The Evolution and Biogeographic History of Metasequoia

Abstract

The fossil record of Metasequoia Miki is extensive and demonstrates that the genus was widely distributed throughout North America and Eurasia from the early Late Cretaceous to the Plio-Pleistocene. The genus first appears in Cenomanian age deposits from western Canada, Alaska and the Arkagala and Koylma River basins in Russia and indicates that Metasequoia had achieved a wide distribution early in its evolutionary history. Exchange of Metasequoia between Asia and North America probably occurred across Beringia, which had become functional at the Albian-Cenomanian boundary (ca. 100 million years ago). However, if the inter-continental exchange of the early representatives of this genus occurred prior to the establishment of Beringia, migration would have still been possible across the Spitsbergen Corridor, which was functional during the Early Cretaceous. By the early Tertiary, the distribution patterns do not appear to have changed considerably from that seen during the Late Cretaceous, except that Metasequoia became a dominant constituent of the polar Broad-leaved Deciduous Forests. More importantly, the distribution of Metasequoia indicates that the genus grew and reproduced under a diverse range of climatic and environmental conditions throughout geologic time, including the cold and unique lighting conditions of the polar latitudes. Of particular interest is the apparent lack of Metasequoia fossils in Europe despite the presence of two land bridges linking North America and Europe throughout the early Tertiary and the drying of the Turgai Straits that separated eastern and western Asia up until Oligocene time. Metasequoia persisted in western Siberia and the Canadian Arctic until late Pliocene time, and in western Georgia and Japan until the late Pliocene-early Pleistocene. Following the apparent early Pleistocene extinction, Metasequoia re-appeared in southeastern China. The pronounced reduction in distribution during the Miocene appears to be coupled with increasing global aridity and cooling and increased competition for resources and habitat from representatives of the Pinaceae. With few exceptions, the bulk of the Metasequoia fossils described in the literature indicate that the fossils assigned to M. occidentalis are indistinguishable from the living species. The remarkable morphological stasis observed in Metasequoia demonstrates that the genus has remained unchanged, at least morphologically, since the early Late Cretaceous.