«To cite this version: Aihua Yuan. Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China. Pa- leontology. Universit´ Pierre et Marie ...»
In the Shaiwa Section, there is a rather low percentage of filter-feeders (16.2%). Four of the eight sub-beds are chosen here for statistics (Tab.4-3-C). The sub-bed SW13-12, yielding 28 species in total, has a percentage of 14.3% filter-feeders. The other two sub-beds have respectively 25% and 33% of 121 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) filter-feeders. All of them indicate an oxygen level of more than 5ml/l which indicates a normal marine environment.
For the Chaohu Section, 12 of all the 14 species belong to the Bairdoidea and Bairdiocypridoidea and thus are deposit-feeders. The other 2 species are polycopids and not considered here. Then this means that the benthic Chaohu fauna are completely deposit-feeding and represents an oxic environment.
In general, from the ostracod fauna, three points can be summarized from the oxygen level
reconstruction as follows:
(1) A general oxic condition was indicated with unique exception in the bed 03DP4 of the Dongpan Section, yielding 62.5% of filter-feeders. The dysoxic event might have occurred in this bed.
(2) The oxygen concentration is not stable in studied area. Although there is possibly not evident dysoxic or anoxic event have fallen, the relatively low oxygen level existed discontinuously.
(3) It is notable that in the relatively “dysoxic” beds here, the Myodocopida rarely appeared.
Especially in the Chaohu Section, which did not yield any filter-feeder, two polycopids were discovered.
Thus the here discovered Myodocopid species may not be the inhabitants in oxygen deficient conditions.
Compared with the other results mentioned in 4.3.1, the interpretation on ostracods seems more in accordance with results of trace elements and foraminifers (Fig.4-3-D). According to these analyses, the bed 03DP4 has the lowest oxygen concentration and return to normal upwards in the Dongpan Section, however, the bed 03DP6 seems more possibly to suffer from the anoxic event in the view of Ce abnormity or trace fossils. In the Dongpan Section, the mineralogical analyses show abundant tuff yielding in the beds 03DP4 and 03DP6. The claystones just overlying the tuff composed of the zircon, apatite, hexagonal dipyramide quartze and other materials origining from the acidic lava. Thus the occurrence of volcanic activities could be inferred for the two horizons, 03DP4 and 03DP6. In addition, the above discussed paleobathymetric evaluation showed that the regression occurred in the bed 03DP6 122 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 and the influx of terrestrial materials consequently increase upwards. That is to say the two special horizons were conducted by different events. The bed 03DP4 was mainly influenced by the volcanic activities and 03DP6 by both the regression and volcanic activities. In this view, the different results of oxygen deficiency could be understood as the variable response of the different groups of fossils, although the reason needs more detailed investigations. Anyhow, the first attempt of FF% model in oxygen level reconstruction seems reliable.
Fig.4-3-D Comparison of oxygen-level reconstruction interpreted by ostracods and other analyses (grey zones indicate the interval of oxygen deficiency; the data of oxygen level in the column of ostracod according to Lethiers & Whatley, 1994).
As the other marine organisms, ostracods suffered great change during the end-Permian events (Wang, 1978; Hao, 1994, 1996; Wang & Wang, 1997; Crasquin-Soleau et al., 2004, 2007; Yi, 2004).
The change of the ostracod fauna in the assemblage, diversity, abundance and measurements of individuals brought some researchers the idea of “sudden extinction”. Jin et al. (2000) applied the statistical method on the ostracod fauna of the Changxing Formation, Meishan Section (GSSP), Zhejiang. They found that the sudden extinction of ostracod fauna occurred at the base of the bed 25 with an extinction rate of 83.3% in specific level (Fig.2 (C) in Jin et al., 2000). But the original data on ostracod taxa were not mentioned and thus it is far from clearly understanding the ostracod extinction process. In addition, the Changhsingian ostracods in the Meishan Section have been systematically studied and abundant data were available, whereas the data on Permian-Triassic boundary and lowermost Triassic ostracods were very poorly known. Thus the “sudden extinction” is really questionable and needs validating at least. In fact, the only achieved data of ostracod faunas, spaning the Permian-Triassic boundary in South China, were only those reported from western Guizhou, northeastern Yunan, south Guizhou, Jiangxi and Fujian (Wang, 1978; Yi, 1992, 2004; Hao, 1994, 1996;
Wang & Wang, 1997). And only the base of the early Triassic was sampled. During this boundary interval, the other data were only focused on either the Late Permian (Chen & Shi, 1982; Shi & Chen, 1987, 2002; Becker & Wang, 1992; Hao, 1992a, 1993; Yuan et al., 2007) or the Triassic (Zheng, 1976, 1988; Wei, 1981; Guan, 1985; Hao, 1992b; Crasquin-Soleau & Kershaw, 2005; Crasquin-Soleau et al., 2006). Thus, the relatively poor knowledge on ostracods during the Permian-Triassic event in South China makes it difficult to confirm the extinction process in this group.
Crasquin-Soleau et al. (2007) proposed the extinction and recovery patterns of ostracods through the Permian-Triassic boundary. According to their opinion, the extinction in ostracod faunas has begun ∗ There is not unified opinion on general extinction in ostracod faunas at present due to the lack of data and quantitive analyses. Most published literatures were focused on local or regional change during the Permian-Triassic boundary interval. To some extent, these changes are not strictly equal to the real extinction before the comparison of worldwide data is carried out. So the quotation mark is used here.
124 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 since the early Changhsingian (the end of extinction stage is undetermined for the time being) before the appearance of the Mesozoic forms. During the later period of the Changhsingian, ostracods were represented by the mixed fauna, which was defined as the survival stage, and then experienced the maximum of poverty in the Early Griesbachian (early Induan). From the Spathian (late Olenekian) to Anisian, the ostracods entered into the recovery stage and finally into the radiation stage. The last Paleozoic survivors disappeared in the Spathian. The patterns represent the most systematic cognition in ostracod extinction and recovery during the Permian-Triassic interval up to date. However, the precondition of these patterns is based on and thus applied in the neritic ostracod faunas, as mentioned by the authors “In this paper, we take into account only the neritic forms.” (p.14 in Crasquin-Soleau et al., 2007). The reasons, that the deep water benthic ostracods were excluded, are the relative stability of the deep water habitats and lack of data in this boundary interval.
To sum up the previsous studies, the data on ostracod faunas during the Permian-Triassic boundary interval are insufficient to make the general conclusion on extinction/recovery. Nevertheless, some common understandings on ostracods during the Permian-Triassic boundary have been acquired by previous studies (Wang, 1978; Yi, 1992, 2004; Hao, 1994, 1996; Wang & Wang. 1997;
Crasquin-Soleau et al., 2004, 2007).
(1) The faunas in the Permian-Triassic boundary interval were mixed with Paleozoic and Mesozoic forms. The Paleozoic forms did not totally disappear at the Paleozoic-Mesozoic boundary and survived after the mass extinction. The last Paleozoic survivors were found in the early Anisian.
The typical Mesozoic forms, as Petasobairdia, Lobobairdia, Mirabairdia, appeared during the Wuchiapingian and completely replaced the Paleozoic relicts in the Anisian (Crasquin-Soleau et al., 2007). The Changhsingian faunas discovered in Jiangsu and Hubei (Chen & Shi, 1982), Zhejiang (Shi & Chen, 1987) and Guangxi (Shi & Chen, 2002) also display the phenomenon of “mixed fauna”.
(2) In the view of the fauna in each locality, the “extinction” is evident both in generic and specific levels.The general visage of the lowermost Triassic ostracod fauna was poorly yielded in very low diversity and abundance. However, if consider the survivors occurring in other localities, the incomplete statistics at present shows that the extinction rate in generic level is less than 50% in the mentioned faunas. Thus, the real extinction nature and mode are still not clear, even for the relatively well studied shallow water ostracods. In addition, Shi & Chen (2002) demonstrated that the diversity and abundance have begun declined from the Wuchiapingian through the Changhsingian. In their studies, only 6 of 22 genera and 7 of 75 species continued to the Changhsingian of Heshan, Guangxi. It seems that the extinction began much early than other localities. Then the extinction duration is also pendent.
(3) The changes of faunas in the boundary interval are represented by the decline not only in diversity and abundance but also of measurements and ornamentations in some faunas. Hao (1994, 1996) found that the ostracods surviving in the Triassic have small size and simple ornamentation, whereas Wang (1978) discovered the “miniaturization” has begun in the late Changhsingian. Some 125 2008/5 PhD dissertation of University of Pierre Marie Curie & China University of Geosciences (Wuhan) Healdia species were smaller than 0.5mm. But in the Meishan Section, the ostracods were small in the lower Changxing Formation, dominated by larger ones in the middle part and composed of individuals with different sizes in the upper Changxing Formation.
In a word, the known discussion about extinction of ostracod faunas during the Permian-Triassic boundary interval in South China was only concentrated in the shallow-water environment (nearshore-platform-upper part of slope, mainly carbonate facies). Many materials have been achieved by previous studies, although are still insufficient for drawing a general conclusion on ostracod extinction mode. The further investigations on more faunas completely spaning the boundary interval and from different facies are indispensable.
§5.2 “Extinction event” in the studied ostracod faunas 5.2.1 Discussion on several hotspots related to the extinction event Presence of Mesozoic forms 43 genera and 128 species, belonging to the Palaeocopida, Podocopida and Myodocopida, were recognized in the four studied faunas. The Podocopida took up the dominant place by the average percentage of 75%. The Palaeocopida held an average percentage of 20.3% and Myodocopida only 4.7%. 16 of 43 genera have been reported from the Triassic strata in other localities by previsous studies (Tab.5-2-B). In specific level, neither one of the 128 species survived into the Triassic. Three species were compared to Early Anisian elements from Romania (Bairdia cf. szaszi, Bairdia aff. mirautaae and Polycope aff. baudi). Similar to the shallow water faunas, the Mesozoic forms of Bairdiidae (heavily shelled and strongly ornamented) are also found here, such as Mirabairdia comisa Chen, 1982 also found in the Late Permian of Jiangsu and Guangxi (Heshan, Yishan) and Petasobairdia bicornuta Chen, 1982 widely reported from the Changhsingian strata of South China, including the Meishan Section, and regarded as a Changhsingian species (Chen & Shi, 1982; Shi & Chen, 1987; Hao, 1992a, 1994, 1996; Yi, 2004). Except the Petasobairdia spp., other Mesozoic forms had a single occurrence or a very short stratigraphical distribution in the studied faunas. In addition, some of the common species have long stratigraphic repartition from the Late Devonian- Carboniferous (Mississippian) to the Late Permian (Tab.3-2-C). In brief, the discovered faunas are still dominated by typical Paleozoic species but accompanied with some Mesozoic forms. Thus the same as the shallow water faunas, the character of “mix” is also present in the deep water faunas.
However, the Paleozoic elements in the mixed faunas here are different from those in shallow water faunas. In the discovered shallow water faunas, the Paleozoic forms were dominated by the nearshore-neritic Palaeocopids (e.g. Kirkbya, Amphissites). But in the deep water faunas here, the 126 Yuan Aihua: Latest Permian Deep-Water Ostracod (Crustacea) Fauna from South China 2008/5 Paleozoic elements are composed of the same paleopsychrospheric species as those in the Late Devonian deep-water strata of Poland, France, Germany and Massif Rhenish and the Wuchiapingian deep-water strata of of Italy and Indonesia (Gründel, 1961, 1962; Gründel & Kozur, 1975; Olempska, 1981, 1997; Bless, 1987; Becker, 1990; Lethiers & Feist, 1991; Kozur, 1991a, b; Blumenstengel, 1993, 1994; Crasquin-Soleau et al., 2008). As described in chapter 4, the paleopsychrospheric species discovered from the latest Ordovician to the Middle Triassic and had a so slow evolution that there was not evident change between the Devonian and latest Permian species. The different compositions of the deep water and shallow water assemblages implies that the ostracod faunas yielded from different environments would vary in response during the crisis and thus displayed different “extinction” modes.