Biology Reference
In-Depth Information
arctic Mountains by a combination of diatoms, palynomorphs, mosses, os-
tracodes, and insects dated at between 14.07 and 13.85 Ma. They reveal a
cooling of at least 8°C and record “the last vestige of a tundra community
that inhabited the mountains before stepped cooling that fi rst brought a
full polar climate to Antarctica” (Lewis et al. 2008, 10676). Tillites embed-
ded in basalts of Miocene to Quaternary age in Patagonia document that
glaciation in the lowlands was underway there in the middle Pliocene by at
least 3.5 Ma and mark the time when tundra elements, and probably early
versions of tundra vegetation, were present in the lowlands of far southern
South America. Farther north, the combination of high elevations and cool-
ing global climates resulted in glacial conditions in the Andean uplands in
the later Pliocene and Pleistocene. These conditions are widely established
to have fl uctuated within the past 2 million years as a result of various
climate-forcing mechanisms discussed below (the Younger Dryas, a mid-
Holocene dry period, the Little Ice Age, Heinrich events). In terms of indi-
vidual lineages, the interactions of climate, geology, and time are proving to
be of increasingly well-documented evolutionary importance. For example,
Ribas and colleagues (2007) studied the parrot genus
Pionus
using cyto-
chrome
b
(
cyt b
) and NADH dehydrogenase 2 (
ND2
) mitochondrial genes.
Their results showed that diversity in the montane lineages is “directly at-
tributable to events of Earth history,” that “the three lineages were trans-
ported passively to high elevations by mountain building, and that subse-
quent diversifi cation within the Andes was driven primarily by Pleistocene
climatic oscillations and their large-scale effects on habitat change” (Ribas
et al. 2007, 2399; see also Ohlemüller et al. 2008).
In terms of ecosystem history, with a few notable exceptions, middle
Miocene through Pliocene fossil fl oras are widely scattered, and many have
not been studied recently using modern methodologies or interpreted within
a global context of available information. Others, such as the Solimöes Ba-
sin of Brazil and the Amazonas Basin of Colombia, give a better indication
of conditions at specifi c places and processes infl uencing ecosystem devel-
opment (Hoorn 1993, 1994a-b, 1996). In these fossil assemblages,
Rhi-
zophora
is important because it identifi es the time of marine incursions into
the basins. A reconstruction of the middle to late Miocene setting is shown
in fi gure 7.6. It was a dynamic landscape as revealed both by frequent
changes in the vegetation and by alternating sequences of freshwater and
brackish-water fi sh faunas (Monsch 1998). Fluctuations continued until
the late Miocene, when inundations ceased except in the borderlands. This
event identifi es the time when uplift of the Northern Andes Mountains had
tilted the lowlands to the point that widespread tectonic-induced marine
