| The unique Permian bivalve family alatoconchids is the largest bivalve group in the Paleozoic, with aberrant shell forms and extraordinary size up to 1 m. alatoconchids have been reported from 10 areas in the world that cover both the Tethyan and Panthalassan domains, including Japan, Philippines, Malaysia, Thailand, Afghanistan, Iran, Oman, Croatia, Tunisia and Alaska, Their occurrence environments are dominated by Lower and Mid-Permian shallow-marine carbonate environments, thus, research on their spatial and temporal distribution will help us to better understand the response of the tropical shallow-marine environment around the Guadalupian-Lopingian boundary (GLB). In spite of well-studied in these mentioned regions, no investigation on this aspect has been carried out yet in South China. To fill this information gap, we reported four occurrences of the Permian bivalve family alatoconchids to date, and discusses the environments where they lived and their geological implications.In this thesis, alatoconchids have been investigated on five geological sections in different paleogeographic settings. Among of them three occurrences are located around the Yangtze carbonate platform southern margin, including Guiding and Pu’an sections in Guizhou Province (inter-platform), Luodian section in Guizhou Province (carbonate slope). The rest one, Zigui section, Hubei Province, is located at the northern margin of the Yangtze carbonate platform.Based on detailed observation and description, the characteristics of alatoconchids can be summarized into five main points:1) Aberrant shell forms. The ventral surface forms a large planer surface, whereas the dorsal side forms a crest extending perpendicular to the flat ventral surface. An overall profile of the bivalve across the main axis forms a triangle in the anterior and an extremely flat oval in the posterior half.2) Extraordinary size. The length of individual alatoconchids shell mostly ranges from 20 to 30 cm with a maximum-70 cm.3) Double-layered shell. The external layer consists of prismatic calcite aligned perpendicularly to the shell surface, whereas the internal one is composed of mosaic granular calcite. The calcite commonly reaches 2 mm in length,6 mm sometimes. Due to their three-dimensionally twisted and fragile shell most of alatoconchids observed in South China are broken fragments, accumulated as beds of storm deposits in thickness from 10 to 90 cm. It is thus hard to get complete specimen for further morphologic identification. However, taxonomic characters, including the huge dimension, unique lateral flange and prominent prismatic external layer, are obvious. According to the morphological characteristics, alatoconchids found in South China might belong to the Shikamaia Ozaki and Saicraconcha (Yancey & Boyd). In addition, coeval similar fossils, previously interpreted as phylloid algae from three localities in Guangxi and eastern Yunnan, probably belong to this unique bivalve group.Combined with previous studies on Permian Yangtze Platform, we suggest that the southern margin of the Yangtze platform belong to rimmed carbonate platform margin. Based on field observations and laboratory microfacies analysis, from the shallow platform to the deep sea, several depositional environments were developed successively, including open platform (shallow and deep platform, shore), platform margin (reef, bank), platform slope and open shelf (upper and lower shelf). alatoconchids occurred in the shallow environments, such as open platform and upper shelf. The alatoconchids fossils are usually preserved in storm deposits, thus we infer that their living environment might be shallow subtidal environment, near the storm wave base and above the fair-weather wave base. And probably they are most developed in the vicinity of the fair-weather wave base, with a gradual decrease downwards. According to their morphology, we infer that alatoconchids respect to bivalve’s ecological habit of photosymbiosis. Thus, they lived in the warm photic zone, and were epifaunal suspension-feeders.This thesis firstly reported this bivalve family (alatoconchids) in South China. Their paleogeographic distributions were limited to low-latitude warm shallow environment, and all of them were extinct before the GLB. This special geographical distribution and extinction timing indicates that alatoconchids could carry environmental information on mass extinction event occurred at the end of Maokouan Age. As they appeared in a temporal and spatial framework with well biostratigraphic and paleogeographic constraints, further study of alatoconchids will promote our understanding about the giant clam’s living environments and its evolution. |
PDF Full Text Request