Evidence of Environmental Mosaic in Toarcian Jurassic China

A late Early Jurassic flora from Xilinhot Basin of Inner Mongolia,with discussions on coeval climate in China

Authors:

Shenghui et al

Abstract:

The flora from the Jurassic Hongqi Formation of the Xilinhot Basin,Inner Mongolia,North China,is composed of 37 species in 21 genera:Equisetites sp. and Neocalamites sp. of Equisetales;Hausmannia ussuriensis Kryshtofovich,H. leeiana Sze,Coniopteris bella Harris,C. sp.,Eboracia lobifolia(Phillips)Thomas,Todites williamsonia(Bronign.)Seward,Cladophlebis angutula (Heer)Fontaine,Cl. asiatica Chow et Yeh,Cl. cf. asiatica Chow et Yeh,Cl. haiburnensis(L. et H.)Bronigniart,Cl. hsiehiana Sze, Cl. lobifolia(Phillips)Bronigniart, Cl.ingens Harris,Cl.pseudoraciborskii Srebrodolskaja,Cl. shansiensis,Cl. scariosa Harris Sze and Cl. sp. nov. of Filicopsida;Anomozamites turkmenicus Burakova,Nilssoniopteris angustifolia Wang,Ptilophyllum contiguum Sze,Pterophyllum sp.,Nilssonia sp. nov. and Ctenis sp. of Cycadopsida;Ginkgoites ferganensis Brick,Ginkgoites hottonii (Sternb.)Heer, Phoenicopsis angustifolia Heer,Czekanowskia setacea Heer, Cz. latifolia Turutanova-Ketova and Sphenobaiera sp. of Ginkgopsida;Elatocladus sp.1, Elatocladus sp.2,Podozamites eichwaldi Schimper,Pityophyllum longifolium(Nath.)Moeller and Pityophyllum sp. of Coniferopsida and seed Carpolithus sp. The flora is characterized by abundant ferns and appearance of Coniopteris and Eboracia of the Dicksoniacea and some thermophilous elements. The thermophilous plants include dipteridaceous fern Hausmannia and Ptilophyllum of Bennettitales. The age of the flora is late Early Jurassic and most possibly the Toarcian. The age of the Hongqi Formation is chiefly late Early Jurassic and its upper part is possibly the early Middle Jurassic. The appearance of the climate sensitive plants,particularly the south-type(thermophilous)elements,indicates that the Xilinhot Basin was located in the warm temperate zone-subtropic zone during the late Early Jurassic. It was warmer than that of the early Early Jurassic and early Middle Jurassic,indicating a temperature raising event in the late Early Jurassic. Based on the palaeontological and sedimentary data, there existed 5 palaeoclimatic regions during the late Early Jurassic in China,namely the eastern Heilongjiang warm-cool climatic region,North China warm-temperate and warm-humid climatic region,central China tropic-subtropical semi-arid and semi-humid climatic region,central-southern China tropic-subtropical arid climatic region and the Tibet-western Yunnan tropical arid climatic region. The boundary between the warm-temperate warm-humid climatic region and the tropic-subtropical semi-arid and semi-humid climatic region was about 4-8 degrees of latitude northwards than both in the early Early Jurassic and early Middle Jurassic,also indicating a temperature arising event occurred in the late Early Jurassic. This climatic event most possibly happened in the Toarcian of Early Jurassic and is likely the response of the terrestrial ecological system to the Toarcian Oceanic Anoxic Event(T-OAE).