Department of Biological Sciences, Kent State University, Kent, Ohio 44242.
Am J Bot. 1999 Jan;86(1):32-8.
The time of origin of cool-to-cold-temperate plants of northern affinities in the Latin American biota is unsettled. Two models have been proposed-a Paleogene origin from a once widespread temperate rain forest, and a Neogene origin by introductions from the north which is best supported by new evidence. Fourteen palynofloras of Tertiary age are now available from Mexico and Central America, in addition to numerous others from the southeastern United States and northern South America. Pollen of cool-temperate plants occurs in the Eocene of southeastern United States, but not in northern Mexico, central Panama, or northern South America. In the Miocene this pollen is sparse in deposits from Mexico and Guatemala, rare in Panama, and absent from northern South America. In the Pliocene pollen representing a diverse northern temperate element of ten genera is present in the Pliocene of southeastern Veracruz, Mexico, five in northeastern Guatemala, and two (Myrica, Salix) first appear in northern South America; Alnus and Quercus are added in the Pleistocene. This north-to-south and early-to-late pattern is consistent with the appearance of highlands in southern Central America and northern South America in the Neogene, closure of the isthmian marine portal between 3.5 and 2.5 Ma (million years ago), and the late Cenozoic cooling trend evident in the O/O-based paleotemperature curve.
北方亲缘关系的冷温带植物在拉丁美洲生物区系中的起源时间尚未确定。有两种模式被提出——古近纪起源于曾经广泛分布的温带雨林，以及新近纪由北方引入，这一模式得到了新证据的最佳支持。除了来自美国东南部和南美洲北部的众多花粉化石外，现在还可以从墨西哥和中美洲获得 14 个第三纪花粉化石。在古近纪，美国东南部有冷温带植物的花粉，但在墨西哥北部、中巴拿马或南美洲北部没有。在中新世，这种花粉在墨西哥和危地马拉的沉积物中稀少，在巴拿马则罕见，在南美洲北部则不存在。在更新世，代表十个属的多样化的北温带成分的花粉出现在墨西哥东南部的维拉克斯，在中美洲东南部的墨西哥有五个，在东北危地马拉有两个（杨梅、柳树）首次出现在南美洲北部；在更新世，桤木和栎木出现了。这种从北到南、从早到晚的模式与中新世中南美洲和南美洲高地的出现、350 至 250 万年前（百万年前）海峡海洋门户的关闭以及 O/O 基古温度曲线中明显的晚新生代冷却趋势一致。
