«To cite this version: Aihua Yuan. Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China. Pa- leontology. Universit´ Pierre et Marie ...»
Diversity and abundance in general In the studied faunas, the diversity and abundance per bed/sub-bed were generally low (Fig.3-1-A, B, C, D). There were some diverse and abundant∗ horizons, such as 03DP3-14 and 03DP5-17 of the Dongpan Section, LQ16 of the Liuqiao Section and SW13-12 of the Shaiwa Section. In these beds, both the diversity and abundance reached a prominent peak value and declined rapidly to the low value upwards. From the diversity curve, except the “peak” horizons, no evident decreasing trend was observed along the whole section. However, the disappearance of ostracods from below the PTB to the very early Triassic did demonstrate the decline of ostracods during the boundary interval. In the Dongpan Section, the samples from the bed 03DP10 to 03DP14 were processed but no ostracod was found. Only several sponge spicules and pyritized small bivalves and gastropods were yielded in the basal of Triassic. In the Chaohu Section, the strata near the Permian-Triassic boundary had very high content of mudstones. Nothing was found in the latest Permian strata above the bed CH3-2 (only one fragment) and the basal of Triassic strata. This may indicate the catastrophic environment, began since the Late Permian, still continued in the early Early Triassic of the Dongpan and Chaohu Sections.
As mentioned above, the discovered ostracod faunas are dominated by small, thin-shelled individuals with length between 0.2mm-0.5mm, except some Kirkbyoidea and ornamented Bairdioidea larger than 0.6mm (1.1mm). The change in individual size was not observed along the sections studied here. As mentioned above, there are 19 common species between the studied sections and other occurrences. Some of the common species were reported from the Late Devonian-Carboniferous strata.
Was there the “miniaturization” in these long-ranging species? For finding out the answer, the measurements of the 19 common species in this study were compared with the previous data (Tab.5-2-A).
From the data listed in Tab.5-2-A, the long-range species have smaller size than the specimens in the Late Devonian-Early Carboniferous (? Libumella athabascensis Green, 1963, Microcheilinella cf.
elatus (Lethiers, 1978) sensu Lethiers, 1981, Microcheilinella aculeata Buschmina, 1975, Healdiopsis
thuringensis thuringensis (Gründel, 1961), Waldeckella ? sphaerula (Gründel, 1961) sensu Becker, 1999 ). But the Late Carboniferous individuals of Pseudobythocypris concava (Cooper, 1941) and Discoidella xingyangensis Zhang, 1987 were not larger than the latest Permian ones found here. The Pseudobythocypris concava (Cooper, 1941) specimens even have larger size than their ancient congeners. Thus along the geological time, no unified change can be concluded from the present data.
Most of the common species ranging in the Permian have a comparable size with individuals from other localities of shallow and deep environments. The specimen of Roundyella ? oblonga Wang, 1978 is larger than the Guizhou ones. The specimens of Cyathus caperata (Guan, 1978) sensu Shi & Chen 1987, ? Bairdia anbeedi Belousova, 1965, Bairdia sp.1 sensu Gründel & Kozur, 1975, Bairdia altiarcus Chen 1958, Mirabairdia comisa Chen, 1982 and Fabalicypris parva Wang, 1978 have a common size-variation interval or comparable measurements with the previous reported specimens.
The exception is the specimens of Bairdia ? sp. 6 sensu Bless, 1987 which have evidently smaller size than the Early and Middle Permian individuals. But it is noticeable that the Middle Permian specimens from Sicily (Crasquin-Soleau et al., 2008) had a median size between the Early Permian individuals of Indonesia (Bless, 1987) and the specimens here. All the three localities were deep water environments which are characterized by the strong stability. Thus I consider the variation in size as the differences between instars and adults rather than the miniaturization. In the species Bairdia altiarcus Chen 1958, the similar phenomenon can be observed: the Late Permian specimens from the shallow water environment of South China (Chen, 1958) were larger than the Middle Permian specimens discovered in deep water strata of Sicily (Crasquin-Soleau et al., 2008). So it is more reasonable to regard the small individuls as the instars. The presence of different sizes in one species reflects the ontogenic lineage among different occurrences.
Thus, even though the specimens here are small in general, compared with the contemporary specimens from different environments or to the very ancient specimens, there is not general miniaturization occurring. The no-miniaturization was also observed in the reported neritic ostracod faunas in Turkey (Crasquin-Soleau et al., 2002). In addition, the coexistence of instars and adults supports the preservation in situ of the studied faunas.
5.2.2 Concrete discussion on Dongpan and Liuqiao faunas
∗ Forel M-B. Les ostracodes margueurs des modifications paléoenvironmentales lors des événements de la limite Permien-Trias en Chine du Sud: [mémoire de maîtrise]. Université Bordeaux 1: Bordeaux. 2007. (in French with english abstract).
130 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 Dongpan and Liuqiao faunas - generic level The stratigraphical distribution of all genera (37 genera) in the joint section is showed as Fig.5-2-A.
From this figure, three horizons are prominently special, i.e. the bed LQ16, the top to 03DP5 to bottom of 03DP6 and the bed 03DP10.
In the bed LQ16, the Mesozoic forms of bairdiids Lobobairdia, Mirabairdia, Petasobairdia, Abrobairdia and other genera surviving into the Triassic (Fabalicypris, Healdia) appeared at one time.
This bed is the only occurrence of the genus Lobobairdia in studied sections. Abrobairdia and Mirabairdia disappeared until the bed LQ22. In addition, it is noticeable that the bed LQ16 had highest diversity and abundance in the Liuqiao Section. The abundant occurrence of bairdiids implies the normal environment in this horizon. Thus it can be inferred that this bed is cofessedly below the major episode horizon of the mass extinction (suppose the mass extinction existed).
The second special horizon, the top of 03DP5 to bottom of 03DP6, experienced a rapid decline in diversity. A quarter of the total genera, including three palaeocopid genera (Kirkbya, Libumella, Permoyoungiella), disappeared above this horizon in the Dongpan Section. It seems well accordant to the data in other ways. The first crisis of radiolarians occurred in the bottom of 03DP6 (Feng et al., 2007b). In addition, the paleoenvironmental analysis shows the presence of the regression and possible anoxia (dysoxia) in this bed. The studies on sedimentology, mineralogy and geochemistry indicated the occurrence of strong volcanic activities. And the TOC also suggested the mass burial of organisms by its largest positive excursion (Zhang, 2006; Zhang et al., 2007a, b). All studies highlighted this “event horizon”. However, for ostracods, it is too early to draw a conclusion on extinction because the genera disppeared in the Dongpan Section have been reported from the Triassic strata. Paraberounella has been found in the Early Anisian of Romania (Crasquin-Soleau & Gradinaru, 1996). Monoceratina and Bairdia was widely discovered from the Triassic strata and even still exists at present (references see Tab.5-2-B). Bairdiacypris was also very common Triassic species. This means that these genera, disappeared in the studied sections, still survived into the post-Permian in fact. Consequently, the extinction in the top of 03DP5 to the bottom of 03DP6 belongs to the apparent extinction rather than the real extinction for ostracods. And the apparent extinction was well accordant to the paleoenvironmental change.
In the bed 03DP10, it appears the last occurrence of all genera surviving after the apparent extinction horizon in the Dongpan fauna. Macronotella, the only palaeocopids surviving the “extinction” but has not been found in the Triassic, could be the relict. Fabalicypris, Cavellina, Microcheilinella and Pseudobythocypris have been reported from the Triassic strata (Tab.5-2-B). Thus the disappearance of all taxa in 03DP10 of the Dongpan Section was also apparent extinction. In addition, it should be noted that the surviving genera are mostly deposit-feeders. Their sudden disappearance in 03DP10 may indicate the enhancement of anoxia/dysoxia, which could have continued until the early Early Triassic suggested by the presence of the pyritized bivalves and gastropds in the Dongpan Section.
131 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan)
100 30 200 0
7 8 3 8 9 5 4 9 2 3 6 12 4-7 24-36 15 16 19 20 22 23 37 43 47 50 53 11 41 10 12 13 14 11
Dongpan and Liuqiao faunas – specific level The stratigraphical distribution in specific level (82 species) displays both accordance and difference with generic distribution (Fig.5-2-B).
Of course, the mentioned three special horizons were also well reflected in specific change. In the bed LQ16, 21 species appeared for the first time in the Liuqiao fauna. In this horizon, the fauna was dominated by the bairdiids including both smooth and ornamented shelled species. The present data are insufficient to determine this horizon as the first occurrence for the Mesozoic forms in ths studied faunas. However, their abundant yielding undoubtedly implies the normal marine environment.
Above this horizon, the environments appeared worsening stepwisely. In the bed LQ22, only the species, which continued to the apparent extinction horizons, were yielded. Several samples from the 5m gap strata between the Dongpan and Liuqiao Sections were also picked. Only the long-ranging species in the studied faunas, such as Spinomicrocheilinella anteropressa and Bairdia dongpanensis, were found. In the top of 03DP5 to bottom of 03DP6, most of species disappeared and only six species survived this “apparent extinction horizon”. Thus the“apparent extinction” of ostracod faunas existed in both generic and specific levels. Here this event horizon is considered as the First (major) apparent extinction horizon. The survived 6 species finally disappeared in the bed 03DP10, which is regared as the Second apparent extinction horizon for ostracods.
However, compared to the generic distribution, it appears more phases of “extinction” took place in specific level than in the generic level. There are other two evident “extinction” horizons, the lower 03DP3 and 03DP4. If we correlate with the paleoenvironmental analysis of the previous chapter, these two “extinction” events seem more “apparent”. As mentioned above, in the lower part of 03DP3, there was an influx of turbid current. The sudden increase of many neritic species could be explained by transported through turbid current from the shallower environments. Fig. 5-2-B shows that all the extincted species in the lower 03DP3 were neritic inhabitants. Thus it can be inferred that the environment then was not proper for those species, which disappeared rapidly after being transported by the turbid current. So the “extinction” in lower 03DP3 was actually the reflection of local events. The extincted taxa were not the original members. Consequently, the lower 03DP3 is here excluded from the apparent extinction horizon. The “extinction” in 03DP4 is probably associated with the oxygen deficiency, which is suggested above by the analyses on ostracods, foraminifera and trace elements.
Above this horizon, still more than 20 species survived. Thus the extinction in this horizon was not general. It is presumable that the dynoxia did not destroy the whole ostracod faunas. The regression (bottom of 03DP6) finally inflicted on the ostracod faunas and caused the First apparent extinction.
5.2.3 Comparison and Discussion Interestingly, if we compared the Dongpan & Liuqiao faunas with the Meishan and Chaohu faunas, the ordinal “extinction” in the different faunas can be concluded.
134 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 Comparison with Meishan fauna If we follow Jin et al. (2000), the ostracod “extinction” in the Meishan Section occurred at the bottom of the bed 25. In the Dongpan Section, the First and Major apparent extinction took place at the bottom of 03DP6, which is corresponding to the boundary of 24d and 24e in the Meishan Section (Feng et al., 2007; Zhang et al., 2007a, b) (Fig.1-2 & Fig.5-3-B). This means the ostracod “extinction” happened earlier in the Dongpan Section than in the Meishan Section. This phenomenon was also observed in the radiolarian extinction: the first crisis in radiolarian faunas occurred at the bottom of 03DP6 (Feng et al. 2007), which is earlier than the major extinction in the Meishan Section. In addition, the extinction in radiolarian faunas had a “paleoecological rule”. The deep water indicator Albaillellaria disappeared at first, and then the shallower representive Latentifistularia, and finally Entactinaria and Spumellaria. This implies that the extinction could earliest take place in the deep environment and gradually spread to the shallow environment. In fact, the “successive phenomenon” was proposed by some researchers. Isozaki (1997) held the opinion that the anoxia had begun in the deep water area since the Middle Permain but spread to the shallow water until the late Late Permian. All of these indicate that the deep water seems affected foremost so that the inhabitants thereinto suffered from the crisis earliest. Then the catastrophe spread upwards and resulted in the extinction in the shallow water environments. The extinction observed in ostracods and radiolarians implies that the organisms with different lifestyles (benthic, pelagic) did not escape the drastic crisis.