«To cite this version: Aihua Yuan. Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China. Pa- leontology. Universit´ Pierre et Marie ...»
Note that both the major “extinction” horizon in the Dongpan and Meishan Sections are underlied the volcanic claystone layer. Thus it seems the volcanic activity played an important role in the “extinction”, but not all volcanic activity could induce the “extinction”. In the studied interval of the Dongpan Section, 21 claystone layers with volcanic origins were yielded. But the claystone layers were thicker and more frequently yielded in the two “extinction” horizons, which may indicate the strong and frequent volcanic activities. Thus the volcanic activity was not always the definitive factor in the “extinction”, only the strong and/or frequently multi-phased ones took an important place. In addition, in the Dongpan Section, the First apparent horizon is accordant to the regression event, whereas the major “extinction” in the Meishan Section took place in the shelf-margin sequence tract above the second-order sequence boundary (regression event) (Zhang et al., 1996; He et al., 2007). This implies that the “extinction” in different environments was resulted from variable origins, which further reveals the complexity of the extinction.
Comparison between Chaohu, Meishan & Dongpan faunas The comparison with the Chaohu Section shows another interesting phenomenon. In the Chaohu Section, ostracods were mainly yielded below the bed 5 (accordant to the bed labelled in figure and different from the sample number, see Fig.3-1-D & Fig.5-2-C). Above the bed 5, only several ostracod fragments were discovered in the bed 8. In the samples collected from bed 9 to bed 13, neither one fragment was found. Based on the present data, it is insufficient to delimit the ostracod “extinction” horizon in the Chaohu Section, but the bed 5 can be regarded as an “event horizon” for ostracod fauna 135 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) Fig.5-2-C Comparison of the extinction horizon between the Dongpan, Meishan and Chaohu Sections.
due to the rapid decrease both in diversity and abundance. According to the stratigraphical correlation, the bottom of bed 5 is corresponding to the bottom of bed 25 in the Meishan Section. Thus the ostracod “event horizon” in the Chaohu Section was comparable to the major extinction horizon in the Meishan Sections but delayed than the First apparent extinction horizon in the Dongpan Section (Fig.5-2-C). As mentioned above, the Chaohu fauna is dominated by bairdiids. The absence of paleopsychrospheric species implies a shallower environment than the Dongpan Section. But the bairdiids had very thin and elongated shells which may still indicate a relatively deep environment. The radiolarians yielded in the Chaohu Section also suggested that the Chaohu Section was deposited in the paleobathymetry between the Dongpan and Meishan Sections. The comparison between the Dongpan and Meishan Sections implies that the extinction took place earlier in the deep water environments. Then here the delay of the “event” in the Chaohu Section strengthens this proposition. In addition, the synchronous occurrence between the Chaohu and Meishan Sections seems indicating another character of the “extinction”.
Maybe this synchronousness implies that the Chaohu Section was deposited in a very similar environment to the Meishan Section. However, the paleogeographical locality and geological settings can not be ignored. The Chaohu and Meishan Section were situated in the Lower Yangtze Region 136 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 during the Late Permian. Although the former was deposited in basin environment and the latter in upper part of slope, the similarity in ostracod “extinction” suggests that the paleogeographical locality played an important role in influencing the extinction process. This further displays the complexity of the mass extinction induced by various factors.
Discussion In general, the discussion and comparison above provide some informations on the “extinction” of deep water ostracod faunas during the latest Permian.
(1) The deep water faunas included typical Paleozoic and some Mesozoic forms and thus were presented as mixed faunas.
(2) The comparison of 19 common species with other localities indicates that no miniaturization occurred in the deep water ostracod faunas during the “extinction”. Some specimens found in this work were larger than their ancestors. The smaller individuals are here considered as the instars according to the reason mentioned above. Thus the coexistence of instars and adults strengenthen the in situ preservation of the studied faunas.
(3) Two “extinction” horizons were proposed for the Dongpan & Liuqiao joint Section. The First apparent extinction horizon is at the top of 03DP5 to the bottom of 03DP6. In this horizon, the first crisis of radiolarians, the largest positive excursion of TOC, regression, volcanic activities and possible dysoxia/anoxia occurred at one time, which greatly highlighted this “event horizon”. The Second apparent extinction horizon is located at the bed 03DP10. Above this horizon, all ostracod taxa disappeared.
(4) The two apparent extinction horizons were well reflected both in the generic and specific distributions. However, the specific distribution displayed more detailed information upon the influence of some paleoenvironmental changes (anoxia, influx of turbid current, etc.). Although in this work the specific distribution is well accordant to the generic one after eliminating the influence by the local events, it also reveals that it is indispensable to point out in which level when the “extinction” is mentioned.
(5) In the studied faunas, except few peak values, no evident decreasing trend was observed in diversity and abundance during the “extinction”. The abundance curve even showed a small positive excursion above the First apparent extinction horizon. Thus it seems that the ostracod faunas were not affected in abundance as much as in the diversity, although both decreased to zero above the Second apparent extinction horizon. Anyhow, it is strict to clarify the response of faunas in diversity or abundance. Take this work for example, the two apparent extinction events proposed for the Dongpan Section, were reflected in diversity other than in abundance.
(6) The comparison of “extinction” horizon between the Dongpan and Meishan Sections indicate the diachronism of the “extinction” in different environments. The “extinction” might have begun foremost in the deep water area and then spread into the shallow water area. This indicates the concrete extinction process greatly varies for faunas from different paleoenvironments. In addition, 137 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan)
In this work, four sections, the Dongpan Section, the Liuqiao Section, the Shaiwa Section and the Chaohu Section, were selected for thorough studies on deep water ostracod taxonomy, paleoecology and “extinction” process. This is the first systematic study on ostracods from the latest Permian deep water strata worldwide. The acquired cognition during this work will be summarized as follows.
1. General visage of the latest Permian deep-water faunas In general, the faunas are abundant and diverse although there are many barren horizons (especially in the Shaiwa Section). There are some heavily shelled and strongly ornamented representatives, but the ostracod faunas are dominated by small and thin-shelled individuals. Many specimens are in very poor preservation and did not provide information for identifying.
In total, 43 genera and 128 species have been identified. 2 new species Bairdia dongpanensis and Spinomicrocheilinella anterocompressa were described and one new genera Denticupachydomella n.gen. and 2 new species Pseudobythocypris guiqianensis n.sp. and Denticupachydomella spinosa n.sp.
are proposed. Majority of the ostracod species are endemic although 19 common species were reported from the Late Devonian-Late Permian strata of Europe, North America and Southeast Asia. The ostracod faunas, including Palaeocopida, Podocopida & Myodocopida, are dominated by typical Paleozoic species accompanying with several Mesozoic forms (e.g. Abrobairdia, Lobobairdia) and thus appeared as mixed faunas.
2. Paleobathymetric evaluation The studied faunas mix neritic and paleopsychrospheric species. The discussion on the term “paleopsychrospheric” is reviewed. This proposition of Kozur (1991b) to replace “Thüringian” by “paleopsychrospheric” is adopted here. The ostracods with the following characters, archaic, smooth/delicately ornamented, thin-shelled and/or having one to four spines, can be considered as the paleopsychrospheric ostracods. The reasons of adopting this term are also demonstrated in the text.
According to the definition, in the studied faunas, 38 species are regarded to be paleopsychrospheric.
They belong to the spinose Bairdiidae, Bythocytheridae, Tricorninidae, Berounellinidae, Rectonariidae, 140 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 Pachydomellidae, Healdiidae, Quasillitidae, Polycopidae∗, Discoidella∗ and the two undetermined podocopid species.
The paleobathymetry is generally interpreted according to the families/superfamilies with the paleoecological significance. Then the triangular model, proposed by Lethiers & Raymond (1991), is adopted for tracing the paleobathymetric variation along each studied section and between studied sections. The beds/sub-beds yielding more than 3 species and 15 individuals are considered as the valid statistic horizon. The statistics by 14 sub-beds in the Dongpan Section indicated the frequent variations of the paleobathymetry. The few/barren beds valid for statistics in the Shaiwa, Liuqiao and Chaohu Sections were insufficient to trace the paleobathymetric variations along the section. But the comparison between sections displays, among the studied faunas, the Dongpan fauna was yielded in the deepest habitats from the outer shelf to bathyal stage, the Shaiwa fauna took the second place and indicated a inner shelf to upper slope environment, then the Chaohu fauna dominated by thin-shelled and elongated bairdiids may represente the open-marine environments, and the last, the Liuqiao fauna indicated the shallowest normal marine environments by the presence of heavily shelled and strongly ornamented bairdiids and other typical neritic species.
The paleobathymetric analyses by different methods were compared. The interpretation based on ostracod was well supported by other evidences (radiolarians, sedimentology, mineralogy and geochemistry). The comparison implies the necessity of integration with other evidence when the triangular model is applied. Some local events may influence the original fauna and thus distort the meaning of the assemblage.
3. Oxygen-level reconstruction The oxygen level is reconstructed according to the alimentation mode of the benthic ostracods.
This is the first time to apply the FF% (percentage of the filter-feeders) model in the deep-water fauna composed of both the neritic and paleopsychrospheric species. The paleopsychrospheric species found here were considered as filter-feeders or deposit-feeders according to the shell morphology. The statistics are carried out for beds/sub-beds with more than 2 species and 10 individuals. The general oxic conditions can be inferred from the present statistics. In the Dongpan Section, 19 bed/sub-beds were analysed the proportion of filter-feeders. The bed 03DP4, yielding 62.5% filter-feeders, was the unique dysoxic horizon according to the relationship between FF% and oxygen level proposed by Lethiers & Whatley (1994). This interpretation appeared accordant with the results by trace elements and foraminifera. Thus the application of FF% model is reliable. But it should be mentioned that the oxygen level reconstruction in deep water environments seems still very difficult, because not all
evidence from different methods support the same horizons. More work is needed in oxygen level reconstructing.
4. Proposed deep water ostracod “extinction” horizons Some hotspots related to the extinction event were evolved in this work. As the contemporary shallow water ostracod faunas, the “mixed” phenomenon was also reflected by the latest Permian deep water ostracod faunas, which were dominated by typical Paleozoic species accompanying with Mesozoic forms. But the deep water faunas differed from their shallow contemporary by including the long-ranging paleopsychrospheric species, reported from the Late Devonian-Early Carboniferous strata.
The difference in composition may result in various “extinction” process. The change of diversity and abundance in studied sections did not show continuous decline upwards along the studied interval. But below the Permian-Triassic boundary, the diversity and abundance showed sudden decline because no ostracod was found from the topmost of Upper Permian and lowermost Triassic. The discussion on miniaturization was also carried out. In the studied faunas, the change in individual size was not observed along the section. The comparison of the 19 common species between the studied faunas and previous occurrences also did not display the miniaturization. Some individuals found in this study were larger than their ancestors. Thus no general miniaturization occurred in the latest Permian deep water ostracods. The smaller individuals than the ancient ones are considered as the instars (detailed discussion see 5.2.1).
The concrete discussion on ostracod “extinction” was involved in the Dongpan and Liuqiao joint section. According to the specific and generic distribution, two apparent extinction horizons were proposed for the Dongpan and Liuqiao faunas. The First/Major apparent extinction horizon was located at the top of 03DP5 to the bottom of 03DP6. Above this horizon only 6 species survived. The Second apparent extinction horizon was in 03DP10, above which all ostracod taxa disappeared in the Dongpan Section. The First apparent extinction horizon is corresponding to the first crisis of radiolarians, the regression, the strong volcanic activities and possible anoxia/dysoxia. And the TOC curve showed the largest excursion in this horizon. All studies highlighted this “event horizon”.