«To cite this version: Aihua Yuan. Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China. Pa- leontology. Universit´ Pierre et Marie ...»
Luolou Formation (T1l) Member IV (1-22) – interbedded siliceous mudstones, mudstones and limestones Member III (23-48) – interbedded siltstones and mudstones with minor siliceous and argillaceous cherts Member II (49-69) – siltstones and pebbly siltsonte with muddy siltstone and siliceous mudstone Member I (70-95) – grey, thin-bedded siliceous mudstones interbedded with mudstones and siltstones ———————— Conformity contact ———————— Sidazhai Formation (P2) The Wuchiapingian age of the Member I is determined by the appearance of ammonoid genus Tauroceras. The conodont Clarkina wangi reported from the top of the Member II indicates the beginning of the Changhsingian. The presence of the zonal conodont species for the late Changhsingian Clarkina changxingensis (Wang & Wang) in the Member III and the typical brachiopod species and radiolarian zone Neoalbaillella ornithoformis confined to the late Changhsingian discovered in the Member IV furtherly determined the age of Changhsingian in the top of the Member III. The bivalve assemblage is also correlated to other Changhsingian bivalve assemblages (Hao et al., 1999; Yang & Gao, 2000; Gao et al., 2001, 2005; Wang & Shang, 2001; Feng & Gu, 2002; Gu et al., 2002; Chen et al., 2006). In this work, all samples were collected from the upper part (Bed 1 to Bed 5 in Chen et al.,
2006) of the Member IV (Linghao Formation). The strata are exposed 11.4m and subdivided into 13 beds and 87 sub-beds (Fig. 1-2). Lithology of each sub-bed has been described in Gu (2002).
1.1.3 Chaohu Section (CH)
The Chaohu Section (i.e.West Pingdingshan Section, the GSSP candidate of the Induan-Olenekian boundary) is situated 5km northwest of Chaohu City, Anhui Province. There are railway and highway to connect with other big cities, e.g. about 60km southeast to Hefei City, the capital of Anhui Province (Fig.1-1-C (a)) (Tong & Zhao, 2005; Gui et al., submitted).
During the Late Permian, the Chaohu area was situated in the northern margin of the Lower Yangtze basin (Fig.1-B). The stratigraphical sequence is exposed from the Upper Precambrian (Sinian) to Middle Triassic except for the absence of the Lower-Middle Devonian. This area began to receive 14 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 terrestrial instead of marine sediments or be eroded since the Middle Triassic. All strata were folded as the Mt.Majiashan-Mt.Pingdingshan Syncline during the Indosinian Movement in the Late Triassic (Tong & Zhao, 2005; Tong et al., 2005; Li et al., 2007; Zhao et al., 2008).
Fig.1-1-C: (a) Geographic location of the Chaohu Section (modified after Tong & Zhao, 2005); (b) The photo of the Chaohu Section; (3) Geologic location of the Chaohu Section (modified after Tong & Zhao, 2005).
The Chaohu Section is situated at the western slope of Mt. Pingdingshan. The strata from the Middle Permian (Gufeng Formation) through the Lower Triassic (Yinkeng Formation, Longshan Formation and lower Nanlinghu Formation) are well exposed (Fig.1-1-C (c), Fig.1-2). The Permian-Triassic boundary, at the middle the bed CH12 (i.e. Bed 5 in Zhao et al., 2007), is determined by the lithostratigraphic correlation of “Permian-Triassic boundary stratigraphic set” (PTBST) (Peng et al., 2001). The Late Permian Dalong Formation is mainly composed of the thin-bedded siliceous mudstones and mudstones. Radiolarians, foraminifera, ostracods, sponge spicules and small bivalves and brachiopods are yielded and abundant in some beds (Tong et al., 2005; Zhao et al., 2007; Gui et al., submitted).
15 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) In this work, 24 samples of siliceous mudstones and 8 samples of mudstones were collected from the upper Dalong Formation. The descriptions of lithology for each bed are listed as follows from top to bottom (Fig.1-1-C (b)). (Remarks: The sample were not numbered by the bed number) Yinkeng Formation (T1y)
Remarks: The field work and sample processing have also been conducted in some other sections (see table below). However, they are not systematically included in this thesis except that some specimens from the Tieqiao Section and the Xichang Section are presented in the plate (Plate 2, figs 5-6, Plate 5, fig.8 and Plate 6, fig.15), the brief information about these sections are listed in the table below (Tab.1-1) and the detail descriptions are omitted here.
16 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5
1.2.1 Permian-Triassic Boundary This study is focused on the latest Changhsingian strata. Thus the determination of Permian-Triassic boundary (PTB) is the first important problem.
In the Dongpan Section, the presence of the Neoalbaillella optima radiolarian zone in the beds from 03DP2 to 03DP 6 and ammonoids Huananoceras cf. perornatum Chao and Liang, Laibinoceras cf. compressum Yang, Qianjiangoceras sp. at the top of the bed 03DP12 indicates the Changhsingian age of the Dalong Formation. The first occurrence of the typical Triassic ammonoids Ophiceras tingi Tien and bivalve Claraia dieneri Nakazawa at the base of the bed 03DP13 gives the Luolou Formation Early Triassic age. Consequently, the PTB in DP Section is placed between the beds 03DP12 and 03DP13 (Feng et al., 2004, 2006, 2007a; He et al., 2005; Meng, 2005; Jin et al., 2007; Zhang et al, 2007a, 2007b) (Fig. 1-2).
In the Liuqiao Section, the radiolarian Neoalbaillella optima zone assigns the strata to the Changhsingian.Albaillellaria is present from the bottom up along the whole section, which disappears above the bed 03DP6 of the DP Section. Furtherly, Albaillellaria flabellata was recognized in the top of the bed 03DP2 of the Dongpan Section but not found in the Liuqiao Section. Thus the Liuqiao Section should be underlying the top of the bed 03DP2 of the Dongpan Section. In addition, our investigation in the studied area (group work, unpublished data) indicates that there are still at least 5m strata between the bottom of Dongpan Section and the top of the Liuqiao Section. In another word, all the strata of the Liuqiao Section underlie the Dongpan Section and belong to the Changhsingian (Fig. 1-2).
The Member IV of the Shaiwa Section is constrained to the late to latest Changhsingian based on biostratigraphical studies. The presence of the ammonoids Ophiceras sp. and the bivalve Claraia sp.
determines the PTB between the Shaiwa Group and the Luolou Formation (Yang et al., 2000; Gao et al., 2001, 2005; Feng & Gu, 2002; Chen et al., 2006).
17 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) Fig. 1-2: Stratigraphic correlation between the Liuqiao Section, Dongpan Section, Meishan Section and Chaohu Section.
The Meishan Section shares the common scale with the Chaohu Section (Meishan Section after Yin & Lu, 2006) The PTB of the Chaohu Section is assigned according to the lithological “sandwich” PTBST which is characterized by the composition of the bottom clay (Bed 25 and 26 in the GSSP Meishan (MS) Section), the boundary rock and the top clay (Bed 28 in the Meishan Section) (Yin & Lu, 2006). The beds CH6 and CH7 can be considered as the bottom clay, which is constituted of “white claystone” and “black claystone”, the bed CH12 is composed of the marls and hence considered as the boundary rock, the bed CH13 which is composed of the mudstones as the top clay. Thus the PTB is designated at the middle of the marl bed CH12 (the Bed 5 in Zhao et al., 2007) (Fig. 1-1-C (b), Fig. 1-2).
18 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 1.2.2 Section Correlation The correlation between the Dongpan Section, the Chaohu Section and the Meishan Section has been established based on the lithology, biostratigraphy, geochemistry and event-stratigraphy.
Dongpan Section & Meishan Section:
In the Dongpan Section, Albaillella yao zone in the beds from 03DP2 to 03DP5 can be correlated to the Clarkina changxingensis – C. postwangi – C. postsubcarinata – C. deflecta assemblage in Bed 23 to Bed 24d of the Meishan Section (Feng et al., 2007a; Zhang et al., 2007a, 2007b). The multi-layers of claystones in the Dongpan Section provide the research possibility of the event-stratigraphy. Studies on claystones in the bed 03DP9 indicate the volcanic origin and suggest the correlation with the lower claystones of the PTB in the Meishan section (Bed 25- Bed 26). The stratigraphic trend of the organic carbon isotope curve is accordant with that in the Meishan Section. The evident negative excursion of the organic carbon isotope also supports the correlation between the bed 03DP9 of the Dongpan Section and Bed 25 of the Meishan Section (Zhang et al., 2007a, 2007b) (Fig. 1-2).
This chapter is composed of three main parts. The object of the first and the second parts is to know what ostracods are and how to carry out the taxonomic research on fossil ostracods. With this understanding, in the third part, I systematically present all ostracods identified during this thesis (43 genera, 128 species).
(1) “Ostracoda” is the formal taxonomic name when referred to the class. From the beginning of ostracodology, there are two other different vernacular words “ostracode” and “ostracod”. “Ostracode” is the usual spelling in North America and France. “Ostracod” is normally used elsewhere in Europe and in Australia (Holmes & Chivas, 2002, p.1). In this thesis, I uniformly use “ostracod”.
(2) My thesis follows Bowman & Abele (1982) and Holmes & Chivas (2002, p.7) in considering the Ostracoda to be a Class.
(3) “Shell” is used here for an indefinite and general name either referring to the carapace or valve.
§2.1 Generality on ostracods
Ostracod is a small bi-valved Crustacea with a general size of 0.15-2mm. There are some representatives with large size of 80 mm for Paleozoic species and 32 mm for recent swimming forms (Pokorný, 1978; Holmes & Chivas, 2002). The soft parts are protected in a bivalve carapace hinged dorsally and closed by adductor muscles. Chemical composition of the carapace is low-magnesium calcite.
Ostracoda is one of the most widespread and diverse crustaceans with 33,000 living and fossil species (Holmes & Chivas, 2002, p.5). This kind of small crustacean spans vastly both in spatial and temporal distribution.
The earliest fossil record of ostracods can be traced to the Ordovician. Ostracoda is divided into two subclasses, Myodocopa and Podocopa, which have been recorded from the Ordovician and are still very prosperous at present.
Ostracod can live in nearly all aquatic and some near/semi-terrestrial environments. Living ostracods can be found from damp leaf litter and fen soils, from small ponds to deep sea and from fresh to marine water. Fossil ostracods have been discovered in limestones, mudstones, siliceous and other rocks sedimented from the Ordovician onward. The ostracod soft parts, measurements, outline and ornamentation as well as its assemblage greatly vary in different regions and environments. Ostracods 20 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 are effective (paleo) ecological markers and play an important role in paleoenvironmental and paleogeographical reconstructions.
The great majority of ostracods are benthic inhabitants and their larvae have the same ethology.
The lack of planktonic mode means that marine ostracods can not get across the geographical barrier (vast ocean, temperature, etc.). The geographical isolation limits the biostratigraphical correlation between ostracods in different continents even regions. However, this disadvantage is largely outweighed by its powerful application in paleoecology and paleogeography (Moore, 1961, Q2;
Pokorný, 1978; Holmes & Chivas, 2002, p.1, 5) (detailed introduction on paleoecological application see Chapter 4).
1993; Holmes & Chivas, 2002) As all other crustacean, ostracods have a segmented body protected by a carapace. The ostracod body is divided into cephalic and thoracic segments. The segmentation of the body is only observable on appendages. All ostracod appendages are pair and typical biramous (Fig. 2-1-1) except the antennule which is uniramous.
The cephalic segment is composed of the forehead, upper lip, lower lip, hypostome and four pairs of appendages respectively termed as antennule (the first antenna, sensory and locomotory organ), antenna (the second antenna, locomotory or burrowing organ), mandible (locomotory, digging and feeding organ) and maxilla (feeding and respiratory organ) from front to back (Fig. 2-1-1).
The thoracic segment contains the digestive and reproductive systems and usually one to four pairs of thoracic limbs named as fifth limb (feeding, walking, respiratory and clasping organ), sixth limb (walking leg), seventh limb (walking leg) and eighth limb (male copulatory limb) (Fig. 2-1-1).
One pair of furcae (uropod) have unsegmented rami and functionate as locomotory organ when well developed (Fig. 2-1-1). The furca is mostly absent in some taxa.
Hinz-Schallreuter & Schallreuter, 1999; Ikeya & Kato, 2000; Holmes & Chivas, 2002) Ostracods reproduce in three modes: gamogenic, parthenogenetic and mixed. Most of marine ostracods reproduce sexually. Few species are thought to be parthenogenetic by ostracodologists due to not finding male specimens in the investigations. In non-marine ostracods, all three modes exist but the main way of reproduction is parthenogenetic. Some fresh-water species reproduce in different modes according to the environments. In fitting conditions, they can reproduce sexually, whereas in not good conditions reproduce asexually. This phenomenon is known as “geographical parthenogenesis”.
Ostracods usually have a life of several weeks to 4 years. They can oviposit at anytime of a year.