«To cite this version: Aihua Yuan. Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China. Pa- leontology. Universit´ Pierre et Marie ...»
According to the fundamentals above, the proportion of the families/superfamilies found in this work can be grouped as below in the table (Tab.4-1-A) (at specific level).
The Bairdioidea are the most abundant in the studied faunas. In this superfamily, some recognized species are thick-shelled (e.g. Bairdia dongpanensis) or strongly ornamented (e.g. Petasobairdia bicornuta, Abrobairdia sp.1, Pustulobairdia spp.), whereas the others are acuminate and thin-shelled (e.g. Acratia sp.1, Cooperuna cf.tenuis). This may suggest an alternant environment of inner shelf and outer shelf with normal salinity. The low percentages of the Kirkbyidae, Amphissitidae, Kellettinidae, Scrobiculidae, Youngiellidae, Paraparchitidae and Cavellinidae indicate that the environments were not so favorable for their development. The proportion of Bythocytheridae, Tricorninidae, Berounellidae and Rectonariidae is low but gives the signal of the presence of the deep-water environment, mainly in the Dongpan and the Shaiwa Sections. Consequently, on the whole, the ostracod faunas here were inhabitants from the outer of inner platform to the upper part of bathyal environments.
In the four studied faunas, the Dongpan fauna contains the highest percentage of the paleopsychrospheric species and the lowest Bairdioidea which include some acuminate and thin-shelled species. This may indicate that the Dongpan fauna was the deepest inhabitants. The Shaiwa fauna seems the second deepest among the four faunas by the absence of typical neritic species and presence of paleopsychrospheric elements. The Chaohu fauna takes the third place due to the nearly absolute composition by the Bairdioidea. But the Bairdioidea is dominanted by the acuminate and thin-shelled Bairdiacypris and Fabalicypris and thus may imply an outer shelf environment. The Liuqiao fauna may present the shallowest environment by the common presence of the kirkbyid and amphissitid species and ornamented bairdiids. Interestingly, the ornamented Bairdioidea with the Mesozoic characters have occurred in the lower part of the Liuqiao Section but were rarely discovered in the Dongpan Section, the overlying strata of the Liuqiao Section.
In this section, the adopted concept of paleopsychrospheric species and triangular model will be introduced. Then the paleobathymetric evaluation will be carried out according to the model and integrated with other data from geochemistry, sedimentology and other fossils.
4.2.1 Introduction of concept
Kozur (1972) initially introduced the word “psychrospheric” to indicate the Triassic ostracods of Alps which present strong similarities to the modern psychrospheric ostracods (presence of the archaic forms, thin-shelled specimens and /or very well-developed spines). Gründel & Kozur (1975) also found these similarities when studied the Early Permian ostracods of Timor, Indonesia. When Kozur (1991a, b) studied the Middle Permian ostracods of Sicily, Italy, he erected the name “paleopsychrospheric” for distinguishing the pre-Jurassic ostracod assemblage from the modern psychrospheric ostracod faunas.
The ocean was thermospheric during the Jurassic and Cretaceous. The paleopsychrospheric assemblage corresponds to the Thüringian ecotype. Then why the term “paleopsychrospheric” is adopted instead of the Thüringian ecotype? A brief review on the Thüringian ecotype and its related ecotypes are presented and then the rejected reasons will be explained.
The Thüringian ecotype is mainly mentioned by Bandel & Becker (1975), Olempska (1979, 1997), Bless et al. (1987), Casier (1987, 2004), Wang (1988b), Lethiers & Feist (1991), Wei (1993), Becker (2000, review) and Casier & Préat (2003). These different authors proposed different nomenclatures and more transitional ecotypes or subecotypes. Becker (in Bandel & Becker, 1975) firstly proposed the Eifelian ecotype, Thüringian ecotype and Entomozoacean ecotype for the Carboniferous ostracods.
Based on the Carboniferous-Permian ostracod studies, Bless et al. (1987) divided five marine groups:
brackish, shallow marine, deep marine shelf, Thüringian and Entomozoan. Wang (1988b) concluded the paleoecologic significance of the Late Paleozoic ostracods in South China and proposed also five associations∗, i.e. leperditiid association, palaeocopid association, smooth-podocopid association, spinose-podocopid association and entomozoacean association. The “spinose-podocopid association” is equivalent to the Thüringian ecotype. Then during the studies on the Devonian-Carboniferous ostracods in southwestern China, Wei (1993) furtherly subdivided into eight ecotypes which are leperditiid ecotype, leperditiid-Eifelian ecotype, Eifelian ecotype I, Eifelian ecotype II, Eifelian-Bairdioidean ecotype, Bairdioidean-Thüringian ecotype, Thüringian-Entomozoacean ecotype and Entomozoacean ecotype. The distribution and composition of each ecotype of the Late Paleozoic ostracods in South
China can be synthetized as below (Tab. 4-2-A) (Wang, 1988b; Wei, 1993). However, Casier (1987,
2004) and Casier & Préat (2003) demonstrated that the word "ecotype" describes the plants within a single species in a particular habitat and thus proposed a new system i.e. Eifelian Mega-Assemblage (=Assemblages 0-III in Casier & Préat, 2003), Thüringian Mega-Assemblage (=Assemblage IV ibid) and Myodocopid Mega-Assemblage (=Assemblage V ibid).
Among the mentioned ostracod ecotypes, the division criteria are the the composition, diversity, abundance, shell morphology, life style and their favorable ecotope, which is mainly associated with the paleobathymetry and the water-energy. The three key concepts are Eifelian ecotype, Thüringian ecotype and Entomozoacean ecotype. The Eifelian ecotype is dominated by the diverse, thick-shelled and sculptured Palaeocopida, Platycopida and Metacopida in a benthic life and indicates the shallow water and high water-energy environment. The Thüringian ecotype is composed of the thin, smooth or delicately spinosed species with relatively low diversity. During the Late Devonian–Early Carboniferous, such cosmopolitan forms in the Paleotethys are associated with the bathyal facies, in low energy cold water, and probably with low oxygen content and high amount of dissolved silica (Lethiers & Crasquin, 1987; Crasquin-Soleau et al., 1989; Lethiers & Feist, 1991). But some authors considered this ecotype was more indicative of low-energy environments than the bathymetry since this ecotype was also found from shallower environments (Bless, 1987; Becker & Bless, 1990; Olempska, 1997;
Becker, 2000). In South China, this ecotype was discovered in the Devonian of lower part of slope to shallow basin (trough) environments (Wang, 1988b and personal communication). The Entomozoacean ecotype is formed by the pelagic finger-print entomozoids. The bathymetric significance for this ecotype is ambiguous. Some researchers considered it as deep basin indicator (e.g. Bandel & Becker, 1975), whereas the others reported this ecotype from the shallow environments (e.g. Lethiers, 1982).
For adopting the term “paleopsychrospheric”, there are three important reasons.
(1) This term is chosen to point out the analogies in morphology (i.e. archaic, thin-shelled, smooth or delicated ornamented and with one or more (generally 4) developed spines) and assemblage (high diversity, few individuals per species) with the the modern psychrospheric ostracods occurring from the 104 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 Eocene to Present. In another word, instead of the “locality designation” (both the Thüringian ecotype and the Eifelian ecotype were named by their first reported locality in Germany), this nomenclature is considered as genetic designation (Kozur, 1991b).
(2) The presence of paleopsychrosphere has been proved by the paleoceangeographic studies.
Raymond & Lethiers (1990) proposed that the installation of the Thüringian ecotype in the Late Famennian Paleotethys was correlated to an invasion of cold deep-water currents that originated in the glacial north-Gondwanian area. Kozur (1991b) demonstrated that the Thüringian ecotype began to settle down during the glaciation in the latest Ordovician to Silurian and disappeared during the Jurassic/Cretaceous, since when the world ocean became to the whole thermospheric. Until the Eocene, the modern two-layered striated ocean was reestablished∗. Thus the appearance of the Thüringian ecotype and the existent psychrosphere pre-Jurassic seems closely correlated. Then the analogies between the Thüringian ecotype and the modern psychrospheric ostracods could be easily understand due to the similar habitat.
(3) The “Eifelian” and “Thüringian” have been more early designated as the stage name respectively in the Devonian and Permian (Tab.4-2-B (a), (b)). Then the name of the “Eifelian” and “Thüringian” could be misunderstanded as the assemblage belonging to the correspondent geological interval and lost their paleoecological significance. And the term “paleopsychrospheric” can avoid the unnecessary confusion with the stratigraphic unit.
Tab.4-2-B (a) International Stratigraphic Chart; (b) European Stratigraphic Chart by Bless et al., 1987
∗ The modern ocean is thermally stratified into two-layers, thermosphere (surface layer) and psychrosphere (deep ocean), separated by the thermocline (metalimnion). In the thermosphere, with the water lighter and warmer (more than 10°C), the physicochemical factors are influenced by the external conditions. The depth of the thermocline is not fixed which is variable along with the latitude and the season. In the psychrosphere below the thermocline, the temperature decreases rapidly to lower than 8°C and the water is denser. This cold layer is very stable (e.g. the temperature around 4-8°C, without variation in salinity, etc.) and connected with a global ocean supplied by cold water from ice caps (Benson & Sylvester-Bradley, 1971; Benson, 1972, 1975).
105 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) Becker, 1975; Gründel & Kozur, 1975; Bless, 1987; Lethiers & Crasquin-Soleau, 1987; Wang, 1988b;
Becker & Bless, 1990; Becker, 1991; Kozur, 1991b; Lethiers & Feist, 1991; Lethiers & Raymond, 1991;
Becker & Wang, 1992; Becker & Blumenstengel, 1995a, b; Casier, 2004). The paleopsychrospheric taxa reported in this work and the above mentioned references are synthesized as follows (Tab. 4-2-C).
4.2.2 Adopted model The discovered paleopsychrospheric species are usually accompanied with the neritic species. The triangular model proposed by Lethiers & Raymond (1991) is applied to evaluate the paleobathymetry by virtue of the relative proportion of paleopsychrospheric and neritic species (Fig.4-2-A).
Fig.4-2-A Diagram showing the triangular model proposed by Lethiers & Raymond (1991) (after Lethiers & Raymond, 1991).
According to this model, the benthic ostracod species are divided into three ecological groups, the paleopsychrospheric species (P) (indicator of deep-water environments, bathyal to abyssal), the bairdiid species (B) (inhabitants of neritic to open-marine environments, the bathymetry varies with the different morphology as mentioned above) and other neritic species (N) (preferring to shallower environments, littoral to inner shelf), which are corresponding to three points of the triangular. The relative proportion of P, B and N can give the paleobathymetric interval. A percentage of P higher than 50% is indicative of a bathyal to abyssal environment. With a percentage of P between 15% and 50%, the environments of outer platform to the upper part of slope can be presumable. If the percentage of P is lower than 15% and dominated by B and N, the fauna is considered as living in inner platform. In this instance, the higher the percentage of B is, the deeper the environment may be. In addition, since bairdiids indicate 107 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) the environments with normal salinity and oxygen-level, the environment could have relatively poor oxygen and different salinity if B% is lower.
4.2.3 Paleobathymetric evaluation on ostracod faunas The first step of the paleobathymetric evalution is to group all discovered species respectively into P, B and N. Based on the morphology and referred to the paleopsychrospheric taxa, 38 species in this work are regarded as the paleopsychrospheric species, belonging to the spinose Bairdiidae, Bythocytheridae, Tricorninidae, Berounellinidae, Rectonariidae, Pachydomellidae, Healdiidae, Quasillitidae, Polycopidae, Discoidella and the two undetermined species Podocopida indet.2 and Podocopida indet.3. There are 51 non-spinosed, thick-shelled and smooth/heavily ornamented bairdiid species. The other 39 species, of the family Kirkbyidae, Amphissitidae, Kellettinidae, non-spinosed Pachydomellidae etc., are considered as “other neritic species” (Tab.4-2-D). Then, the proportions of paleopsychrospheric, bairdiid and other neritic species are respectively calculated∗ at specific level for the bed/sub-bed with more than 3 species and 15 individuals. The statistic results are presented here in two forms, the proportion of the P, B and N and their projections in the triangular diagram.
Dongpan Section The 19 paleopsychrospheric species were yielded in 20 sub-beds, whereas in the other 6 sub-beds the fauna is completely composed of the bairdiid and/or other neritic species (and Myodocopid species).