Rhuddanian (Llandovery, Silurian) brachiopod assemblages from the Yangtze Region, South China  

Rong Jia–yu¹, Zhan Ren–bin¹ and David A. T. Harper²  

¹Nanjing Institute of Geology and Palaeontology, CAS, Nanjing, 210008, The People’s Republic of China. E–mail: rongjy@lonline.com

² Geological Museum, University of Copenhagen, DK–1350 Copenhagen, Denmark.

Key words: Brachiopods. Survivor–recovery. Rhuddanian. Silurian. South China.

The decay of the Gondwana glaciation promoted a rise in water temperature, oceanic overturn, and a major rise of eustatic sea level with anoxia in the second phase of the latest Ordovician mass extinction (e.g., Brenchley et al., 1995, 2003; Harper and Rong, 1995; Sheehan, 2001; Rong and Shen, 2002). Rhuddanian (Early Llandovery) black graptolitic shale was widely distributed in many regions of the world, whereas its coeval shelly facies is comparatively poorly known. Brachiopod survival and recovery following the latest Ordovician mass extinction are not well documented partly because of inadequate or poor fossil record (Erwin, 1998).

Brachiopods are dominant members of many marine benthic communities in the Early Silurian. Rhuddanian brachiopods are known from many regions (e.g., Rubel, 1970; Amsden, 1974; Jin and Copper, 1997, 2000; Dewing, 1999; Oradovskaya, in Koren et al., 1983; Williams, 1951; Temple, 1970; Baarli and Harper, 1986; Cocks, 1988; Modzalevskya and Popov, 1995). However, correlation of the brachiopod assemblages with graptolite zonation is usually unclear. Following the latest Ordovician extinction, the vast majority of the Yangtze Platform was covered by anoxia water, except surface water and shallow water seabeds which were inhabited by the benthic brachiopods (mainly BA2–upper BA3) noted below.

A continuous sequence from mid–late Ashgill graptolite shale (Wufeng Fm) through Hirnantian mudstone (the Kuanyinchiao Beds) to Rhuddanian graptolite shale (basal Lungmachi Fm) is well documented in many areas of the Upper Yangzte Region (see Chen et al., 2000). The Rhuddanian shelly facies occurs in the southern marginal belt north of the Central Guizhou Old–land. The Rhuddanian brachiopod data discussed by Rong and Harper (1999) were mainly based on the material from four sections. Brachiopods being most common in this facies are associated with scarce trilobites and gastropods. More collections of the brachiopod assemblages from Tongzi and Meitan counties, northern Guizhou are presented herein. Graptolites from the overlying and underlying beds of the brachiopod bearing strata afford a probability of correlation of the brachiopod assemblages with graptolitic zonation in this region.

Figure 1. Showing stratigraphical range of the Rhuddanian–early Aeronian brachiopod genera in northern Guizhou with the four brachiopod assemblages (1–4) as discussed in the text.

Four Rhuddanian brachiopod assemblages are recognized with a correlation of graptolite zonation based on the data from northern Guizhou. They are as follows in descending order.

1. Earliest Rhuddanian brachiopods are found from the upper Kuanyinchiao Beds (AFA 309–311: A. ascensus Biozone, possibly into lower P. acuminatus Biozone) at Honghuayuan, Tongzi (Chen et al., 2000). New investigation suggests that relicts from the underlying Hirnantia fauna, e.g., Paracraniops sp., Dalmanella testudinaria (Dalman), Eostropheodonta parvicostellata (Rong), Paromalomena polonica (Temple), Leptaena cf. trifidum (Marek and Havlicek), Fardenia sp., Plectothyrella sp., and Hindella crassa incipiens (Williams), are associated with Eospirifer cf. praecursor Rong et al., and Nalivkinia?, of which both are regarded as Lazarus taxa. Eospirifer possibly originated from the mid Ashgill (Rong et al., 1994; Rong and Zhan, 1996), disappears in early–mid Hirnantian, and recovers in late Hirnantian (e.g., Laurie, 1991). Occurrences of the relicts of the Hirnantia fauna, a situation similar to those of the English Lake District (Harper and Williams, 2002) and the Oslo region (Baarli and Harper, 1986), along with Lazarus taxa indicate that environmental condition may have been slightly ameliorated at the beginning of the Silurian. It represents an early survival in the light of the high proportion of relicts. It is worth mentioning that no taxa are known from this interval.

Plate: Figures 1, 2, Levenea qianbeiensis Rong, Xu and Yang, 1974; Dorsal and ventral internal molds, x2.2, x1.9; from the middle Niuchang Fm (late Rhuddanian–early Aeronian), Niuchang, Meitan. Figures 3, 4, 9, Whitfieldella sp.; ventral (x1.5) and dorsal (x1.7) internal molds, and ventral external mold (x1.5); from the basal Lungmachi Fm (early Rhuddanian), Honghuayuan, Tongzi. Figures 5, 8, 10, 23, Athyrisinoidea seclusa (Rong and Yang, 1981); two ventral internal molds (8, 10, x1.7) and dorsal internal mold (5, x1.9) with its posterior view (23, x2.1); from the upper Niuchang Fm (early Aeronian). Figures 6, 7, Zygospiraella cf. duboisi (Verneuil, 1843); ventral (x1.6) and dorsal (x1.9) internal molds; the same locality and horizon as Figure 1. Figures 11–13, Merciella striata Rong, Xu and Yang, 1974; two dorsal internal molds (12, x1.4; 13, x1.9) and external mold (x1.5); the same locality and horizon as Figure 1. Figure 14, Eospirifer sp.; dorsal internal mold (x1.9); the same locality and horizon as Figure1. Figures 15, 17, Strophomena modesta Rong, Xu and Yang, 1974; ventral and dorsal internal molds (both x1.4); the same locality and horizon as Figure 1. Figure 16, Paracraniops sp.; dorsal (?) internal mold (x9.2); from the upper Kuanyinchiao Beds (earliest Rhuddanian), Honghuayuan, Tongzi. Figure 18, Plectothyrella sp.; dorsal internal mold (x2.4); the same locality and horizon as Figure 16. Figure 19, Dalmanella testudinaria (Dalman, 1828); dorsal internal mold (x3.6); the same locality and horizon as Figure 16. Figures 20, 21, Alispira? sp.; ventral (x2) and dorsal (x2.7) internal molds; from the Wulipo Bed (early Rhuddanian), Wulipo, Meitan. Figures 22, 24, Hindella meitanensis (Rong, 1974); ventral (x2.3) and dorsal (x1.5) internal molds; the same locality and horizon as Figure 20. All materials illustrated are deposited in Nanjing Institute of Geology and Palaeontology, CAS.

2. Early Rhuddanian brachiopods are known from the Wulipo Beds at a new section near Wulipo, Meitan. The beds consist of mudstone (AFA600: 20cm) in the lower and muddy limestone (AFA601: 60cm) in the mid–upper parts. The mudstone yields a shelly fauna containing abundant brachiopods with scarce gastropods and trilobites. The brachiopods include Dalmanella, Hindella, Eostropheodonta?, Eospirifer, Alispira?, Mendacella, Rostricellula and a few others (newly established taxa at least at the species level). The former four, as holdovers, are known from South China in the Ordovician, whereas the last two named genera, as holdovers and immigrants, came from outside China. Both Alispira? and Hindella are assessed as disaster or opportunistic taxa with a high abundance. This assemblage is apparently different from the earliest Rhuddanian brachiopods in composition. In the limestone immediately above the mudstone there occur graptolites, e.g., N. normalis, N. angustus, Glyptograptus cf. tamariscus, and Atavograptus sp., suggesting a level most likely of the vesiculosus Biozone. The brachiopod–bearing bed may correspond to upper acuminatus or/and lower vesiculosus biozones. An association of the holdovers and immigrants with a few newly established taxa shows a continuation of the survival period and a transformation of brachiopod faunas through the Ordovician–Silurian transition since the acuminatus Biozone.

3. Mid Rhuddanian brachiopods occur from the basal Lungmachi Fm that overlies the upper Kuanyinchiao Beds at Honghuayuan (see Chen et al., 2000). The brachiopods in the graptolite facies (AFA 313–315) contain a single taxon, Whitfieldella sp., along with Neodiplograptus bicaudatus, Atavograptus gracilis, Dimorphograptus malayensis, C. vesiculosus and others, indicating a level corresponding to the vesiculosus Biozone (Chen et al., 2000). Very low diversity of benthic brachiopods together with a variety of graptolites may suggest poorly oxygenated environmental conditions. The earliest known Whitfieldella in South China occurs in the Hirnantia fauna (N. extraordinarius–N. ojsuensis Biozone) at Zhujiapo, Zunyi, northern Guizhou (Rong, 1979). Whitfieldella sp. may have been linked possibly with the Hirnantian precursors.

4. Late Rhuddanian to early Aeronian brachiopods are found in the Niuchang Fm (new name, formerly as "Lungmachi Fm", Zhang et al., 1964) at Gaotan, Niuchang, Meitan. This formation, 35.5m thick, consists of yellow or gray, thick–bedded mudstone yielding graptolites in the basal and brachiopods and trilobites in the rest. Levenea qianbeiensis (Rong et al.), Merciella striata Rong et al., Strophomena modesta Rong et al., Eostropheodonta sp., Fardenia sp., Athyrisinoidea, Zygospiraella cf. duboisi (Vernuil), Eospirifer sp. and a few others occur from the lower–middle part (AFA 476, 477: 11m thick and AFA 478, 479: 12.5m). In addition to the survival taxa (e.g., Eostropheodonta), there are immigrants (e.g., Zygospiraella) and Lazarus taxa (e.g., Strophomena) that have not been known from the Yangtze Region in the Ordovician, associated with some new genera (e.g., Merciella). A remarkable increase of diversity and decline of holdovers with higher rate of origination repesent a recovery phase (Rong and Harper, 1999). Monograptus sp., Lagarograptus cf. acinaces (Törnquist), Glyptograptus cf. serratus (Elles and Wood), and Rivagraptus insectiformis (Nicholson) occur in the basal Niuchang Fm (AFA 485: 2m), an association suggesting a level slightly higher than C. vesiculosus Biozone. This shallow water brachiopod assemblage is ascribed to the late Rhuddanian (P. cyphus Biozone) and early Aeronian (D. triangulatus Biozone).

Biotic survival and recovery after the latest Ordovician mass extinction are documented based on four successive Rhuddanian brachiopod assemblages from northern Guizhou. Correlation of the assemblages with graptolite zonation is proposed (Figure 1). Relicts of the Hirnantia Fauna in the A. ascensus Biozone (possibly into P. acuminatus Biozone) shallow water regime are associated with Lazarus taxa or crisis–progenitors, showing an early survival phase following the mass extinction. An association of holdovers (including immigrants and Lazarus taxa) and a few new taxa in the early–middle Rhuddanian (P. acuminatus–C. vesiculosus biozones) shallow water environments indicates a continuation of survival interval with an apparent change of taxonomic composition of brachiopod faunas through the Ordovician–Silurian transition. Typical Silurian brachiopod faunas started to be established during the late Rhuddanian when many new comers (e.g., atrypoids, pentameroids and spiriferoids) inhabited shallow water benthic regimes. No pentameroids occur from the late Rhuddanian Niuchang Fm. (mudstone facies), but the Borealis assemblage has been documented from the limestone facies of the coeval lower Xiangshuyuan Formation in Shiqian, northern Guizhou (Rong and Yang, 1981).

This work is funded by the grants (G 2000077700 and DEA20020–4) from MST, China. The authors thank Chen Xu for the identification of graptolite fossils and Wang Yi, Zhang Yuan–dong, Fan Jun–xuan, and Liu Jian–bo for their valuable help in the field.

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Received: February 15, 2003

Accepted: June 15, 2003