长江三峡地区巴东组植物群的生态环境分析

FLORAL PALAEOECOLOGICAL ENVIRONMENT OFTHE BADONG FORMATION IN THE YANGTZEGORGES AREA

对长江三峡地区中三叠世巴东组植物群作了较系统的总结,自下而上首次建立了两个植物组合,即Pleuromeia marginulata-Annalapis sangzhiensis组合和Annalepis latiloba-Scytophyl-lum组合,前者的时代为安尼期,后者为拉丁期.本文从分析巴东组的沉积特征入手,较深入地探讨了该组二段植物组合的生态环境和当时的植被景观,依据组合的重要分子肋木、脊囊等的生态结构,推测它们是适于在水上漂浮的滨海潮坪植物,提出其孢子叶上的叶舌坑和唇瓣等与植物的呼吸和吐水有关等一系列新见解.此外,还对肋木植物的时空分布、迁移和衰亡作了较详细的讨论,指出在长江流域一带安尼期肋木明显由东向西迁移,其原因很可能与当时扬子地台海退作用总体上由东向西周期性发生有关;同时认为安尼期末为全球性肋木生命危机时期,肋木从此告终,就区内资料而言,其绝灭原因明显与海平面迅速上升或下降以及巨型风暴潮事件有关.

In recent years, a flora represented by Pleuromeia and Annalepis has been found from Middle Triassic Badong Formation in the Yangtze Gorges area. The discovery of this flora will be of great significance for the evolution of the Triassic flora, the palaeobotanic geographical regionaliza-tion and correlation of the global Middle Triassic floras.The Badong Formation, consisting chiefly of a set of purple clastic deposits intercalated with carbonate rock in the middle part, is widespread in the Yangtze Gorges area, with a total thickness of about 1000m. This formation may be evidently subdivided into 5 lithologic members. The litho-logic characters and main fossil assemblages of each member are shown in Fig. 1.I . Plant assemblages of the Badong FormationAccording to their changes in time and space, the fossil plants of the Badong Formation may be subdivided into the following two assemblages:1. Pleuromeia marginulata-Annalepis sangzhiensis assemblageThe assemblage, containing 18 genera and 30 species, is represented by the fossil plants from Members 1-2 of the Badong Formation, the main elements of which are: Pleuromeia marginula-ta Meng, ? P. sanxiensis Meng, Annalepis sangzhiensis Meng. A. brevicystes Meng, Calamites shanxiensis(Wang), Eouisetites cf. gracilis ( Nathorst), Crematopteris sp., Neuropteridium voltzii Brongniart, Cladophlebis sp., Peltaspermum mulicostatum Zhang et Shen, P. miracari-natum Meng, Scytophyllum hunanense Meng, ? Thinnfeldia nordenskioldi Nathorst, Nilssonia costanervis Meng, Yuccites anastomosis Wang Z. Q. et Wang L. X., Y. vogesiacus Schimper et Mougeot, Voltzia heterophylla Brongniart, V. curtifolia Meng. Willsiostrobus of. cordiformis (Grauvogel-Stamm), Pagiophyllum sp., etc. It is obvious that the assemblage is dominated by Lycopsida and Pterodosperms, especially Pleuromeia and Annalepis of Lycopsida. The Conifer, a-mounting 3 genera and 5 species, is also developed. The Sphenopsida, Filices and Cycadophytes are quite simple, each only having individual genera and species. Judging from its compsitions, this assemblage could be correlated with those floras from Muschelkalk of Germany[4], the Lattenkohlen of France[5] and the bottom of the Ermaying Formation of North China[3], and thus may be assigned to Anisian in age.2. Annalepis lotiloba-Scytophyllum assemblageThe assemblage represented by the fossil plants from the Members 4-5 of the Badong For-matioa chiefly includes Annalepis lotiloba (MS), Equisetites arenaceus (Jaeger), Neocalamites sp., Scytophyllum sp., Sphenozamites sp., Sinoctenis pulcella Ye, Voltzia sp., Taeniopteris sp.,etc. This assemblage does not have very abundant genera and species and Pleuromeia of Members 1 -2, but evident increase of Scytophyllum in number and the occurrence of Sphenoza-mites and Sinoctenis which are commonly found from Late Triassic in South China. It differs conspicuously from that assemblage of Members 1-2 of the Badong Formation, and is suggested to be of Ladinian age.II. Ecology of the Anisian plant assemblageMost of fossil plants of this assemblage are found from Member 2 of the Badong Formation which is mainly considered a set of shore deposit of the tidal flat. Based on the field observation, all types of plants from Member 2 are different from each other in their burial states. The pleuromeia is generally well preserved, with a complete plant and some appendices showing a growing state almost perpendicular to the bedding plane. This indicates that Pleuromeia may be autochthonous taphocoenose. The Annalepis has both sporadic sporophylls and rather perfect cones. Presumably, it has ever been transported for a short distance, but basically is subautochthonous taphocoenose. All other plants, preserved as fragments, might have been transported for a long distance.The Pleuromeia of Member 2 consists of 3 species of dwarf bushes, representing a mono-genetic association. It might grow in a relatively flat shoreline area, frequently influenced by waves and tides. When the tide was coming in, the plants might be submerged partly or totally;and while the t