Biology Reference
In-Depth Information
are indicated by an increase in conifers typical of higher elevations in the
western cordillera. Drying at the lower elevations in the latest Miocene is
revealed by the Teewinot Lake fl ora at Jackson Hole, Wyoming (8 Ma).
It is shrubland to near desert with some scattered arid elements such as
Ephedra
and
Sarcobatus
(greasewood). Pollen from the Idaho Group on the
Snake River Plain extends the drying record to 3-2 Ma near the Pliocene-
Quaternary boundary. By this time, deciduous hardwoods are rare. There
is an impoverished conifer forest, as well as components of a Great Basin
desert-shrubland (
Artemisia
,
Sarcobatus
), with up to 60 percent grasses.
Typical MATs in the region, still 3°C-4°C warmer than the present, were
declining. Grasslands to the west of the Rocky Mountains, such as the
Palouse Prairie of eastern Washington, Oregon, and adjacent Idaho, appear
around 3 Ma as a result of the uplift of the Cascade Ranges to a height suf-
fi cient to cast a rain shadow. These grasslands developed about 10 million
years later than those appearing around 13 Ma on the drier plains to the east
of the older Rocky Mountains.
Thus, at the beginning of the middle Miocene (16.3 Ma), the ecosystems
of northern North America are shrubland/chaparral-woodland-savanna,
with desert elements; grassland, by about 13 Ma in the Plains; beach/
strand/dune, mostly unrecorded because of habitat and preservation condi-
tions; freshwater herbaceous bog/marsh/swamp; and aquatic communities.
There was a coniferous forest ecosystem, both boreal and montane, and
probably a Gulf Coast pine forest association in sandy habitats (as opposed
to pine as an element on sandy soils in deciduous forest vegetation). This
association would increase after Eocene cooling diminished tropical vegeta-
tion and would develop further during the early Miocene, middle Pliocene,
and interglacial warm periods. There was lowland tundra (transitional and
locally variable at 15 Ma, recognizable and more extensive by 7-6 Ma), and
alpine tundra (earliest in the highlands toward the north beginning around
15 Ma).
Later, near the end of the Pliocene (2.6 Ma), the northern North America
ecosystems are desert, shrubland/chaparral-woodland-savanna, grassland
(with Palouse Prairie added around 3 Ma west of the Rocky Mountains),
beach/strand/dune, freshwater herbaceous bog/marsh/swamp, aquatic, co-
niferous forest, tundra, and alpine tundra. Mangrove with
Rhizophora
is not
widely evident as an ecosystem in the Tertiary of northern North America
(see Jarzen and Dilcher 2006, middle Eocene Florida), and its presence
today in southern peninsula Florida is probably a result of the most recent
of repeated introductions during the interglacial warm periods. Time, as
well as climate, may also be a factor because
Rhizophora
likely came from
