Biology Reference
In-Depth Information
ent were
Equisetum
,
Ginkgo
,
Metasequoia
, and angiosperm leaves similar to
Corylus
(hazel) and
Platanus
(sycamore) that constituted the warm/moist
phases of a deciduous forest. The recently discovered northernmost rec-
ord of the tapir lineage is from Ellesmere Island in Arctic Canada at 79°N
(Eberle 2005). On the North Slope of Alaska, early to middle Eocene cli-
mates are estimated as warm temperate to subtropical, with some decidu-
ous forest elements present such as
Alnus, Betula, Carya, Juglans
, and
Liquid-
ambar
(Frederiksen et al. 2002). The estimated MAT for northern Alaska in
the late Paleocene is 10°C-12°C.
In the early Eocene, temperatures rose by another 5°C-6°C, and at 55-
50 Ma, global warmth reached its peak for all of latest Mesozoic and Cenozoic
time. Warm temperatures persisted at maximum levels for 170,000220,000
years (Pagani et al. 2006) and remained at elevated levels for about 2-5 mil-
lion years. The atmospheric CO
2
concentration is estimated at 1125 ppmv
(Zachos et al. 2008) compared to the present 380 ppmv. The tropical rain
forest reached its maximum extent to 45°N-50°N, and grew farther north
toward the coasts. Lateritic paleosols are present at the same high latitudes.
Coastal paratropical forests grew at 60°N-65°N, and a notophyllous broad-
leaved evergreen forest occurs on a terrane on the Pacifi c Northwest coast
at 70°N-75°N, although it was transported some distance from the south.
In the Northwest Territories, the Eureka Sound Group preserves remains
of these paratropical and notophyllous evergreen forests. The tropical tree
fern
Cnemidaria
and the liana fern
Lygodium
make their fi rst appearance at
these latitudes in North America in the early Eocene.
During the Paleocene and early Eocene, a few deciduous forest trees
grew in the highest lands of the eastern United States. In the U.S. Geologi-
cal Survey's Oak Grove core from northern Virginia, there is pollen simi-
lar to
Alnus
(alder),
Betula
(birch),
Carya
(hickory, pecan), and
Ilex
(holly;
Frederiksen 1991). Migrations across the North Atlantic from Asia and
Europe were possible until about the middle Eocene, and North Ameri-
can fl oras include a number of warm-temperate species extinct there now
but still found in eastern Asia (e.g.,
Platycarya
,
Pterocarya
of the Juglan-
daceae, the pecan/hickory/walnut family). Climatically, the opportunity
for interchange between the New World and the Old World, particularly of
megathermal (tropical) and mesothermal (warm-temperate to subtropical)
organisms, was greatest in the late Paleocene and early Eocene. Physically,
land connections across the North Atlantic were nearly continuous, and a
northern DeGeer route (after Baron Gerard DeGeer, Swedish geologist)
and a more southerly Thulean route (from Thule, northwest Greenland)
accommodated warm-temperate and tropical taxa (Tiffney 1985; Graham