POPULATION STATISTICS OF TWO LATE FRASNIAN (LATE DEVONIAN)BRACHIOPOD FAUNAS FROM CENTRAL HUNAN OF CHINA AND ITS PALAEOECOLOGICAL SIGNIFICANCE
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Abstract:
Survivorship curves and size-frequency distributions of 10fossil species populations have been depicted based on abundant small-sized brachiopod specimens of various growth stages of two samples (C1-0 of Jiangjiaqiao and S-Y-4 of Shetianqiao sections)from Hunan Province of southern China．None of the size-frequency distri- butions of the C1-0fauna are of a normal bell-shaped distribution pattern；only those of Thoma-saria？baii，Thomasaria？liangi，Xiangia liaoi，Cyrtinaella？houi and Gypidula xui somewhat resemble a bell-shaped distribution，whereas Qi-dongia tani，Athyris supervittata and Levipugnax？liui display a distinct right-skewed pattern．The survivorship curves of various populations are of the concave type of various degrees，which was probably related with their living on the muddy substrate．Survivorship curve of Thomasaria？baii is slightly concave，whereas those of other species are of either moderately concave type，e．g．，Levipugnax？liui，Xiangia liaoi，Gypidula xui，Qidongia tani and Cyrtinaella？houi，or strongly concave type，e．g．，Athyris supervittata and Thomasaria？liangi，suggesting a high mortality rate during the juvenile stage and decreasing mortality rate towards the gerontic stage．The above differences in size-frequency distributions and survivorship curves of various species are probably a result of their adaptational differences to the muddy environments．For example，diffe-rences in shell shape might be related with their adaptational differences to both muddy substrate and feeding efficiency． Study of Iowatrypa pseudobodini and Hunanotoechia tieni shows that size-frequency distributions and survivorship curves of brachiopod populations can be affected by sampling methods to various degrees in case of fewer specimens by hand-picking．Bulk sampling is recommended for population analysis，in combination with hand-picking in some cases for more actual survivorship curves．
