Rudists of Tibet and the Tarim Basin, China: Significance to Requieniidae Phylogeny
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J. Paleont., 84(3), 2010, pp. 000–000 Copyright ’ 2010, The Paleontological Society 0022-3360/10/0000-0000$03.00 RUDISTS OF TIBET AND THE TARIM BASIN, CHINA: SIGNIFICANCE TO REQUIENIIDAE PHYLOGENY ROBERT W. SCOTT,1 XIAQIAO WAN,2 JINGENG SHA,3 AND SHI-XUAN WEN3 1Department of Geosciences, University of Tulsa, Tulsa, Oklahoma 74104, ,[email protected].; 2China University of Geosciences, Beijing, China; and 3LPS, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing ABSTRACT—Rudists are a principal biotic component of Cretaceous carbonates in Tibet and in the Western Tarim Basin. Barremian to Maastrichtian carbonate units are widespread on the northern margin of the Indian Plate and in Tethyan tectonic slices that were welded onto Eurasia in successive stages during the Late Cretaceous and Paleogene. In far northwestern Tibet, Barremian-Cenomanian endemic rudists and cosmopolitan orbitolinid foraminifera occupied isolated carbonate platforms in the eastern Tethys. Rudists, corals, and stromatoporoids composed bioherms up to 10 m thick and several kilometers in lateral extent. A unique endemic requieniid rudist, Rutonia, is compared to morphologically similar but older, less derived genera. Associated specimens in this assemblage are indeterminate requieniid valves, monopleurids, and two genera with three radiolitid species that are re-described and taxonomic positions re-evaluated. In southern Tibet, mainly endemic Campanian-Maastrichtian radiolitid rudists and cosmopolitan larger benthic foraminifera contributed to carbonate shelves on the northern Indian Plate near the Cretaceous equator. In the Western Tarim Basin Cenomanian strata yield Tethyan rudist species. Coiling morphometric analysis using the three-dimensional morphology Raup diagram shows that Requieniidae valves in contact with the substrate are convergent with the basic gastropod shell. More derived strongly coiled, younger requieniids were adapted to encrusting or semi-infaunal habits. Stratigraphic analysis confirms that Requieniidae diversity crises coincided with Cretaceous oceanic anoxic events Two end members of valve geometry each appear to be primitive and derived characters respectively and separate the family Requieniidae into two clades that are here recognized as two new subfamilies. The end members are defined by the coiling geometry, whether the spire is close to the plane of commissure or it is translated along the coiling axis and by myophore structures. The older matheroniform clade has a low spirogyrate LV that is translated slightly from the commissure along the coiling axis; this group is composed of Matheronia (and its subgenus Monnieria), Hypelasma, Lovetchenia, Rutonia, and Kugleria. Genera in the younger clade have a tall trochospiral LV that is translated along the coiling axis and consists of Requienia, Toucasia, Pseudotoucasia, Apricardia, Bayleoidea, and Bayleia. Claditics support these relationships. INTRODUCTION Mesorbitolina texana (Roemer, 1849) (senior synonym of RETACEOUS CARBONATE shelves and platforms in the Tethys Orbitolina kurdica Henson, 1948, according to Simmons et al., C Realm were populated by marine benthic communities 2000) and Mesorbitolina pervia (Douglas, 1960) together with dominated by unconventional rudist bivalves, scleractinia, and Praeradiolites hedini Douville´ (1917), Sphaerulites [formerly various types of larger benthic foraminifera (Masse et al., Praeradiolites] biconvexus (Yang et al., 1982), and Praeradio- 1995). Rudist distribution and species composition are rather lites ngariensis Yang et al. (1982). The Cenomanian Gamba- well known in the western part of the Tethys Realm of North cunkou Formation yields Mesorbitolina aperta (Erman, 1854), Africa, southern Europe, Turkey, and the Middle East Orbitolina concava (Lamarck, 1816), and Conicorbitolina (Steuber and Lo¨ser, 2000). However, rudists in the eastern conica (D’Archiac, 1837) with the rudists Praeradiolites hedini part of southern Asia are poorly known. In Tibet/Xizang, and Rutonia bangonghuensis Yang et al. (1982), and mono- pleurids. Six additional rudist species were created by Gou and Cretaceous carbonates are quite thick and widespread and Shi (1998): list them. rudists are a principal biotic component. The Chinese The objectives of this contribution are to demonstrate the geosciences community has described many rudist species geological importance of Tibetan rudists and to synthesize (Qian, 1993; Gou and Shi, 1998; Wen, 1999; Wen et al., 2000). their distribution, biostratigraphy, paleoecology, and phylo- Douville´ (1916) first reported Upper Cretaceous rudists genetic considerations. A modern description of the locally from southern Tibet collected during a 1903 British expedition known requieniid genus, Rutonia, prompts a morphological (Fig. 1). The Campanian assemblage consisted of Bournonia analysis and a cladistic hypothesis of the rudist Family haydeni Douville´ (1916) and Bournonia tibetica Douville´ Requieniidae that suggests the Family Requieniidae be divided (1916) associated with Biradiolites. The Maastrichtian rudist into two subfamilies. This paper follows phylogenetic research is Plagioptychus tibeticus Douville´ (1916) with the larger of Upper Jurassic and Lower Cretaceous species by Masse benthic foraminifers Orbitoides, Lepidorbitoides and Ompha- (1994, 2002) and Gourrat et al. (2003). locyclus. These assemblages are part of the Campanian- Maastrichtian Zongshan Formation (Willems et al., 1996). MATERIAL Albian and Cenomanian rudists and orbitolinid foraminif- Large collections of rudists and other megafossils as well as era are significant components of thick carbonate units in far samples for microfossil thin sections were collected during the northwestern Tibet in the Ngari, Xizang area (Fig. 1) (Wan et 1980 expedition. These materials were distributed between the al., 2003). The Albian Lanshang Formation is from 1,000 to University Museum at China University of Geosciences 3,000 m thick (Fig. 2) and is dated by the foraminifers Beijing and the Paleontological Institute in Nanjing. Speci- 0 Journal of Paleontology pleo-84-03-04.3d 31/3/10 16:48:52 1 Cust # 09-137 0 JOURNAL OF PALEONTOLOGY, V. 84, NO. 3, 2010 FIGURE 1—Major sedimentary terranes of the Tibetan-Qinghai-Xizang Plateau in present-day position. Legend: 1(marine shelf facies, 2) deep sea facies, and 3) continental facies (from Wen, 1999). The key rudist localities are 1) Ngari region (Yang et al., 1982): type area of Radiolites bangonghuensis, Praeradiolites biconvexus, and Praeradiolites ngariensis; and 2) Zongshan area with villages of Kampa Dzong and Tuna, (N28u to 28u209 by W88u309 to 89u159) (Douville´, 1916): type area of Bournonia haydeni and Bournonia tibetica, and Plagioptychus tibeticus. mens collected by later expeditions are at the Chengdu and extends for many kilometers across the Ngari region in Institute of Technology. western Tibet (Fig. 1, site 1). In places rudist-coral bioherms Dr. Zetong Nie (personal commun., 2007), who, with Dr. up to 10 m thick and several tens of meters wide are encased in Xiaqiao Wan, participated in the geological expedition to marl. Rudists and corals dominate the bioherms and algae are Tibet, reviewed for us the rudist occurrences. The rudist- accessory biota. At the bioherm margins orbitolinids are bearing Langshan Formation carbonate is up to 1,000 m thick common and are intermixed with rudist talus debris. Overlying the buildups at the base of the marl, orbitolinids and intraclasts of the rudist-coral bioherm are abundant and diminish up section (Yang et al., 1982, fig. 2). BIOGEOGRAPHY OF TIBETAN RUDISTS The Tibetan Plateau today is a complex tectonic province composed of several terranes or blocks sandwiched between the Indian Plate to the south and the Eurasian Plate on the north (Fig. 3) (Huang and Chen, 1987; Qian, 1993; Li and Xiao, 1995; Wang et al., 1997; Gou and Shi, 1998; Wen, 1999; Wen et al., 2000; Sha et al., 2002; Wan et al., 2003; Sha et al., 2004). North of the Indian Plate in northwestern Tibet the Lhasa Block is bounded by the Yarlung-Zangbo and the Bangong-Nu River suture zones (Fig. 1). Albian-Cenomanian carbonates with rudists (Yang et al., 1982) are located at the northwest end of the Lhasa Block in the Ngari district (Fig. 1, site 1). During the middle Cretaceous this tectonic slice was positioned between approximately 10u and 20u north latitudes (Fig. 3; Golonka, 2002). A second tectonic slice, the Qiang- tang block, lies north of the Lhasa Block and south of the Eurasian Plate. This latter block hosts Cretaceous carbonates FIGURE 2—Generalized stratigraphic classification of rudist-bearing in the Karakorum and Kunlun mountains and the Tarim formations in Tibet (modified from Wen, 1999; Wang et al. 2002). Basin along its southern margin (Wen, 1999). The Qiangtang Journal of Paleontology pleo-84-03-04.3d 31/3/10 16:49:19 2 Cust # 09-137 SCOTT ET AL.—TIBETAN AND CHINESE RUDISTS 0 rudists are known from this subprovince; however, the Tithonian megalodontid, Protodiceras lanonglaensis Li and Grant-Mackie (1994), occurs west of Mount Everest/Qomo- langma in the Nyalam district at the base of the Xuomo Formation. By Campanian-Maastrichtian times the Indian Plate had moved into the tropical zone and bivalves including rudists were part of the Tethyan Eastern Mediterranean Subprovince (Gou and Shi, 1998; Wen, 1999). Common rudists were endemic species of Biradiolites, Bournonia, Lapeirousia, Plagioptychus, and Praeradiolites (Appendix 1). North of the Yarlung-Zangbo