A genus of a single extant species of coniferous tree (and several extinct species described from fossils); deciduous, monoecious; main branches irregularly whorled; branchlets of several kinds, persistent or deciduous, developing from axillary buds. Branchlets, buds, and leaves opposite or sub-opposite. Seed cones terminal, with scales in deccusate pairs. (Fu, Yu & Mill 1999).
The genus Metasequoia was described by the Japanese botanist Shigero Miki in 1941, from fossils discovered in Japan in Lower Pliocene strata (Bean 1981). Several samples studied by Miki had previously been assigned to the related genus Sequoia, but Miki realised these and other unnamed samples differed from Sequoia in several respects, notably the opposite, not alternate or spiral arrangement of the leaves, buds and branchlets, which also helps to separate Metasequoia from two of its winter-deciduous cousins, Glyptostrobus and Taxodium. The deccusate arrangement of the seed cone scales are another distinction (Farjon 2005; Bean 1981).
Living trees of Metasequoia were discovered in China in 1941 just months after Miki had published the new genus, but it would be several years before the link between the two events was made and its significance realised – see M. glyptostroboides for further detail.
Most contemporary descriptions of the genus are in effect descriptions of M. glyptostroboides, the only extant species and a well known example of a deciduous conifer, but ancient Metasequoia species may have been evergreen: there are theories that winter-deciduous races evolved in response to seasonal fluctuations in light availability after ancient species were able to colonise northern latitudes in the much warmer past (e.g. Chaney 1948); and others that the trait evolved prior to the colonisation of northern latitudes, and that this gave Metasequoia a competetive advantage (e.g. Jagels & Equiza 2005).
Although over 20 species have been described from fossils, contemporary paleobotany considers that these probably represent no more than four species: M. disticha; M. foxii; M. milleri; and M. occidentalis (Wikipedia 2021).