The Bivalve Fauna From Deep-water Facies Of South China During The Permian-Triassic Interval

The Permian-Triassic mass extinction (PTME) altered the marine ecosystem from the Palaeozoic-type to the Mesozoic/Modern-type. Associating with PTME, mollusks, mainly bivalves and gastropods, took over brachopoids to be the dominating benthic invertebrates. Herein seven distinct sections of three different sedimentary facies, i.e. deep-water siliceous facies (including the Dongpan section in Guangxi Province, the Xinmin section in Guizhou Province, the Sangzhi section in Hunan Province, the Duanshan section in Guizhou Province, the Paibi section in Guangxi Province), shallow carbonate facies (the Huangzhishan section in Zhejiang Province) and neritic clastic facies (the Zhongzhai section in Guizhou). This study focused on systematic study, biostratigraphic correlation and palaeoecological community comparion, involving with 1031 bivalve specimens (77 species of 32 genera). Thirty four species of 18 genera are described herein in details. One new genus, i.e., Yinipecten gen. nov., and two new species, i.e., Palaeolima xinminensis sp. nov. and Yinipecten prosoclinus sp. nov. were established. In addition, the morphological evolution of Claraiidae, including ligament structure, byssal notch, size changes, as well as the potential paleoecological and paleoenvironmental significances, are investigated and discussed in this paper.Referring to the standard zonations of bivalves in South China, several bivalve zones were recognized:for Zhongzhai section, Changhsingnian Pernopecten huayingshanensis Zone, Griesbachian Eumorphotis venetiana Zone and Claraia wangi-C. griesbachi Zone; for Xinmin section, Changhsingnian Hunanopecten exilis Zone and latest Changhsingnian to earliest Griesbachian Claraia liuqiaoensis acme zone; for Sangzhi section, Changhsingnian Hunanopecten exilis Zone and Griesbachian Claraia wangi-C. griesbachi Zone; for Dongpan section, Changhsingian Hunanopecten exilis Zone and Griesbachian Claraia griesbachi-C. dieneri Zone; for Duanshan section, Changhsingian Hunanopecten exilis-H. qujiangensis Zone.The Changhsingian bivalve zones recognized from deep-water siliceous facies sections can be directly correlated with the typical Hunanopecten exilis zone of South China. For Zhongzhai section, the coexisting genera such as Tambanella constrained the age of Pernopecten huayingshanensis Zone to Changsingnian. The Eumorphotis venetiana Zone of the Zhongzhai section and the Eumorphotis venetiana-Towapteria scythica-Pteria ussurica variablis Zone of the Huangzhishan section correlate well with Pteria ussurica variablis-Towapteria scythica Zone, the typical Permian-Triassic transitional bivalve zone. Although the Claraia liuqiaoensis acme Zone of the Xinming section can correlate with the Pteria ussurica variablis-Towapteria scythica Zone in age, the zone lacks common taxa, such as Pteria, Towapteria and Eumorphotis venetiana. Meanwhile, no coeval Permian-Triassic transitional bivalve zone was recognized in other deep-water siliceous facies sections. The Griesbachian Claraia zones of the Dongpan, Sangzhi and Huangzhishan sections herein were approximately equivalent to the cosmopolitan Claraia wangi Zone.Besides the bivalve zonations and correlations, community analysis of the latest Permian bivalves within the studied sections was also investigated to reveal the paleoecological characteristics and paleoenvironmental significances:1. There are 32 species of 16 genera within the Hunanopecten exilis-Euchondria fusuiensis Community at the Dongpan section, dominated by Euchondria fusuiensis, Hunanopecten exilis, H. qujiangensis, Parallelodon changhsingensis, Yinipecten intermedia, Claraia liuqiaoensis and Palaeoneilo guizhouensis. The Euchondria fusuiensis, as an endemic species, is especially abundant (30% in abandunce), suggesting an evident characteristic of endemic fauna.2. There are 18 species of 16 genera within the Paradoxipecten jiaheensis-Euchondria jiaheensis Community at the Xinming section, dominated by Paradoxipecten jiaheensis, Euchondria jiaheensis, Pteria? sp., Nuculopsis wymmensis, Hunanopecten exilis and Claraia liuqiaoensis. With respect to the diversity indices, the community of the Xinmin section is as diverse as the Hunanopecten exilis-Euchondria fusuiensis Community of Dongpan section, but the trophic nucleus elements here lack endemic species as Dongpan.3. There are 9 species of 8 genera within the Paradoxipecten jiaheensis-Pteria? sp. Community at the Duanshan section. The community was dominated by Paradoxipecten jiaheensis, Pteria? sp., Phestia hunanensis, and shared many components with the Paradoxipecten jiaheensis-Euchondria jiaheensis Community of the Xinming section.4. There are 7 species of 6 genera within the Hunanopecten exilis Community at the Duanshan section, dominated by Hunanopecten exilis, Euchondria jiaheensis and Hunanopecten qujiangensis. Comparing to other communities, it’s marked by the highest D (D=0.4253) and lowest S (S=1.249), suggesting a bivalve community under anoxia marine condition.5. There are 15 species of 13 genera within the Pernopecten-Streblochondria Community at the Zhongzhai section, dominated by Astartella symmetrica, Streblochondria sp., Pernopecten huayingshanensis, Tambanella cf. gujoensis and Palaeoneilo guizhouensis. Although the specimens are limited, with largest Eveness values among studied sections suggests the most even composition. Futhermore, the dominating element, Pernopecten huayingshanensis, also occurred in deep-water sections whereas the typical deepwater elements, Hunanopecten, Euchondria and Paradoxipecten, are poorly reported at the Zhongzhai section.Three earliest Triassic bivalve communities were recognized, including the Claraia griesbachi-C. dieneri Community at the Dongpan section, the Claraia wangi-C. griesbachi Community at the Sangzhi section and the Claraia griesbachi-C. stachei Community at the Zhongzhai section. These three communities were characterized by the dominance of Claraia and similiar to the typical earliest Triassic community elsewhere. The difference is the absence of Claraia species with radial costae in the Claraia wangi-C. griesbachi Community of Sangzhi, whereas abundant taxa with radial costae, e.g., Claraia stachei and Eumorphotis, were found in the Zhongzhai section.Dongpan section was chosen to investigate faunal composition changes within the local habitats. The cluster analysis for each sampled bed suggested no significant turnover until the Permian-Triassic boundary. The composition of the bivalve assemblage was relatively stable throughout Changhsingian, marked by the constant abandance of Euchondria fusuiensis. However, previous studies with palaeontological and geochemical approaches indicated that there were multiple environmental deteriorations, such as remarkable sea level changes and constant productivity declines, occurring throughout Changhsingian to earliest Triassic.The study results suggested that occurrence of epifaunal bivalves was quite stable in each bed within the Changhsingian bivalve community at Dongpan. Whereas, infaunas and semi-infaunas occurred only in Beds 2,3 and 8 for their high sensitity to environmental changes, such as food supply reduction and frequent anoxia scenarios. That is to say, infaunal bivalves migrated elsewhere and got absent in most beds above Bed 3 when poor environments occurred. Interestingly, bivalves and brachoipods were extremely abundant in the Bed 8 though the productivity stayed in a low level. Charcoal fragments peaks in the Bed 8, indicating more terrestrial trophic inputs by enhanced terrestrial weathering. However, these terrestial inputs made the seawater turbid, which negatively affected the filtering rate of suspension feeding brachiopods and contributed to the miniaturization of brachoipods. In contrast, the infaunal bivalve would not be affected by the turbid seawater for the deposit feeding life habit.Previous study on δ13Corg based stratigraphic correlation correlated the Bed 9 of the Dongpan section with the Bed 25 of the GSSP Meishan section (the first phase of mass extinction). However, the bivalve and brachoipod distributions of the Dongpan section suggested no significant taxa loss occurring in Bed 9. The Changhsingian Hunanopecten exilis-Euchondria fusuiensis Community was eventually replaced by the earliest Triassic Claraia griesbachi-C. dieneri Community owing to the PTME.The flourishing of Claraia during Early Triassic was frequently discussed in the past decades. Previous studies of the Early Triassic Claraia λ,-shaped duplivincular (chevrons) ligament structure indicated its late Paleozoic Dunbarella (Pterinopectinidae) origin. In addition to the complete chevrons, a series of incomplete chevrons also developed in the posterior of ligament area in Claraia. This kind of ligament structure was more complex than Pterinopectinidae which only developed with complete chevrons. Therefore, Claraia could be independently assigned to Claraiidae rather than Pterinopectinidae.The ligaments of Permian Claraia has been less figured and discussed in previous studies. Some specimens of Claraia liuqiaoensis showed four pairs of complete chervons and some wave-like incomplete chevrons in the posterior. These materials indicated that there was no difference between Permian and Early Triassic Claraia in the ligament structure. Two kinds of ligament structures, i.e., chevron duplivincular ligaments (characteristic of Claraiinae) and parallel-form ligaments (characteristic of Chuluariinae) were observed in some specimens of Claraia wangi in the Early Triassic. This was normally treated as subfamily level difference in fossil identification. Herein it is proposed that subfamily Chuluariinae probably derived from some species of Claraia with variable ligament structures, e.g., Claraia wangi. Other descendants of Claraia might also include Crittendenia-Eobuchia sequence.Morphologically, the byssal notch of Claraia could be categorized into three types, i.e., Pseudoclaraia type, Claraia type and Claraioides type, and previously identified as three different genera, respectively. It was confirmed that there existed transitional types between Pseudoclaraia type and Claraia type. Thus Pseudoclaraia should be treated as a junior synonym of Claraia. Likewise, both Claraia type and Claraioides type occurred on specimens of Claraia liuqiaoensis at the Xinming and Dongpan sections, so Claraioides was also a junior synonym of Claraia. It was very likely that Claraioides type forms were evolved from the older Claraia type forms.A systematical revision of Claraia species was conducted herein. Forty two valid species ranging from Changhsingian and earliest Triassic were included in this study. According to our statistics, there were only 13 species of Claraia in the late Permian and the number remarkably increased and reached its acme (32 species) during the Griesbachian. Then the number constantly decreased from Dienerian (16 species) to Smithian (8 species), and Claraia eventually went extinct near the Smithian-Spathian boundary.During the Permian-Triassic interval, there was no evident selectivity on the development of radial ornamentation but the body size of post-extinction individuals was smaller than pre-extinction ones. Higher mobility, associating with the dominance by Claraia with narrow byssal notch, required more food supply, while decreased body size in return enhanced its mobility for its better adaptation to fluctuated environment of the Early Triassic. In one word, high variability of the morphology, e.g., ligaments and body size, occurring within interspecific and intraspecific Claraia species was attributed to environmental perturbations during the earliest Triassic.