Diversity Changes in Cretaceous Inoceramid Bivalves of Japan
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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 extinc- tion 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 combi- nation 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 À PN FR ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi FN Á 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)