Late Early–Late Cretaceous floras of the North Pacific Region: florogenesis and early angiosperm invasion

Abstract

During the early–middle Albian, a single uniform flora existed in the North Pacific Region. In the middle Albian, structural and landscape modification of the region resulted in its subdividing into several floristic-palaeogeographic subregions. Latest Albian and Late Cretaceous floras of coastal plains of North-eastern Asia and Alaska were dominated by angiosperms, whereas within the Asiatic continental interior depressions and the highlands of the Okhotsk–Chukotka volcanogenic belt angiosperm-dominated floras coexisted with floras dominated by ferns and gymnosperms. The observed phenomenon can be explained by the early angiosperm ability to colonise disturbed coastal plain environments due to their abbreviated reproductive cycle and generalised methods of pollination and seed dispersal. In North-eastern Asia and Alaska the first angiosperm remains were found in deposits of an alluvial–deltaic origin. During the early–middle Albian these plants probably occupied disturbed near-channel environments. Within the coastal plains and lowlands of the North Pacific Region, repeating transgressions and regressions and lateral river channel migration constantly disturbed environments and provided fresh sedimentary surfaces that were colonised by angiosperms more readily than by other plants. Climatic fluctuations and proximal volcanic eruptions probably contributed to the formation of habitats favourable for early angiosperms. Angiosperm-rich plant communities gradually invaded the Asiatic continental interiors along river valleys and other disturbed habitats and replaced previous fern–conifer-dominated mature climax vegetation. The data presented here do not support the hypothesis of the initial dispersal of early Cretaceous angiosperms along mountain ranges but support the hypothesis of angiosperm dispersal around coastal margins and into continental interiors along disturbed river habitats.