Biology Reference
In-Depth Information
there is a line of volcanoes that later will accrete onto the mainland as the
Cordillera Occidental. On the ascending slopes of the moderately elevated
uplands, there are Araucariaceae, Podocarpaceae, and cycadlike gymno-
sperms, an understory of ferns, some early angiosperms of the magnoliid
clade, and the beginnings of a lower to upper montane broad-leaved forest.
In far southwestern South America, the Cretaceous landscape consists of
low offshore volcanic island arcs slowly being added to the mainland as the
proto-Southern Andes and coastal islands. To the east of the mountains,
basins are forming as South America pulls apart from Africa, and these are
accumulating early angiosperm plants such as
Nymphaea
-like water lilies
and animal remains that will later constitute the extensive fossil fl oras and
faunas of Patagonia.
At 100 Ma, South America has been separated from Africa for about
20 million years along its southern coast, but it is still connected or nearly
so to the north with Africa, and to the far south through Antarctica with
Australia. This defi nes the broad land mammal fauna of South America of
the Cretaceous as Gondwanan, with its prominent Southern Hemisphere
dinosaurs and other reptilian components. A titanosaur named
Argentino-
saurus huinculensis
reaches a length of 40 meters and weighs 90 tons. Later,
while South America drifts as an isolated island continent, endemic therian
mammals (marsupials and placentals) evolve that defi ne the mammal fauna
as distinctly South American rather than broadly Gondwanan. Mammals
will include giant anteaters, tree sloths, ground sloths, armadillos, opos-
sums, porcupines, and a rhinoceros-like ungulate. In the Miocene deltaic
deposits of the proto-Orinoco River at La Venta, Colombia, there are dis-
tinctive fossils of the giant (for a rodent)
Phoberomys pattersoni
, weighing
some 1500 pounds, 3 meters in length, with a 1.5 meter tail, and the 40-
foot-long crocodile
Purussaurus
; while in Argentina and Brazil live fl ightless
predatory birds of the Phorosrachidae. The fossils also include lungfi shes
and the freshwater, long-necked turtles of the Chelidae. The transition be-
tween the Gondwanan and South American paleofaunas is especially well
preserved in Patagonia.
The MAT is 23°C-25°C between 10°N and 10°S, 15°C between 10°S
and 30°S, and 13°C south of 30°S. By the end of the Cretaceous, MAT will
decrease by about 4°C. In southernmost South America, the vegetation in
the Middle Cretaceous is mostly gymnosperms, with the angiosperms fi rst
appearing in the region sometime in the Aptian and Albian (120-110 Ma).
Nothofagus
is a temperate deciduous angiosperm tree or shrub growing
here in the Maestrichtian. Later in the Paleogene, with the onset of warm
climates,
Nothofagus
will disappear; then with cooler conditions after the
