedros includes tropical to sub-tropical specimen. The authors are also thankful to Barnes, shark fossils, such as Carcharinus and Carcharodon L. G. and Furusawa, H. for access to the specimens megarodon（Barnes 2008）. These data suggest that under their care. The authors thank an anonymous overall marine climate of the area is warm-temperate reviewer for detailed reviews that enhanced the or temperate during the period. The Late Miocene manuscript. kentriodontids and phocoenids Salumiphocaena have been described from California and these References localities locate 33°N–34°N（e.g., Barnes 1985b）. Amano, K.（1983） Palaeontological study of the The horizons of these fossil localities were under Miocene Togeshita molluscan fauna in the Rumoi the influenced under the cool-temperate waters, district, Hokkaido. Sci. Repts. Inst. Geosc. Tsukuba based on the evidence of diatom and foraminiferal Univ., Sec. B, 4 : 1-72. fossil assemblages（e.g., Keller and Barron 1981）. Amano, K.（1986） Age and characteristics of the so- Consequently, these fossil faunas may be correlated called "Atsunai-Togeshita Fauna" in Hokkaido. with those of Tohoku or Hokkaido. However, Palaeont. Soc. Japan, Spec. Papers, 29 : 187-198. delphinoid fossil from the Pliocene are so far scarce, Barnes, L. G.（1977） Outline of eastern North Pacific

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北西太平洋初の鮮新統産アルビレオ科の耳周骨と千葉県銚子市の名洗層産マイルカ上科耳骨群集の古動物地理学的意義

村上瑞季１）国府田良樹２）

要旨千葉県銚子市の鮮新統名洗層から産出したアルビレオ科の耳周骨を記載した。アルビレオ科化石は北西太平洋では 2 例目で鮮新統からは初の産出となり、本標本の発見は同科の古生物地理分布の知見を大きく向上させた。この耳周骨はタイプ種であるAlbireo whistleriと以下の形質を共有する : 大きな外リンパ管口；台形の蝸牛殻 ; 尾側突起と内耳部の外側縁が大きな角度を形成する ; 耳周骨後突起の関節面は内外方向に狭く、浅く幅広い溝がいくつか存在する。しかしながら、本標本の内耳道と前方突起の形態はタイプ種とは大きく異なるので未記載種に属すると考えられる。名洗層の古環境は北西太平洋域の他の主要な化石産地に比べ、東部太平洋域における主要化石産地の古環境により近い。したがって、そのマイルカ上科耳骨化石群集は鮮新世における北西太平洋域と太平洋内の他地域の動物群集との比較を行う上で非常に重要である。

1）早稲田大学教育・総合科学学術院〒 169-8050 東京都新宿区西早稲田 1-6-1 2）茨城県自然博物館〒 306-0622 茨城県坂東市大崎 700