Plant fossils from the Lower Jurassic coal-bearing formation of central Inner Mongolia of China and their implications for palaeoclimate

Abstract

The plant macrofossils from the Lower Jurassic coal-bearing Hongqi Formation in the Xilinhot Basin, central Inner Mongolia of North China, are systematically documented for the first time. The flora contains 40 species of 21 genera, belonging to horsetails, ferns, Cycadopsida, Ginkgopsida, conifers, and isolated seeds of unknown affinity. The Filicopsida, predominant in the flora, are characterized by high species diversity of form genus Cladophlebis (with 12 species), abundant Hausmannia (Protorhipis) of the Dipteridaceae, and existence of Coniopteris and Eboracia of the Dicksoniaceae. Among the gymnosperm group, the Ginkgopsida rank the second of the flora, with nine species representing Ginkgoites, Sphenobaiera of ginkgoales and Phoenicopsis, Czekanowskia of czekanowskiales. The Cycadopsida consist of six species, four of which belong to genera Anomozamites, Pterophyllum, Nilssoniopteris, and Ptilophyllum of bennettitales and the other two to Nilssonia and Ctenis of cycadaleans. Conifers, with a relatively smaller number of species, are characterized by long and broad leaves, including genera Podozamites, Elatocladus, and Pityophyllum. Based on the stratigraphical ranges of the known species, the existence of Coniopteris and Eboracia, and the abundance of Hausmannia and Cladophlebis, the age of the studied flora can be determined as late Early Jurassic and possibly the Toarcian. The existence of climate sensitive elements, particularly the thermophilous plants such as the dipteridaceous fern Hausmannia, bennettitales Ptilophyllum, further demonstrates that the climate of Xilinhot area during the late Early Jurassic was warm and humid, but could be hot and even arid as in the subtropic zone at short intervals. It was warmer than during both the early Early Jurassic and early Middle Jurassic, indicating a temperature rising event occurred in the late Early Jurassic. Palaeobotanical and sedimentary data also support the conclusion that a warming and somewhat arid climatic event occurred in the terrestrial area of North China. This climate change event was coincided possibly with the Toarcian Oceanic Anoxic Event (T-OAE), and could be the response of the terrestrial ecological system to the great and rapid change of palaeoenvironment in the marine ecosystem.