Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China
Faune d'Ostracodes (Crustacea) d'eau profonde du Permien Terminal de Chine du Sud
Résumé
As the other marine organisms, ostracods suffered drastic change during the end-Permian mass extinction, the largest event among the "Big Five" in the Phanerozoic history. In South China, Late Permian strata are well and widely exposed, which provides the great availability for related studies. Previous studies on Late Permian shallow water ostracod faunas in South China have been evolved in the taxonomy, biostratigraphy and palaeoenvironment. These studies have greatly increased our knowledge on Late Permian shallow water ostracods. However, the absence of studies on contemporary deep water ostracods makes it difficult to comprehensively understanding the Late Permian ostracods during that significant geological time. This dissertation will focus on the latest Permian deep water ostracod faunas in South China and aims to enrich our understanding on Late Permian ostracods. This is the first study on the Late Permian deep water ostracods in China and on the latest Permian deep water ostracods worldwide. For the time being, the only available data on Permian deep water ostracods were from the Early Permian of Indonesia and Middle Permian-Wuchiapingian of Italy. Thus this work will fill the gap in the history of lastest Permian deep water ostracod study. In this dissertation, four sections from the deep water strata in South China (Hunan-Guizhou-Guangxi basin and Lower Yangtze basin) are studied in detail on ostracod taxonomy, paleoecology (paleobathymetry and oxygen level) and "extinction" process. The ostracod taxonomy was carried out as the preliminary work. The diverse faunas are represented by 43 genera and 128 species. Two new species Bairdia dongpanensis and Spinomicrocheilinella anterocompressa were described. One new genus Denticupachydomella n.gen. and two new species Pseudobythocypris guiqianensis n.sp. and Denticupachydomella spinosa n.sp. are proposed. In general, the ostracod faunas are dominated by small and thin-shelled individuals, although there are also some heavily shelled and strongly ornamented representatives. Many specimens are in very poor preservation and did not provide information for identifying. The recognized ostracods belong to Palaeocopida, Podocopida and Myodocopida. The typical Paleozoic species dominated the faunas accompanying with several Mesozoic forms (e.g. Abrobairdia, Lobobairdia). Compared with previous studies, 19 common species were reported from the Late Devonian-Late Permian strata of Europe, North America and Southeast Asia. Thus the studied faunas have a rather high endemic rate (85.2%). The paleobathymetry is generally interpreted according to the families/superfamilies with the paleoecological significance. The triangular model, proposed by Lethiers & Raymond (1991), is adopted for precisely tracing the paleobathymetric variation along each studied section and between studied sections. In this model, the paleobathymetry is suggested by virtue of the relative proportion of paleopsychrospheric and neritic species. The paleopsychrospheric species mean the ostracods with the following characters: archaic, smooth/delicately ornamented, thin-shelled and/or having one to four spines. 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, Pachydomellidae, Healdiidae, Quasillitidae, Polycopidae, Discoidella and the two undetermined podocopid species. The beds/sub-beds yielding relatively abundant and diverse ostracods are analysed. The analyses based on 14 sub-beds in the Dongpan Section indicated the frequent variations of the paleobathymetry. The few/barren beds 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 environments, the Shaiwa fauna was in the next place and indicated a inner shelf to upper slope environment, then the Chaohu fauna dominated by thin-shelled and elongated bairdiids may represent 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 interpretation based on ostracods was well supported by other evidences (radiolarians, sedimentology, mineralogy and geochemistry). The comparison also implied the necessity of integration with other evidence when the triangular model is applied. Some local geoevents (e.g. turbid current) may influence the original fauna and thus distort the meaning of the assemblage. For the oxygen level reconstruction, the FF% (percentage of the filter-feeders) model is attempted here for the first time in the deep-water fauna. This model is based on the alimentation mode of the benthic ostracods. According to this model, the oxygen level is associated with the percentage of the filter-feeders. In this study, the analyses are carried out for beds/sub-beds with relatively abundant and diverse individuals. The general oxic conditions can be inferred from the analyses. 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 seems 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. Before discussing the concrete "extinction" process, some hotspots related to the extinction event were evolved in this dissertation. 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 along the studied interval from the bottom up. 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 even larger than their ancestors. Thus no general miniaturization occurred in the latest Permian deep water ostracods. The presence of different sizes of intraspecies individuals reflects the ontogenic lineage among different occurrences. The smaller individuals than the ancient ones are considered as the instars. The coexistence of instars and adults supports the preservation in situ of the studied faunas. The concrete discussion on ostracod "extinction" was involved in the Dongpan and Liuqiao joint section due to the clear stratigraphical relationship between the two sections. 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 ostracod 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 positive excursion in this horizon. All studies highlighted this "event horizon". The "extinction" process was compared between the Dongpan and Meishan Sections. The delay of the "extinction" in the Meishan Section was revealed. In the Dongpan Section, the First/Major apparent extinction took place at the bottom of 03DP6, which is corresponding to the boundary of 24d and 24e in the Meishan Section. In the Meishan Section, the major ostracod "extinction" occurred at the bottom of the bed 25. The delay of "extinction" in the Meishan Section may indicate that the deep water area was earlier affected by the catastrophic events, such as the anoxia. Both the major "extinction" horizon in the Dongpan and Meishan Section was underlied the volcanic ash bed. This may indicate that the ostracod "extinction" in both sections were associated with the volcanic activities. The comparison between Dongpan, Meishan and Chaohu faunas displays the role of paleogeography in the "extinction". In the Chaohu Section, the bed CH5 is corresponding to the bed 25 of the Meishan Section (stratigraphical correlation based on the "sandwich" PTBST) and ostracods rapidly declined in diversity and abundance at the bottom of bed CH5. This synchronism between the Chaohu and Meishan indicated that the paleoenvironment was not the only definitive factor for the "extinction" process. The paleogeography also played an important role. The reason resulting in the similarities and differences by the paleogeography is still pendent. However, without doubt, the variety of "extinction" between the different paleoenvironments and paleogeographical localities, indicates the necessity of the research on deep water ostracods, comparing the shallow water contemporaries with relatively abundant data (although systematic collation and revision are necessary).
Comme tous les organismes marins, les ostracodes subissent un changement drastique au cours de l'extinction en masse de la fin du Permien. En Chine du Sud, le Permien supérieur affleure largement. Des études précédentes ont été conduites sur la taxonomie, la biostratigraphie et le paléoenvironnement des ostracodes néritiques du Permien supérieur. C'est ici la première étude sur les ostracodes profonds du Permien supérieur de Chine et la première dans le monde sur le Permien terminal. A ce jour, les seules données disponibles sur les ostracodes profonds du Permien proviennent du Permien inférieur d'Indonésie et du Permien moyen - Wuchiapingien d'Italie. Dans cette thèse, quatre coupes de séries profondes de Chine du Sud (Bassin du Hunan-Guizhou-Guangxi et Bassin du Yangste inférieur) sont étudiées en détail pour la taxonomie, le paléoenvironnement (paléobathymétrie et niveau de paléo-oxygénation) et les processus "d'extinction". La taxonomie des ostracodes est le travail de base. Une faune diversifiée est représentée par 43 genres et 128 espèces. Deux nouvelles espèces Bairdia dongpanensis et Spinomicrocheilinella anterocompressa ont été décrites. Un nouveau genre Denticupachydomella n.gen. et deux nouvelles espèces Pseudobythocypris guiqianensis n.sp. et Denticupachydomella spinosa n.sp. sont proposés. Beaucoup de spécimens ont une conservation médiocre et ne présentent pas suffisamment de caractères pour une identification précise. Les spécimens déterminés appartiennent aux Palaeocopida, Podocopida et Myodocopida. Les espèces typiquement Paléozoïque dominent la faune et sont accompagnées de quelques formes à affinité mésozoïque (i.e. Abrobairdia, Lobobairdia). 19 espèces ont été rattachées à du matériel précédemment décrit du Dévonien supérieur au Permien supérieur d'Amérique du Nord, d'Europe et d'Asie du Sud-Est. La faune étudiée a un taux d'endémisme de 85.2%. La paléobathymétrie est analysée à partir du modèle de Lethiers & Raymond (1991) qui utilise les proportions d'ostracodes psychrosphériques/Bairdiidea/autres espèces néritiques. 38 espèces sont considérées comme psychrosphériques. Elles appartiennent aux Bairdiidae épineux, Bythocytheridae, Tricorninidae, Berounellinidae, Rectonariidae, Pachydomellidae, Healdiidae, Quasillitidae, Polycopidae, Discoidella et deux espèces indetminées de podocopid. La coupe de Dongpan est la plus profonde et présente des variations de bathymétrie depuis la plate-forme externe jusqu'au milieu bathyal. Ces interprétations sont bien en adéquation avec les résultats d'autres analyses (radiolaires, sédimentologie, minéralogie et géochimie). Quelques événements locaux (courants de turbidité) peuvent perturber le signal. Pour l'évaluation de la teneur en oxygène, le modèle FF% (pourcentage d'ostracodes filtreurs) de Lethiers & Whatley (1994) est appliqué pour la première fois à des faunes profondes. Un seul horizon dans la coupe de Dongpan présente un niveau d'oxygénation proche de la dysoxie. En milieu néritique, un intervalle de transition, avec des faunes typiquement paléozoïques et des nouveaux venus mésozoïques a récemment été mis en évidence. Il semble que ce phénomène soit observable également en milieu profond. Il ne semble pas y avoir de phénomène de miniaturisation chez les ostracodes profonds à l'approche de la limite Permien-Trias. Dans la coupe groupée Liuqiao-Dongpan, deux horizons d'extinction sont mis en évidence. Le premier horizon correspond à la crise majeure chez les radiolaires, à la régression, à une activité volcanique intense et peut-être un niveau disoxique. Ces niveau d'extinction sont corrélés avec le GSSP de la limite Permien-Trias à Meishan. Il semblerait que les événements d'extinction soient plus précoces en milieu profond qu'en milieu néritique.