Biology Reference
In-Depth Information
Rhizophora
(Germeraad et al. 1968). A late Eocene assemblage of permin-
eralized woods and other fossils from 39.4 Ma, the El Bosque Petrifi cado
Piedra Chamana in the Huambos Formation of Peru, is under study (Wood-
cock et al. 2009).
The presence of a lowland neotropical rain forest is suggested by logs
up to 4 m in length in the late Eocene Itaquaquecetuba Formation of the
São Paulo Basin (Fittipaldi et al. 1989). Outcrops of sandstone, and sandy
coastal areas, may have supported communities of savanna or grassland by
the end of the early Miocene, but following the sequence elsewhere, their
development and expansion was favored by enhanced cooling, drying, and
greater seasonality beginning in about the middle Miocene.
At the base of the moderately elevated Andes Mountains in the early
Miocene, there were likely wet and super-wet areas resulting from the con-
fi guration of the east-facing slopes of the mountains vis-à-vis winds angling
southwest across the Amazon lowlands (Killeen et al. 2007; fi g. 6.7). For
most of the length of the Northern and Central Andes, the winds intercept
the mountains at an angle. Recall that at the “elbow” of the Andes in Bo-
livia, however, the interception is more at a right angle, and precipitation
is especially high on the slopes and on the lands below. On these slopes,
there was likely a tropical rain forest or the wet phase of a lower to upper
montane broad-leaved forest, with drier vegetation in the Andean valleys,
as at present, where soils, slope, exposure, and rain shadows augment the
reduced moisture. To the west of the mountains, there were other dry ar-
eas, in part, as the result of a limited rain shadow, and in part to the cooling
Humboldt Current that was beginning to form because of closure of the
Drake Passage, thermal isolation of Antarctica from warmer equatorial wa-
ters, southern drift of the continent, early continental glaciations, and cold
meltwaters starting to fl ow into the ocean. The dryness along the western
coast appearing about or just prior to the middle Miocene, circa 15 Ma, is
evident from sediments in the central Andean fore-arc basins of Chile (Sáez
et al. 1999). Slightly older sediments from Peru of early to middle Miocene
age are alluvial deposits and do not show appreciable dryness. Early Mio-
cene water temperature along coastal Chile around 24 Ma was 5°C higher
than at the present mean annual minimum of 20°C at 45°S (Nielsen and
Glodny 2009). Thus, the Atacama Desert was just beginning to form in the
early Miocene. There is no tundra or páramo evident at this time.
There is a quantum leap from the meager information available at scat-
tered sites in northern South America to the paleobotanical “hot spot” of
Argentina. Assemblages ranging in age from the Cretaceous through the
Oligocene are known from Santa Cruz Province (Hunicken 1955), and
