Biology Reference
In-Depth Information
erosion of silicate rocks, further reducing CO
2
concentration, and provid-
ing new temperate habitats in the highlands. The Rocky Mountains and the
Sierra Madre were at least half their modern elevations by the end of the Eo-
cene, and early uplift in the Sierra Nevada began at the Eocene-Oligocene
boundary at 33 Ma. The principal rise of the Andes Mountains would be
later in the Miocene. During this interval of the middle Eocene through
the early Miocene, South America separated from Antarctica through the
Drake Passage (32-28 Ma), and it remained an island continent for some
29 million years until reconnected with North America around 3.5 Ma.
With cooler temperatures, there was also less evaporation of water from
the ocean surface, resulting in less atmospheric moisture and greater sea-
sonality. The continued drainage of epicontinental seas further promoted
continentality. Paleosols from the Badlands of South Dakota show a de-
crease in annual precipitation from 1000 mm in the late Eocene, to 500-
900 mm in the early Oligocene, to 450-500 mm later in the Oligocene, to
250-450 mm at the Oligocene-Miocene boundary (Retallack 1983, 1990).
Sea levels were falling because reduced amounts of crustal material were
being generated at the midocean ridges, and because early glacial ice was
forming in the Eocene on Antarctica, Greenland, and in the Arctic. More
widespread glaciations began around 35 Ma (fi g. 3.4), especially on Antarc-
tica, accounting for the notable drop in sea level around 30 Ma (fi g. 3.1).
Furthermore, the inexorable process of evolution continued to introduce
new forms with varied ecological characteristics. Thus, the hallmarks of
this interval were cooler and more seasonal climates, increased topographic
heterogeneity, and continued phylogenetic and ecologic diversifi cation of
plant lineages and communities
In addition to these trends of a global scope, there were others that af-
fected more specifi cally the biotas of northern North America. Spreading
along the Mid-Atlantic Ridge continued into the North Atlantic until by
around 45 Ma the connection between North America and Europe was be-
coming signifi cantly disrupted. This was at the same time that climates were
cooling. The greatest impact of the cooling was in the polar regions because
greater amounts of solar energy are absorbed during the longer distance
traveled to the Earth's surface at the poles than at the equator (I, 28-29).
Also, a given amount of energy is distributed over the increasingly slanted
surface toward the poles than at the equator, which is more perpendicular
to the sun. Palynofl oras in Greenland across the Eocene-Oligocene bound-
ary (33.5 Ma) indicate winter temperatures declined from more than 5°C
to 0°C-2°C, and MAP fl uctuated around 120 cm (Eldrett et al. 2009). The
combined effect of land disruption and cooling climates was to dramatically
