Diversity Changes in Cretaceous Inoceramid Bivalves of Japan

Paleontological Research, vol. 9, no. 3, pp. 217–232, September 30, 2005 6 by the Palaeontological Society of Japan

Diversity changes in Cretaceous inoceramid bivalves of Japan

AKINORI TAKAHASHI

JSPS Research Fellow, Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan (e-mail: [email protected])

Received January 27, 2005; Revised manuscript accepted May 11, 2005

Abstract. Temporal species-diversity changes in Japanese Cretaceous inoceramid bivalves were analyzed from an extensive literature survey and statistical analysis, with the following results: (1) Species diversity increased gradually from the Upper Albian to Lower Campanian, and then dropped suddenly across the Lower/Upper Campanian (LCa/UCa) boundary; (2) There is no statistical correlation between ammonoid and inoceramid diversity changes in Japan, which must reﬂect the different ecologies of both groups; (3) Relatively high extinction ratios occurred at boundaries near Oceanic Anoxic Events (OAEs). The extinction events at the Albian/Cenomanian, Cenomanian/Turonian, and Turonian/Coniacian boundaries were potentially caused by OAE1d, 2 and the onset of OAE3, respectively. The drastic diversity decrease at the LCa/UCa boundary probably resulted from an abrupt and large-scale relative sea-level fall in the Yezo forearc basin; and (4) The pattern of diversity changes is similar to that of long-term (2nd-order) eustatic sea-level changes. The following hypotheses are presented as the cause of these phenomena: changes in shelf area, the primary inoceramid habitat, controlled their diversity, or changes in the Cretaceous outcrop area (rock volume) associated with sea-level changes controlled their diversity. It is possible that a combination of both factors controlled diversity patterns.

Key words: Cretaceous, eustasy, inoceramids, Japan, Oceanic Anoxic Event, species diversity

Introduction Marine invertebrates are strongly influenced by oceanic environmental changes. Toshimitsu et al. Cretaceous global events and paleoenvironments, (2003) surveyed temporal species-diversity changes especially Oceanic Anoxic Events (OAEs; Schlanger among Japanese Cretaceous ammonoids, and dis- and Jenkyns, 1976), are a major focus of attention cussed the biotic responses of ammonoids to marine for interdisciplinary researchers in earth sciences and paleoenvironmental changes (e.g., OAEs and sea allied fields. Investigating Cretaceous paleoenviron- level). Yazykova (2004) also illustrated Upper Creta- ments is essential to understanding aspects of current ceous ammonoid species diversity for Far-Eastern global greenhouse effects (e.g., the trigger mechanisms Russia, and discussed their relationship to marine for OAEs) and attendant biotic responses. There paleoenvironmental changes. The immobile benthos, are few detailed Cretaceous paleoenvironmental data however, is probably more strongly controlled by from the northwestern Pacific margin, although re- marine paleoenvironmental changes than the nekton, searchers have undertaken a variety of geochemi- such as ammonoids, because nektonic organisms can cal, biostratigraphic, sedimentologic, and paleoenvir- migrate to escape out of environmental deterioration. onmental studies (e.g., Hasegawa and Saito, 1993; Therefore, the inference that most inoceramid adults Hasegawa, 1997; Ando et al., 2002; Ando, 2003; were immobile epibyssate or endobyssate benthos Takashima et al., 2004; Takahashi, 2005) in this area (e.g., Stanley, 1972) suggests that they were strongly in recent years. As a result, knowledge of global and affected by marine paleoenvironmental changes. Ac- northern Pacific Cretaceous paleoenvironments has cordingly, inoceramids, which thrived during the Cre- increased significantly in recent years. Unfortunately, taceous worldwide and went extinct in the latest Cre- the relationship between bioevents and Cretaceous taceous except for the enigmatic genus Tenuipteria, paleoenvironments in the northern Pacific regions has are appropriate for evaluating biotic responses to hardly been clearly elucidated. Cretaceous marine paleoenvironmental changes. Ja- 218 Akinori Takahashi pan contains extensive Cretaceous marine sediments cies diversity in Japan was re-counted and tallied up that yield abundant inoceramids, making this region from the database of Toshimitsu and Hirano (2000). ideal for such a study. Extinction ratio (ER) and origination ratio (OR) Since temporal diversity changes in Japanese are defined as: inoceramids and their relationship to paleoenviron- ER ¼ (Number of preexisting species absent above mental changes have not been previously evaluated, each boundary)/(Total number of species the main intent of the present study is to clarify the below each boundary) relationship between Japanese Cretaceous inoceramid diversity and faunal changes, and to learn which paleo- OR ¼ (Number of successor species not present environmental factors controlled this diversity, based below each boundary)/(Total number of on an extensive literature survey and statistical analy- species above each boundary) sis. In addition, the relationship between Cretaceous The following formula has been devised to calculate ammonoid and inoceramid diversity changes was ex- the fluctuation ratio (FR). amined statistically, based on detailed data from Japan. FN PN FR ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi FN PN Material and methods Here FN is the total number of species above each Ten genera and 94 species of inoceramid bivalves, boundary and PN is the total number of species below which have been reported from the Japanese Creta- each boundary. FR > 0 means that the number of ceous in 112 publications, are analyzed in the present species in a given substage increases in the follow- study. The objectives are to clarify temporal (chrono- ing substage, whereas FR < 0 indicates the reverse. logical) diversity changes, ratios of extinction, origi- FR ¼ 0 means that the number of species in two suc- nation and fluctuation, and changes in generic com- cessive substages are equal. The absolute value of FR position (the ratios of species in each genus). indicates a changing scale for the number of species Generally, the biological concept of ‘‘species diver- across each boundary. Basically, the criteria for cor- sity’’ includes the elements of ‘‘species richness’’ and relation in the present study are based on Toshimitsu ‘‘evenness’’. However, we cannot learn the number of et al. (1995), so Japanese domestic substages (e.g., occurring individuals based on a literature survey, es- K5b2, K6a3) are utilized in addition to the standard pecially for all of Japan, so it is difficult to document European stages and substages. evenness. Therefore, the present paper equates ‘‘spe- As the main purpose of the present research is to cies diversity’’ with ‘‘species richness’’, namely, the clarify the aspect of Japanese ‘‘species’’ diversity, I do ‘‘number of species’’. not treat ‘‘subspecies’’. Although some Japanese re- I compiled the number of species (species diversity) searchers treat Cremnoceramus, Cordiceramus, Platy- of inoceramids from Japan for each substage of ceramus, Volviceramus etc. as subgenera (e.g., Noda, the Cretaceous from the Upper Albian to Maas- 1986, 1996; Noda and Matsumoto, 1998), I treat those trichtian, based on previously published biostrati- taxa as the independent genera following the recent graphic and taxonomic studies (see Appendix 1). Al- international standard (e.g., Dhondt, 1992; Voigt, though species designated as ‘‘aff.’’ were not counted 1995). in general, those with systematic descriptions in pub- The correlation coefficient between values for sea lished studies, and age-diagnostic species assigned by level, which were determined both at the midpoint of Toshimitsu et al. (1995), were treated as independent the substage (SLmid) and also for maximum sea-level species in the present analysis. Inoceramus aff. con- during a substage (SLmax) using the Haq et al. (1987, centricus, which was collected by me (see Takahashi 1988) curve (long-term ¼ 2nd-order), and inoceramid et al., 2003), is an exception and has been counted as diversity changes were calculated for the ages ranging a species. The ratios of extinction, origination and from the Albian to Maastrichtian. The regression line fluctuation at each stage/substage boundary, and the was calculated and drawn using the reduced-major generic composition for each substage, were calcu- axis method. lated from these results. The number of species in each substage was divided by the duration (m.y.) of Results the substage, so that the result is normalized for time (1 m.y.). A radiometric time scale for each substage is The occurrence of inoceramids has been confirmed adopted from Gradstein et al. (1995). Ammonoid spe- in Japanese strata ranging from the Upper Albian to Cretaceous inoceramid diversity changes 219

Figure 1. Chronological species-diversity changes in inoceramids and ammonoids of Japan (A), and chronological changes in inoceramid species-diversity normalized for time (1 m.y.) of Japan (B). The ammonoid species diversity was re-counted and tallied up by the author based on the database of Toshimitsu and Hirano (2000). K3b1–K6b2 are domestic substages shown by Toshimitsu et al. (1995). A radiometric time scale for each substage is adopted from Gradstein et al. (1995).

Upper Maastrichtian. Lists of inoceramid species and High extinction ratios were present at the Albian/ their stratigraphic distribution are shown in Appendix Cenomanian (A/C) (60.0%), Cenomanian/Turonian 2. The number of inoceramid species is approximately (C/T) (100.0%), Lower/Middle Turonian (LT/MT) a quarter that for ammonoids (Figure 1A). There is (77.8%), Turonian/Coniacian (T/C) (75.0%), Lower/ no statistical correlation between ammonoid and Middle Coniacian (LCo/MCo) (70.0%), Lower/Upper inoceramid diversity changes from the Albian to Campanian (LCa/UCa) (85.7%) and Lower/Upper Maastrichtian (r ¼ 0:241, p > 0:05; r: correlation co- Maastrichtian (LM/UM) (90.9%) boundaries (Figure efﬁcient) (Figure 1A). Inoceramid species diversity 2). The ﬂuctuation ratios at the LCa/UCa and LM/ increased steeply from the Upper Albian to Middle UM boundaries were strikingly low compared to the Cenomanian, and then was fairly stable during the other boundaries (Figure 2). In other words, high ex- Middle Cenomanian to Middle Coniacian. Sub- tinction ratios associated with low origination ratios sequently, it increased steeply from the Middle Con- prevailed at the LCa/UCa and LM/UM boundaries iacian to Lower Campanian, then dropped suddenly (Figure 2). The generic composition changed drasti- across the Lower/Upper Campanian boundary (Fig- cally across the above-mentioned seven boundaries, ure 1A). Species diversities normalized for time are except for the LCo/MCo boundary (Figure 3). Al- high in the Lower Coniacian and Lower Campanian though diversity did not decrease abruptly across the (K5b2), whereas low normalized diversities are rec- C/T boundary (FR ¼0:095), all Late Cenomanian ognized in the Upper Albian, Lower Cenomanian, inoceramid species became extinct by the boundary Upper Campanian (both K6a3 and K6a4) and Upper (i.e., the extinction ratio was 100%) and were replaced Maastrichtian (Figure 1B). by newly originated Early Turonian species (i.e., the 220 Akinori Takahashi

Figure 2. Ratios of extinction and origination (A), and fluctuation ratio (FR; B) for Cretaceous inoceramid species in Japan at stage and substage boundaries. FR ¼ðFN PNÞ=ðFN PNÞ1=2 (FN is the number of species above each boundary and PN is the number of species below each boundary.) FR > 0 means that the number of species in a previous substage increased in a following substage, whereas FR < 0 means the reverse. FR ¼ 0 indicates that the number of species in two successive substages are equal. The absolute value of FR indicates a changing scale for the number of species across each boundary. The chronology of OAEs by Jenkyns (1980), Erbacher et al. (1996), Kauffman and Hart (1996) and Leckie et al. (2002) is adopted. origination ratio was 100%) (Figure 2). Species diver- to Maastrichtian interval are 0.533 (SLmid) and 0.483 sity did not decrease abruptly in the intervals during (SLmax), both of which are significant statistical cor- which OAEs prevailed (the latest Albian: OAE1d, the relations (p < 0:05) (Figure 5). latest Cenomanian: OAE2, and the Coniacian to San- tonian: OAE3; the chronology of OAEs by Jenkyns, Discussion 1980; Erbacher et al., 1996; Kauffman and Hart, 1996; Leckie et al., 2002). However, relatively high extinc- Comparison of patterns of species-diversity changes tion and origination ratios are detected at the A/C, C/ between ammonoids and inoceramids in Japan T and T/C boundaries near OAEs. As the statistical results indicate, given that the The diversity changes are comparable overall fluctuation patterns of ammonoid and inoceramid di- with the long-term (2nd-order) eustatic sea-level versities in the Japanese Cretaceous were completely changes shown by Haq et al. (1987, 1988) (Figure 4). different (Figure 1A), it is clear that their responses to The correlation coefficients between 2nd-order sea- the factors which controlled their diversities could be level changes and inoceramid diversity in the Albian different. The results also strongly imply that patterns Cretaceous inoceramid diversity changes 221

Figure 3. Generic composition (ratios of species in each genus) for each substage in Japan. N: number of species. of ammonoid-diversity changes in Japan throughout the Cretaceous were unaffected by eustatic sea-level changes, although inoceramid diversity changes were at least partially controlled by long-term (2nd-order) eustasy on the whole, as stated in the ‘Results’ and the section (below) on ‘Interrelationship between sea- level changes and inoceramid diversity’. Toshimitsu Figure 4. Relationship between inoceramid diversity et al. (2003) stated that species-diversity changes for changes in Japan and eustatic sea-level changes. Eustatic curves Japanese Cretaceous ammonoids are similar to the are adopted from Haq et al. (1987, 1988). pattern of sea-level changes in the Haq et al. (1987, 1988) curve. However, these authors did not provide supporting ﬁgures and statistical results. They also monoids in Far Eastern Russia. My own statistical stated that the high diversities of ammonoid species in calculations, based on ﬁg. 7 in Zonova and Yazykova the Turonian, Middle Coniacian, and Santonian are (1998), verify that both diversities correlate extremely not in agreement with the pattern of Haq et al. curve. well (r ¼ 0:963, p < 0:01). However, extensive analy- In fact, based on my own calculations for the Albian sis in the present study suggests that the close corre- to Maastrichtian, there is no statistical correlation lation between ammonoid and inoceramid diversity between sea-level changes (SLmid) and Japanese changes shown by Zonova and Yazykova (1998) was ammonoid-diversity patterns (r ¼ 0:448, p > 0:05). due to the relatively limited scope of the outcrop area Different vertical distributions in the water column (rock volume) they studied. Either this, or there were and different modes of life (e.g., nektonic vs. immobile different paleoenvironmental factors between Far epibenthic) and trophic level in ammonoids and in- Eastern Russia and Japan that caused such different oceramids evidently were responsible for the different results. patterns of diversity in Japan. Zonova and Yazykova (1998) showed that diversity Effects of Oceanic Anoxic Events changesseeninLateCenomaniantoLateConiacian The A/C and C/T boundaries roughly correspond inoceramids are concordant with those seen in am- with the timing of OAE1d and OAE2, respectively 222 Akinori Takahashi

Figure 5. Diagram showing correlation between species diversity (number of species) in Japan and the sea-level curve of Haq et al. (1987, 1988). The regression lines were drawn using the reduced-major axis method. Values for sea-level were determined at the midpoint of a substage (SLmid; A) and for maximum sea-level during a substage (SLmax; B) using the long-term (2nd-order) curve.

(e.g., Erbacher et al., 1996; Leckie et al., 2002). The pronouncedly) were also features of the A/C and T/C T/C and LCo/MCo boundaries also roughly coincided boundaries (Figures 1–3). In contrast, the LCo/MCo with the beginning of OAE3 prevalence (e.g., Jen- boundary was characterized by a drastic diversity drop kyns, 1980; Kauffman and Hart, 1996) (Figure 2). As (fairly low fluctuation ratio; Figure 2) and relatively stated in the chapter of ‘Results’, relatively high ex- stable generic composition (Figure 3). Although the tinction ratios are detected at these four boundaries. high extinction ratios detected at the A/C, C/T and It is well known that major paleoenvironmen- T/C boundaries were probably caused by OAEs, only tal disturbances, mainly oxygen depletion, took place the high extinction ratio at the LCo/MCo boundary during the latest Cenomanian (e.g., Jenkyns, 1980; may have been affected by conditions that differed Jarvis et al., 1988; Elder, 1989; Kaiho and Hasegawa, from those at the A/C, C/T and T/C boundaries. 1994; Takahashi, 2005). These paleoenvironmental Toshimitsu et al. (2003) showed that the timing disturbances resulted in high extinction ratios for of ammonoid species-diversity minima approximately many marine molluscs at the C/T boundary (e.g., corresponded to that of OAEs, and concluded that Elder, 1989; Sepkoski, 1989, 1996; Harries and Little, diversity was strongly influenced by OAEs. Although 1999). Results of the present study and Takahashi many inoceramid species also became extinct prior to (2005) suggest that inoceramids in Japan were the A/C, C/T and T/C boundaries, many species or- also dramatically affected owing to conditions of iginated and evolved rapidly just after these bound- oxygen depletion that prevailed during the latest aries, as seen in the high origination ratios (Figure 2). Cenomanian. Accordingly, there was a pronounced In consequence, low inoceramid diversities were not change in generic composition across the C/T bound- observed across the boundaries near which OAEs ary, and many Mytiloides species originated and radi- prevailed. The ability to recover rapidly seems to be ated in the Early Turonian (Figure 3). Kauffman and characteristic of inoceramids, and ammonoids may not Hart (1996) stated that paleoenvironmental conditions have had this ability. This aspect of inoceramid diver- at the T/C boundary were very similar to those at the sity differs from ammonoid diversity, which drastically C/T mass-extinction boundary. The T/C boundary is decreased at the time of OAEs (Toshimitsu et al., characterized by a pronounced change in inoceramid 2003). faunas (Collom, 1998; Walaszczyk and Cobban, 2000; Harries, 2003), and by a significant geochemical per- Interrelationship between sea-level changes and in- turbation (e.g., Arthur and Sageman, 1994). Salient oceramid diversity phenomena at the C/T boundary (the diversity did The pattern of inoceramid species-diversity changes not decrease abruptly, extinction and origination is similar to that for long-term (2nd-order) eustatic ratios were high, and generic composition changed sea-level changes (Figure 4), and there are significant Cretaceous inoceramid diversity changes 223 statistical correlations between them on both the et al., 2003). The Japanese sequences were clearly de- SLmid and SLmax (Figure 5). Signor and Lipps (1982) posited within an active margin setting. Accordingly, showed that the major peaks and valleys of ammonoid there is a possibility of some differences between a generic diversity generally corresponded to those for Japanese relative sea-level curve and the Haq et al. sedimentary rock area through the Mesozoic. Harries’ (1987, 1988) sea-level curve, as exemplified by the (2003) cyclothem-level analysis of Upper Cretaceous presence of the ‘forced regression’ shown in the pres- inoceramid species in the Western Interior of North ent study and by the so-called Miyako transgression America pointed to a significant role for sea level in (Aptian—lower Cenomanian) and Urakawa trans- regulating species richness. He stated that the best gression (Coniacian—lower Santonian), which are proxy for habitat area in the marine realm is sea level. Japanese local names. Hence, further attempts to Based on these studies and the present one, there analyze the interrelationship between the diversity are three possible explanations for a link between di- changes and the other curves, which would be more versity and sea level in Japan: (1) changes in shelf reflective of what the actual sea-level history in Japan area, the main habitat of inoceramids, controlled in- (i.e., Japanese relative sea-level curve), might also be oceramid diversity (i.e., species-area relationships), important hereafter. Such appropriate curves are un- (2) changes in Cretaceous outcrop area (rock volume) available at present. due to sea-level changes controlled diversity and In addition, since there are many environmental abundance, and (3) a combination of factors (1) and factors that change through time other than sea level, (2). As a result, it is essential to estimate the exposure an analysis of sea level and diversity which did not areas for each substage in Japan, in order to differen- address those factors might not achieve fully mean- tiate among these three causes. ingful results. Hence, further attempts to compare en- Marine Cretaceous sediments in Japan ranging vironmental factors other than sea level and diversity in age from Albian to Maastrichtian are exposed might also be important in future studies. mainly in Hokkaido, northern Japan. An abrupt relative sea-level fall is recorded near the Lower/Upper Conclusions Campanian (LCa/UCa) boundary in the Yezo forearc basin sediments of Hokkaido. In other words, the Analysis of species-diversity changes and aspects of Hakobuchi Group (Upper Campanian–Paleocene), faunal turnover in inoceramid bivalves is based on 112 composed mainly of inner-shelf to foreshore sand- previously published biostratigraphic and taxonomic stone, overlies the Upper Yezo Group (Coniacian– studies, with the following conclusions: Lower Campanian), which is composed mainly of (1) There is no statistical correlation between outer-shelf to basin-plain mudstones and turbiditic ammonoid and inoceramid diversity changes in sandstones (e.g., Ando, 2003; Takahashi et al.,2003; Japan. This suggests that their responses to the factors Takashima et al., 2004). The boundaries of both the which controlled their diversities were distinct. This Hakobuchi and Upper Yezo groups can be interpreted evidently reflects their different vertical distributions as a forced-regression surface. In sum, an abrupt and in the water column, and different modes of life and large-scale relative sea-level fall, that was not syn- trophic levels. chronous with eustasy, must have taken place in the (2) Elevated extinction ratios were recognized at the Yezo forearc basin near the LCa/UCa boundary. It is Albian/Cenomanian, Cenomanian/Turonian, Lower/ quite likely that the sudden inoceramid diversity drop Middle Turonian, Turoninan/Coniacian, Lower/ across the LCa/UCa boundary (Figure 1A) and the Middle Coniacian, Lower/Upper Campanian and low diversity normalized for time in the Upper Cam- Lower/Upper Maastrichtian boundaries, and generic panian (Figure 1B) resulted from this abrupt and composition changed drastically across all but the large-scale relative sea-level fall in the northwestern Lower/Middle Coniacian (LCo/MCo) boundary. The Pacific margin. high extinction ratios at the Albian/Cenomanian, Since eustatic sea-level fall during the latest Cenomanian/Turonian and Turonian/Coniacian boun- Cretaceous was gradual, the sudden diversity decrease daries evidently resulted from OAE1d, OAE2 and the across the LCa/UCa boundary cannot be explained initial phase of OAE3, respectively. The phenomena solely from a eustatic viewpoint (Figure 4). This in- of diversity not decreasing abruptly, high extinction ference is in accord with the idea that the relationship and origination ratios, and a drastic change in generic between sea-level changes and paleobiodiversity may composition, are evidenced across those boundaries. be obscured by regional or local tectonism in tectoni- The drastic diversity decrease across the Lower/ cally active margins (Carter et al., 1991; Crampton Upper Campanian boundary is probably derived from 224 Akinori Takahashi an abrupt and large-scale relative sea-level fall in the volume bias in paleobiodiversity studies. Science, vol. 301, Yezo forearc basin of Hokkaido, northern Japan. p. 358–360. (3) The pattern of diversity changes is similar to that Dhondt, A. V., 1992: Cretaceous inoceramid biogeography: a review. Palaeogeography, Palaeoclimatology, Palaeo- for long-term (2nd-order) eustatic sea-level changes. ecology, vol. 92, p. 217–232. Three hypotheses are presented as the cause of this Elder, W. 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Appendix 1. List of literature surveyed to elucidate the stratigraphic distribution of inoceramids in this study.

Ando, H., Tomosugi, T. and Kanakubo, T., 2001: Upper Cretaceous Hayakawa, H., Tashiro, M. and Nishino, T., 1994: Colony of In- to Paleocene Hakobuchi Group, Nakatonbetsu area, northern oceramus (Cataceramus) bulticus Boehm and process of preser- Hokkaido—lithostratigraphy and megafossil biostratigraphy—. vation. Research Reports of the Kochi University (Natural Journal of the Geological Society of Japan, vol. 107, p. 142–162. Science). vol. 43, p. 183–191. (in Japanese with English abstract) (in Japanese with English abstract) Hirano, H., 1995: Correlation of the Cenomanian/Turonian bound- Asai, A., 1998: Stratigraphy of the Upper Cretaceous of the north- ary between Japan and Western Interior of the United States in western part of the Ikushunbetsu area, central Hokkaido. Ga- relation with oceanic anoxic events. Journal of the Geological kujutsu Kenkyu, School of Education, Waseda University, Series Society of Japan, vol. 101, p. 13–18. Biology & Geology, vol. 46, p. 7–18. Hirano, H., Ando, H., Hirakawa, M., Morita, R. and Ishikawa, T. Asai, A. and Hirano, H., 1990: Stratigraphy of the Upper Creta- 1981: Biostratigraphic study of the Cretaceous system in the ceous in the Obira area, northwestern Hokkaido. Gakujutsu Oyubari area, Hokkaido part 2. Gakujutsu Kenkyu, School of Kenkyu, School of Education, Waseda University, Series Biol- Education, Waseda University, Series Biology & Geology,vol. ogy & Geology,vol.39,p.37–50. 30, p. 33–45. (in Japanese with English abstract) Asai, A., Mitsugi, T. and Hirano, H., 2000: Upper Cretaceous Hirano, H., Koizumi, M., Matsuki, H. and Itaya T., 1997: K-Ar age ammonoids and inoceramids from the Ashibetsu area, central study on the Cenomanian/Turonian boundary of the Yezo Su- Hokkaido. Gakujutsu Kenkyu, School of Education, Waseda pergroup, Hokkaoido, Japan, with special reference to OAE-2 University, Series Biology & Geology, vol. 48, p. 17–31. and biostratigraphic correlation. Memoirs of the Geological So- Funaki, H. and Hirano, H., 2004: Cretaceous stratigraphy in the ciety of Japan, vol. 48, p. 132–141. northeastern part of the Obira area, Hokkaido, Japan. Bulletin Hirano, H., Matsumoto, T. and Tanabe, K., 1977: Mid-Cretaceous of the Mikasa City Museum, no. 8, 17–35. stratigraphy of the Oyubari area, central Hokkaido. Palae- 226 Akinori Takahashi

ontological Society of Japan, Special Paper,no.21,p.1–10. Maeda, H., 1986: Stratigraphy and fossil molluscan fauna of the Hirano, H., Takizawa, K. and Tsuchida, S., 1989: Biostratigraphy of Upper Cretaceous System in the Bibai area, Hokkaido, north the Upper Cretaceous in the Oyubari area, central Hokkaido, Japan. Journal of the Geological Society of Japan,vol.92, Japan (4). Bulletin of Science and Engineering Research Labo- p. 135–153. (in Japanese with English abstract) ratory, Waseda University, no. 127, p. 8–26. Matsumoto, T., 1942: Fundamentals in the Cretaceous stratigraphy Jimbo, K., 1894: Beitra¨ge zur Kenntniss der Fauna der Kreide- of Japan, Part 1. Memoirs of the Faculty of Science, Kyushu formation von Hokkaido. Palaeontologische Abhandlungen, Imperial University, Series D, Geology,vol.1,p.129–280. Neue Folge,vol.2,p.140–197. Matsumoto, T., 1943: Fundamentals in the Cretaceous stratigraphy Kanie, Y., 1965: The Cretaceous deposits in the Urakawa district, of Japan, Part 2 & 3. Memoirs of the Faculty of Science, Kyushu Hokkaido. Journal of the Geological Society of Japan,vol.72, Imperial University, Series D, Geology,vol.2,p.98–237. p. 315–328. (in Japanese with English abstract) Matsumoto, T., 1957: Inoceramus mihoensis n. sp. and its signifi- Kanoh, M., Toshimitsu, S. and Tashiro, M., 1989: Stratigraphy and cance. Memoirs of the Faculty of Science, Kyushu University, depositional facies of the Himenoura Group in the Koshikijima Series D, Geology,vol.6,p.65–68. Islands, Kagoshima Prefecture. Research Reports of the Kochi Matsumoto, T., 1989: Some inoceramids (Bivalvia) from the Cen- University (Natural Science), vol. 38, p. 157–185. (in Japanese omanian (Cretaceous) of Japan-5. A world-wide species In- with English abstract) oceramus pictus Sowerby from Japan. Transactions and Pro- Katto, J. and Tashiro, M., 1979: A study on the molluscan fauna of ceedings of the Palaeontological Society of Japan, New Series, the Shimanto Terrain, southwest Japan. Part 3: On the bivalve no. 153, p. 12–24. fauna from the Arioka, Nakamura and Susaki Formations in Matsumoto, T., 1996: A little known species of Inoceramus (Bi- Shimanto (Northern) Terrain, Kochi Prefecture. Research Re- velvia) from the Turonian (Cretaceous) of Hokkaido. Trans- port of Kochi University (Natural Science), vol. 28, p. 49–69. (in actions and Proceedings of the Palaeontological Society of Ja- Japanese) pan, New Series, no. 183, p. 540–543. Katto, J., Tashiro, M. and Aoki, T., 1980: Discovery of Inoceramus Matsumoto, T. and Asai, A., 1989: Cenomanian (Cretaceous) in- and its significance from the northern belt of the Shimanto oceramids (Bivalvia) from Hokkaido and Sakhalin-1. 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Appendix 2. List of inoceramid species in Japan and their stratigraphic distribution. Solid circle in each box indicates that the species occurred in the substage. Question mark indicates questionable occurrence of the species or precise horizon unknown but occurrence approximate. Appendix 2. Continued rtcosioeai iest changes diversity inoceramid Cretaceous 231 3 knr Takahashi Akinori 232

Appendix 2. Continued