Biology Reference
In-Depth Information
In summary, the ecosystems of South America from the Middle Creta-
ceous through the early Eocene are similar in kinds and distribution, but
different in composition, to their counterparts in northern North America,
Mexico, the Antilles, and Central America. In the northern, equatorial
parts of South America there were tropical forests and paratropical rain for-
ests that toward the early Paleocene had formed a lowland neotropical rain
forest in response to increasing and uniform warmth and moisture. It ex-
tended north with elements present in the Arctic region and intermingled
with tropical to warm-temperate plants moving across the North Atlantic
land bridge, and to the south where Australasian plants were moving across
Antarctica to Weddellia during the LPCM and EECL. The period around
55 Ma was the most extensive greening of the New World in the last 100 mil-
lion years. Open and drier sites accommodated elements such as ephedroid
and Welwitschia -related plants (in northern South America), while wetter
habitats across the continent had versions of freshwater herbaceous bog/
marsh/swamp and aquatic plant formations ( Equisetum , Isöetes , Azolla , Sal-
vinia ). A kind of mangrove community (without Rhizophora ), and probably
beach/strand/dune vegetation (probably without grasses until about the
middle Eocene), grew along the coast.
In the far south in the Cretaceous through about the middle Paleocene,
there was a limited polar broad-leaved deciduous forest as one component
of what may better be described generally as a lower to upper montane
broad-leaved forest. When this forest is considered throughout its south-
ern range, early on there was a mixture of gymnosperms (e.g., Araucaria ,
Austrocedrus , Fitzroya ), deciduous angiosperms (e.g., Nothofagus ), and some
limited warm-temperate to subtropical species that in aggregate constitute
an austrotropical counterpart to the boreotropical fl ora. In the late Paleo-
cene and early Eocene, if a south-polar broad-leaved deciduous forest (as
opposed to individual elements like Nothofagus ) was present, it would have
been limited to the higher elevations and to southernmost South America
and Antarctica. Both in northern North America and in southern South
America, the tropical component began to diminish after the early Eocene.
In the north, it was eventually replaced by a boreal coniferous forest and
tundra. To the south, a cool to cold-temperate deciduous forest (lower to
upper montane broad-leaved forest) was present to the west of the South-
ern Andes, and there were the beginnings of a cold shrubland/woodland
steppe to the drier eastern side. These trends in climate and vegetation in
the southern fossil fl oras are refl ected in leaf size, which begins to decrease
after the middle Eocene.
Recall that twelve modern ecosystems are recognized in this treatment
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