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From the Early Oligocene of the Nanning Basin (Guangxi, SE China)

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From the Early Oligocene of the Nanning Basin (Guangxi, SE China) N. Jb. Geol. Paläont. Abh. 267/1, 75–87 Article Published online December 2012 Giant Viviparidae (Gastropoda: Architaenioglossa) from the Early Oligocene of the Nanning Basin (Guangxi, SE China) Tian Ying, Franz T. Fürsich and Simon Schneider With 5 figures and 2 tables Ying, T., Fürsich, F.T. & schneider, s. (2013): Giant Viviparidae (Gastropoda: Architaenioglossa) from the Early Oligocene of the Nanning Basin (Guangxi, SE China). – N. Jb. Geol. Paläont., 267: 75-87; Stuttgart. Abstract: The rich freshwater mollusc fauna from the Lower Oligocene Gongkang Formation (Yongning Group) of the Nanning Basin (Guangxi Province, SE China) has yielded shells of two species of giant viviparid gastropods, which are described as Margarya nanningensis n. sp. and Macromargarya aliena n. gen. n. sp. The genus Margarya nevill, 1877 has so far been thought to be endemic to the Pleistocene to Recent freshwater lakes of Yunnan Province (SW China), and is first described from the Oligocene of Guangxi herein. With a maximum height of more than 100 mm, Macromargarya aliena is among the largest freshwater gastropods reported to date. Moreover, M. al- iena displays a strange ontogeny. An early, very low-trochospiral growth stage is followed by a rapid shift of the coiling axis and whorl proportions, leading to regular viviparid morphology in adulthood. Key words: Taxonomy, Margarya, freshwater, East Asia, body size. Introduction With a few exceptions, the Viviparidae are char- acterised by medium high-spired shells with moder- Large freshwater gastropods of the family Viviparidae ately to strongly inflated whorls that are often more are today distributed on all continents except South or less circular in cross-section and separated by deep America (where they are recorded as fossils; Wenz sutures. The large, thin, horny operculum lacks cal- 1938-1944) and Antarctica. They usually occur in careous carbonate and is usually not preserved in fos- lakes, ponds, and lentic rivers of temperate to tropical silized specimens. In most species, the shells are thin, regions (sTrong et al. 2008). Currently, approximately often stabilized by spiral ribs or bands, and relatively 150 species in 30 genera are recognised in the Vivip- large, ranging between 25 and 70 mm in height. aridae (sTrong et al. 2008), several of which have been Based on their near-global distribution and abun- assigned to the three subfamilies Viviparinae, Lio- dance, the Viviparidae are thought to have evolved placinae, and Bellamyinae (BoucheT & rocroi 2005). before the break-up of the Pangaea supercontinent, For more than half of the genera, however, the exact and thus at least during the Jurassic (Wenz 1938-1944; systematic position and phylogenetic relationships are hamilTon-Bruce et al. 2002; sTrong et al. 2008). The unknown. highest diversity of extant Viviparidae is found in East ©2012 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany www.schweizerbart.de DOI: 10.1127/0077-7749/2012/0298 0077-7749/2012/0298 $ 3.25 76 T. Ying et al. Fig. 1. A. Major tectonic units of China. Guangxi is demarcated by a thick black line. CCO = Central China Orogen; CT = Cathaysian Terrane; EG = Er-Guna Orogen; LT = Laoyeling Terrane; QAB = Qaidam Block; QIB = Qiangtang Block; QLB = Qilian Block; QTB = Quinhai Tibet Block; SGB = Songpan-Garze Belt; TID = Tianshan-Inner Mongolia-Daxinganling Orogen. B. Schematic geological map of Guangxi, showing the position of the Nanning Basin. and Southeast Asia, comprising several widespread rope, however, provides several examples of Pliocene as well as endemic genera and species (sTrong et al. endemic lake radiations in this group, leading to an 2008). The rich fossil record of Viviparidae in Eu- extraordinary high diversity of viviparids. In Lake Da- Giant Viviparidae (Gastropoda: Architaenioglossa) from the Early Oligocene 77 cia (Romania, Bulgaria; luBenescu & zazuleac 1985) comprising predominantly Proterozoic to Palaeogene and Lake Slavonia (Croatia, Bosnia, Serbia; e.g., neu- sediments, while the south-eastern part of the prov- maYr 1869; neumaYr & Paul 1875), viviparids were ince is situated on the Cathaysian Terrane, which is represented by 50 and 35 species, respectively (see mainly composed of crystalline rocks (Fig. 1A, B). harzhauser & mandic 2008 for an overview). Nanning, which is the capital of the Guangxi Zhuang Literature on fossil Cenozoic freshwater gastro- Autonomous Region, is situated in the Nanning Basin pods from continental East Asia is scarce (see Yen in the southern part of this province, approximately 1943b; suzuki 1949; and neuBauer et al. 2012 for an 150 km to the northeast of the border to Vietnam (Fig. overview). Fossil Viviparidae have, however, been 1B). The Nanning Basin is a late Mesozoic to early described from several regions, and seem to have Cenozoic pull-apart basin that is filled with an almost been widespread in East Asia since the Palaeogene. 700 m thick succession of Cenozoic sediments (deng Eocene and Miocene viviparids from Thailand have & Wu 1992). Sedimentation started in the Palaeocene, been used in stable isotope studies (udomkan et al. and the basin infill is dominated by deep to shallow 2003; raTanasThien et al. 2008) and several Mi- lacustrine facies (deng & Wu 1992). During the Late ocene mollusc assemblages dominated by viviparids Eocene and Early Oligocene, the sediments of the were reported from northern Thailand (oYama 1978; Yongning Group were deposited. Based on mammal songTham et al. 2005; ugai et al. 2006). Viviparidae faunas (choW 1958; Tang & choW 1964), the group from the Upper Eocene–Lower Oligocene deposits of has been subdivided into the Upper Eocene Nadu For- northern Vietnam (Na Duong Basin, Lang Son Prov- mation and the Lower Oligocene Gongkang Forma- ince) were illustrated by khuc (1991) and Böhme et al. tion, which has yielded the molluscs described below. (2011). Additionally, Oligocene to Pleistocene mollusc Later on, the discovery of several Anthracotheriidae assemblages that yielded viviparid gastropods have confirmed the Early Oligocene age of the Gongkang been described from Myanmar (annandale 1919) and Formation (zhao 1983, 1993). China (e.g., mansuY 1918; odhner 1930; hsü 1935; odhner (1930) and hsü (1935) erroneously dated Yen 1935). Scattered additional records are listed in the sediments of the Gongkang Formation as Pliocene. detailed compilations of fossil freshwater molluscs of They regarded the rich mollusc assemblage as closely China (Yen 1943b) and East Asia (suzuki 1949). related to the superficially similar fauna of the former The freshwater gastropod fauna of the Nanning Levantine Stage of Slavonia (Croatia, Bosnia, Serbia; Basin in Guangxi Province exhibits certain similari- see harzhauser & mandic 2008 for discussion of the ties to contemporaneous faunas at the family and ge- viviparid fauna of Lake Slavonia), and thus determined nus level, but differs from them by a remarkably high a Pliocene age. Today, the Nanning Basin is drained by diversity and extraordinary preservation. More than the Yong River, and is thus part of the southern catch- eighty years ago, odhner (1930) was the first schol- ment area of the Pearl River. ar who described freshwater molluscs from the Oli- The gastropod fauna that yielded the specimens de- gocene deposits of Nanning. Based on shells collected scribed herein was discovered in a construction trench by W.J. ding, odhner (1930) detailed 20 species of near the city of Santang, which is located ca 2 km to freshwater molluscs, of which he regarded 18 species the northeast of Nanning (Fig. 2A; N 22°52.47’, E as new to science. His material yielded fragments of a 108°24.72’). The pit exposed a somewhat more than 6 single large viviparid species, which he described as m thick succession of the Gongkang Formation which Tulotoma gigas odhner, 1930. A few years later, three consisted of well-bedded mud- and siltstones of differ- additional gastropod species, including the small vivi- ent colours (Fig. 2B). The gastropods are derived from parid Viviparus wongi Yen, 1936, were described by different levels in the section (Fig. 2B), and the two Yen (1936). Since then, the mollusc fauna of the Nan- species detailed herein do not co-occur in the same ning Basin fell into oblivion. horizons. Geological overview Material and methods Tectonically, Guangxi Province is part of the South The large gastropods described herein where surface- China Block, which is subdivided into two large subu- collected from the outcrop at Santang by the first au- nits (Fig. 1A). The major part of Guangxi, including thor. Despite their impressive size, both species occur the area of Nanning, belongs to the Yangtze Terrane in high abundance, but are often fragmented due to 78 T. Ying et al. Fig. 2. A. Schematic map of the Nanning area, showing the position of the outcrop at Santang (N 22°52.47’, E 108°24.72’). B. Schematic section of the succession at Santang. Sample levels are indicated by gastropod symbols. C. Photograph of the Santang section. D. Close-up of a claystone horizon with numerous specimens of Macromargarya. E. Weathered claystone block with Macromargarya. compaction of the sediments and weathering at the Repository: The specimens are stored in the collections surface (Fig. 2D, E). Thus, only a minor portion of the of the Dalian Shell Museum (DLSM) at Dalian, Liaon- specimens could be collected with more or less entire ing Province, China. Extant specimens that are figured for comparison are stored in the collections of the Malaco- shells. The shells
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