湘中上泥盆统弗拉斯阶两腕足动物及其地质意义

RESTUDY OF TWO RHYNCHONELLID BRACHIOPOD SPECIES FROM THE UPPER DEVONIAN(FRASNIAN),CENTRAL HUNAN,CHINA

详细描述了湘中上泥盆统弗拉斯阶腕足动物小嘴贝类的两个属种CamarotoechiashetienchiaoensisTien,1938和HypothyridinalinglingensisWang,1955的内外部特征,首次揭示了后者的内部构造。C.shetienchiaoensis曾先后归于不同的属,但因该种不具有盖板而区别于前人所归的那些属,如Ptychomaletoechia及Ripidiorhynchus等属,目前,暂归其于Hunan otoechia属,所修订的H.shetienchiaoensis包括了田氏的α、β、γ3个变种。这两种小嘴贝类分布于湘中浅海凹陷区及浅水混积岩相区弗拉斯阶的上部,均在弗拉斯—法门阶界线之下约20～40m处绝灭。华南的小嘴贝类材料表明腕足动物在F/F集群绝灭中呈一种长期的阶梯式绝灭的方式。

This paper gives full details of the characteristics of two Upper Devonian rhynchonellid brachiopod species,Camarotoechia shetienchiaoensis Tien,1938 and Hypothyridina linglingensis Wang,1955, which are all from the Frasnian of central Hunan, China. For the first time the inner structure of the latter has been investigated in detail. C.shetienchiaoensis was once regarded to belong to different genera. However, because this species has no connectivum, it is different from those genera, such as Ptychomaletoechia, Ripidiorhynchus etc. It is, therefore, regarded to belong to Hunanotoechia for the present, and the emended H.shetienchiaoensis includes the original Tien's three varieties:α,β and γ.These two rhynchonellid species are distributed in the Upper Frasnian in the depressional facies on shallow water carbonate platforms and shallow subtidal facies characterized by mixed carbonate and shaly deposits. The two brachiopod species all became extinct at a level about 20 to 40 m below the Frasnian/Famennian boundary. This indicates that there are at least two extinction episodes for the Brachiopoda during the F/F mass extinction, which implies a longterm, stepwise extinction pattern.