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Origin, Evolution and Distribution of the Taxodiaceae
Yu Yong-fu
Acta Phytotaxonomica Sinica. 1995, 33 (4 ):
362-389.
In the present paper, the distribution center, the place and time of origin, the way of dispersal and the formation of modern distribution patterns of the Taxodiaceae were discussed based upon studies of phylogeny, fossil history and modern distribution, with ref- erence to the paleogeography and paleoclimatology. The Taxodiaceae, containing 9 genera, 12 species and 3 varieties, is a small family of subtropical to Warm temperate trees with highly disjunct and restricted distribution, occur- lng in East Asia, North America and Australia. Of these nine genera, seven are monotypic, i. e., Cryptomeria, Glyptostrobus, Sequoiadendron, Sequoia, Metasequoia, Cunninghamia and Taiwania, while the other two are ohgotypic, i. e. , Taxodium consisting of 2 species and Athrotaris containing 3 species. Moreover, none of the genera is distributed on more than one continent. Athrotaxis, endemic to Tasmania, is the only genus that is found in the Southern Hemisphere. Metasequoia and Glyptostrobus are endemic to China. Cryptomeria is found in China and Japan. Cunningharnia is widely distributed in China and northern Viet Nam. Taiwania is native in the Chinese mainland, Taiwan and northern Myanmar. Se quoiadendron and Sequoia are endemic to western America. Taxodium is distributed in southeastern North America, Mexico and Guatemala. According to Takhtajan's (1986) regionalization of the world flora, the number of species in every region of the world was statistically surveyed. East Asia Region, with 5 genera and 5 species, is most aboundant in number of genera and species. In China, accord ing to Wu's ( 1979 ) regionalization of Chinese flora, Sino-Japanese forest subkingdom, with S genera and 5 species, is richest in genera and species,Sino Himalayan forest subkingdom, with 3 genera and 3 species, ranks second. The region ranging from the Yangtze river, Qinling Mts. to South China (the southern part of East Asia Region) is the modern distri- bution center of the Taxodiaceae, where the genera are found to reflect basically main phylo- genetic stages of the family, including the most primitive genus Cryptomeria, more primitive genus Glyptostrobus, relatively advanced genus Metasequoia, the more advanced genus Cun- ningharnia and the most specialized genus Taiwania. Despite the limited numbers of extant genera and species, and their restricted geographi- cal distribution, the Taxodiaceae, with a great number of fossil remains, was a very large group of plants in geological times and once played an important role in the forest vegetation of the Northern Hemisphere. The fossil distribution maps of all the living genera and several important extinct genera were drawn. The developments of these genera and the family were discussed. The earliest appearance of the Taxodiaceae was in the deposits of the Middle Jurassic in Eurasia, and it began to occur in North America during the Early Cretaceous. In the Cretaceous times, the Taxodiaceae, contaning all the modern genera and a large number of fossil genera with most of them extinct after the Mesozoic, attained its greatest diversity. During the Middle-Late Mesozoic and the Early Tertiary, the Taxodiaceae was highly di- verse and widely distributed throughout the Northern Hemisphere , reaching the higher lati- tude region of the Arctic Circle, such as New Siberian Islands, Greenland, Spitzbergen , Franz Josef Land and Ellesmere˚˚ Islands at about 82˚ N. Most of the extant genera once had three or two distribution centers: Glyptostrobus, Taxodiurn and Sequoia in East Asia, west- ern North America and Europe; Metasequoia in East Asia and western North Americas Cryptomeria, Cunninghamia and probably Taiwania in East Asia and Europe; and Se- quoiadendron in Europe, North America and East Asia. The extant genera, especially Glyptostrobus, Taxodium, Sequoiadendron, Sequoia and Metasequoia, were very important elements of the temperate flora in the Northern Hemisphere during the Late Cretaceous and Tertiary. Later than in East Asia and North America, most of the living genera began to appear in Europe during the Early Tertiary, where taxodiaceous plants increased gradually from the Paleocene to Miocene, but subsequently underwent a drop, as other conifers did. The Taxo diaceae was also more diverse and more widespread in the Southern Hemisphere in the past than at present. The modern distribution of the Taxodiaceae seems to provide very little information as to the determination of its origin place. Based primarily on the fossil evidence, the author considers that the northeastern part of East Asia at higher middle latitudes (about 35˚-60˚ N), embracing northeastern and northern China, southeastern Siberia, Korea, Japan and Sakhalin etc. , might have been the area of the origin and early differentiation of the Taxodi- aceae. The origin may date back to the Early Jurassic or the Late Triassic. After appeared on the earth, the Taxodiaceae dispersed westward to Europe and spread to the Southern Hemisphere by crossing the Tethys Sea because of the less distinct zonation of climate. In the Early Cretaceous, when Lanrasia had not broken up completely, the Tax- odiaceae migrated from the Europe westward to North America, where the absence of Jurassic records might have been the result of its dry environment. Many early genera un- derwent extinction during the Cretaceous. Most of the extant genera, including Cryptomeri- a, Glyptostrobus, Taxodium, Sequoia, Metasequoia, Cunninghamia and Taiwan&, and some fossil genera, might be differentiated from their ancestral complex in the northeastern part of East Asia during the Early Cretaceous or the Late Jurassic. They dispersed to the northeastern Siberia and then migrated eastward to western North American via Bering Land Bridge in the Late Cretaceous, where they flourished and formed a secondary distribu- tion center. As the Mid-Continental Seaway through North American, a successful phyto- geographic barrier during the Late Cretaceous, disappeared in the Latest Cretaceous, the Taxodiaceae spread to eastern North American and then by North Atlantic Land Bridge, which exis
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