Study On The Shallow Marine Ostracod Fossils During The Permian-Triassic Transition From Leye Area,Guangxi Province

The Permian-Triassic mass extinction has been considered the largest bio-crisis of five mass extinction events since the Phanerozoic,over 90% of marine species disappearing.The pattern of this biological extinction has been one of the key scientific questions that scholars around the world have been working on.Extinction patterns vary between different biological phylum.Ostracod provides sufficient and important study materials as one of the common metazoans before and after the mass extinction.Current research about ostracods focused on the systematic identification,stratigraphic distribution,and the reason for their prosperity during the Permian-Triassic transition.However,the pattern of extinction of ostracods during this major geological upheaval remains controversial,and there are different points about the reasons for their prosperity in microbialites.The materials in this paper are mainly collected from 2 typical shallow marine microbialites sections from Leye area,Guangxi province(Youping and Shanggang section).And detailed sampling and fossil identification were carried out here.A total of 3,717 ostracod specimens were collected from the Youping section and were identified as 35 genera and 108 species,including 108 specimens were identified as 22 genera and 45 species from Upper Permian limestone,and 3,609 specimens were identified as 33 genera and 104 species from Lower Triassic microbialites.A total of4,756 ostracod specimens were collected from the Shanggang section and were identified as 28 genera and 102 species,all of them from Lower Triassic microbialites.It was found that ostracods from the Youping section did not become significantly extinct during the first phase of the Permian-Triassic crisis,with only two genera and four species disappearing,while 33 genera and 104 species disappeared during the second phase of the Permian-Triassic crisis.All 28 genera and 102 species of ostracods from the Shanggang section also disappeared during the second phase of the Permian-Triassic mass extinction.It showed that ostracods suffered severe extinction during the second phase of the Permian-Triassic crisis,i.e.,the earliest Triassic mass extinction.Four ostracod communities have been recognized through cluster analysis from the Youping section during the Permian-Triassic interval,including community 1Bairdia-Reviya from the Late Permian,community 2Bairdia-Liuzhinia-Praezabythocypris 、 community 3 Liuzhinia-Praezabythocypris 、community 4 Bairdia-Praezabythocypris-Cavellina from Early Triassic.One ostracod community has been recognized from the Shanggang section during the Permian-Triassic interval,including the community Bairdia-Praezabythocypris-Liuzhinia from Early Triassic.Diversity indexes at the Youping section showed different changes from Late Permian to Early Triassic.The Shannon index decreased,Dominance index increased and Evenness index decreased first and then increased.The Shannon and Dominance indexes of ostracods communities at the Shanggang section are in the middle of the four communities at the Youping section,While the Evenness index is the lowest.It indicated that there are regional differences in the microbialites.Ostracod fossils were exceptionally abundant and diverse in microbialites after the Late Permian mass extinction,and various hypotheses have been proposed to explain,but these hypotheses are all controversial.Based on the statistical data of all ostracod fossils within the time period of microbialites prosperity globally,this study used the S?renson coefficient to analyze the similarity of ostracods fauna at the genus level.The results showed the high similarity of ostracods fauna between microbialites and non-microbialites in the Yangtze Platform,with similarity values greater than 0.5;the moderate to high similarities of ostracods fauna between the isolated carbonate platform within Nanpanjiang Basin and Yangtze Platform,with similarity values greater than 0.3;the moderate to low similarities of ostracods fauna between microbialites and non-microbialites in Western Tethys,with similarity values less than 0.5.It shows that there is no significant difference during the Permian-Triassic transition for ostracods in shallow marine.And does not support the "microbial-related refuge" hypothesis,but supports the "shallow marine refuge" hypothesis.In addition,the moderate to low similarities of ostracod fauna between Eastern Tethys and Western Tethys,with similarity values less than 0.5.It showed remarkable geographical differentiation at both regional and global scales during the Permian-Triassic transitional beds and was presumably controlled by geographical isolation.