Applications of geochronology in biological evolutions in Asia

Abstract

During my PhD study at HKU, I mainly identify invertebrate fossils and establish their radio-isotopic ages from two Devonian floras, two Triassic entomofaunas, and one Cretaceous amber biota. Based on my paleontological and geochronological data, I discuss the importance for these key evolutionary events. The Hujiersite flora (from Xinjiang, NW China) encompasses the most diverse Middle Devonian plant fossils in China. It mostly resembles the Givetian to Frasnian Campo Chico Flora of Venezuela, and the Givetian Xichong Flora of Yunnan, southern China. This flora is herein dated to approximately 385−380 Ma (Givetian−early Frasnian), which is in agreement with the biostratigraphic correlation. The Zhulumute flora (from Xinjiang, NW China) generated the oldest-known Chinese forest in hosting three typical tree groups during Fransnian to Famennian. The maximum depositional ages, 370.7 ± 1.3 and 370.3 ± 1.4 Ma (Famennian), are suggested for this flora. These ages reflect that the Zhulumute flora is coeval with the Frasnian-Famennian boundary, and clarify the long-term controversy about the position of this boundary: whether this boundary lies within the lower part of the Hongguleleng Formation (a unit overlying the Zhulumute Formation). The Zhulumute flora totally differs from the underlying Hujiersite Flora, indicating a flora evolution during Middle to Late Devonian. Fossils representing typical forest-forming plants Lycopsida, Archaeopteridales and Cladoxylopsida indicate forest development in West Junggar coeval with the Kellwasser event. The forest expansion along coastal margins probably enhanced the Kellwasser event. The Triassic Tongchuan (from Shaanxi, NW China) and Karamay (from Xinjiang, NW China) entomofaunas contain some rare insect fossils. The Tongchuan entomofauna is dated ca. 238−237 Ma (Ladinian), and the Karamay entomofauna is assigned a Carnian age. Two entomofaunas comprise of the earliest definite Trichoptera and Hemiptera, diverse Coleoptera and Mecoptera. The Holometabola are dominant in two entomofaunas, revealing a Middle Triassic Holometabola radiation probably triggered by the various plants. This finding suggests that the diversification of aquatic insects (a key event of ‘Mesozoic Lacustrine Revolution’) has already begun by the Middle Triassic, providing new insights into the early evolution of freshwater ecosystems. Latest Cretaceous Tilin amber biota (from central Myanmar) is initially reported after two detailed geological investigations. Isotopic age (~72.1 Ma) and ammonites give a latest Campanian age for this biota. Py-GC-MS analysis indicates a conifer tree source for Tilin amber, reflecting that the gymnosperms were still abundant in the latest Campanian equatorial forests. Eight orders and 12 families of insects have been found in Tilin amber so far, making it the latest known diverse insect assemblage in the Mesozoic. The appearance of the extant ant subfamilies suggests that the ant turnover from stem-groups to crown-groups had already begun at ~72.1 Ma (439 words).