Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XXI, No.4


Phytogeographic and Phylogenetic History of the Genus Nothofagus Bl. (Fagaceae) in the Southern Hemisphere

Tanai, Toshimasa

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The genus Nothofagus Bl. (Fagaceae) is one of the representative tree genera in the temperate forests of the southern hemisphere, and its phytogeographic history has been in controversy regarding the origin and dispersal route. Based on a recent collection from the Tertiary of southern Chile, all the previously-described species of Nothofagus or Nothofagus-like leaf fossils from South America and West Antarctica were reinvestigated, and compared with leaf architecture of nearly all the extant species of the genus in their phylogenetic relationships. The investigation of straligraphic and areal distribution of fossils (leaves, wood and pollen) in South America reveals that Nothofagus gradually dispersed northward from the Antarctic region since Late Cretaceous to Miocene time. Leaves and pollen occur abundantly with various species in the Oligocene and the Lower Miocene of the Patagonia region compared with paucity in the same horizons of northern region. This northward dispersal corresponds with the considerably descreasing temperature since Late Eocene age, which was suggested by oxygen isotope studies of sub-antarctic marine animal fossils. The bibliographic survey of fossil records of Nothofagus leaves and pollen in other southern continents and deep-sea sediments further substantiated a conclusion that this genus seems to have been in southern origin during Late Cretaceous time. The comparison of leaf architecture with all the morphological characters of extant species allows that 36 extant species of Nothofagus are classified into five groups. A scheme of phylogenetic relationships of these extant species is proposed here, and it is consistent with breakup sequences of the Gondwanaland that is generally accepted by most workers.