Biology Reference
In-Depth Information
Only representatives from the lower elevations of the lower to upper
montane broad-leaved forest are present, such as Selaginella , Pteris , Podocar-
pus , Alchornea , Amanoa , Hedyosmum , and Ilex .
There are no members of the cloud forest, elfi n forest, northern temper-
ate deciduous forest, tundra, or higher elevation coniferous forest, ceja, or
páramo.
According to Hoorn (1993), a fl oodplain community called várzea, like
that presently found along the upper Amazon River, is a close modern ana-
log to the middle to late Miocene vegetation and environment of the Ama-
zon Basin (see fi g. 7.6).
Farther south, the eastern cordillera of Ecuador is being uplifted in the
middle Miocene, and by this time about half of the modern elevation of
the Central Andes has been attained. A midaltitude cloud forest of Mio-
Pliocene age is found in the high elevations of the Central Andes, document-
ing substantial uplift at this time (e.g., Graham et al, 2001). A vertebrate
fauna spanning the interval between the early and late Miocene shows a
trend from browsers, implying shrubs and small trees, to grazers and more
grasses (MacFadden et al. 1994). The elevation is suffi cient to begin block-
ing moisture from the east, and together with the cold Humboldt Current
and a global reduction in atmospheric moisture beginning in the middle
Miocene, there is enhanced formation of the Atacama Desert. Independent
evidence for aridity along the coast at this time is provided by sediments in
northern Chile (Sáez et al. 1999), and hyperaridity will become evident at
4-3 Ma.
The inland sea through the Entre Ríos region (Paraguay-Uruguay-Paraná
river basins) extends northward to the Mato Grosso Plateau. The dry region
of the Monte of Argentina began forming earlier with uplift of the Central
Andes, blocking moisture from the Pacifi c Ocean, but the trend at 17 Ma
is now being augmented by increased middle and late Miocene seasonality.
Thus, from the western United States at about 45°N to northern Argen-
tina at about 25°S, the evolution of dry lineages and the coalescence of dry
ecosystem elements toward modern versions is being facilitated by western
mountain ranges attaining at least half of their present elevation and form-
ing rain shadows to the east, edaphically coarse eroding soils, cold coastal
currents, and by the general climatic cooling that is enhancing seasonality.
A possible difference between the fauna at 17 Ma and at present in Argen-
tina is the presence of a diverse cadre of large caviomorph rodents and large
marsupial carnivores (borhyaeneids), endemic ungulates, and giant eden-
tates, and the absence of mammals of the families Cricetidae (Rodentia,
e.g., New World rats) and Mustelidae (Carnivora, e.g., weasels) because
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